The invention relates to an electronic device, and a method of manufacturing an electronic device.

In the context of growing product functionalities of mounting devices equipped with one or more electronic components and increasing miniaturization of such electronic components as well as a rising number of electronic components to be mounted on mounting devices such as printed circuit boards, increasingly more powerful array-like components or packages having several electronic components are being employed, which have a plurality of contacts or connections, with ever smaller spacing between these contacts. Removal of heat generated by such electronic components and the mounting device itself during operation becomes an increasing issue. At the same time, mounting devices shall be mechanically robust so as to be operable even under harsh conditions.

A condenser for an LED substrate has been demonstrated (28th Annual IEEE Semiconductor Thermal Measurement and Management Symposium, SEMITHERM Symposium, 18-22 March 2012, San Jose, California, USA). Heat pipes are commonly used in modules for high power applications, CPU centric PCs, mainframes, etc. Heat pipes are also used in smartphones (NEC, Xperia 2).

Further background art in terms of heat removal is disclosed in US 6,935,022, US 5,268,812, US 5,095,404, US 6,082,443, US 8,072,763 and WO 2007/096313.

It is an object of the invention to provide a compact electronic device which allows to efficiently remove heat during operation.

In order to achieve the object defined above, an electronic device, and a method of manufacturing an electronic device according to the independent claims are provided .

According to an exemplary embodiment of the invention, an electronic device is provided which comprises a support structure comprising electrically insulating material and/or electrically conductive material, and a heat pipe structure synthesized during or after formation of the support structure (in particular so that the heat pipe structure forms part of the correspondingly processed support structure), being integrated in and/or on the support structure and comprising a thermally conductive shell (preferably formed by a single material) which fully surrounds and thereby hermetically seals a heat transporting medium (in particular fluid) in an interior of the shell with regard to a surrounding (in particular with regard to surrounding material of the support structure).

According to another exemplary embodiment of the invention, a method of manufacturing an electronic device is provided, wherein the method com- prises forming a support structure comprising electrically insulating material and/or electrically conductive material, synthesizing (or building up or constructing or structurally forming or composing) a heat pipe structure after or during forming the support structure (in particular the heat pipe structure may be synthesized by processing the support structure), wherein the heat pipe structure is synthesized to comprise a thermally conductive shell which fully surrounds and thereby hermetically seals a heat transporting medium (in particular fluid) in an interior of the shell with regard to a surrounding, and integrating the heat pipe structure in and/or on the support structure.

In the context of the present application, the term "heat pipe structure" may particularly denote a heat-transfer structure that combines the principles of both thermal conductivity and phase transition of a fluid in an interior of the heat pipe to efficiently manage the transfer of heat between two solid interfaces. At a hot interface of a shell of the heat pipe structure the fluid in a liquid phase in contact with a thermally conductive solid surface turns into a gas/vapor by absorbing heat from that surface. The gas/vapor then travels, guided by a guiding structure, along the heat pipe structure to a cold interface of the shell and condenses back into a liquid, thereby releasing the latent heat or phase transition heat. The liquid then returns, guided by the guiding structure, to the hot interface through one or more mechanisms such as capillary action, centrifugal force, gravity, or the like, and the cycle repeats. However, heat pipes may also use another heat transporting medium than a fluid, for instance a solid such as wax.

In the context of the present application, the term "synthesized during or after formation of the support structure" may particularly denote that the manufacturing procedure of forming the heat pipe structure can be integrated in the manufacturing procedure of forming or processing the support structure. The heat pipe structure may therefore be monolithically integrated within or on the support structure of the electronic device. In particular, synthesizing the heat pipe structure and forming the support structure may be performed at least partially simultaneously. Consequently, the heat pipe structure can be formed in particular by a combination of material supply or deposition procedures (such as plating), material removal procedures (such as chemically etching, laser drilling, mechanically drilling, cutting or punching) and pattern- ing procedures (such as the selective removal of only portions of a structure such as a layer, for instance using the concept of masks). Hence, the heat pipe structure may be constituted by the formation and patterning of individual layers which also form part of the support structure or which are arranged above or below the support structure to thereby form a monolithically inte- grated heat pipe structure. Hence, the heat pipe structure may be intrinsically assembled within an interior of the electronic device. Accordingly, an exemplary embodiment of the invention does not use a prefabricated heat pipe for which the manufacturing procedure of forming the heat pipe has already been finished before starting with the connection of the heat pipe to the support structure.

In the context of the present application, the term "hermetically sealed" may particularly denote that fluid communication between an interior of the heat pipe structure and an exterior thereof is disabled. Hence, fluid enclosed in an exterior shell of the heat pipe structure is prevented from leaving the heat pipe structure.

According to an exemplary embodiment, a heat pipe structure may be formed as part of an electronic device and may be intrinsically constituted during a procedure of processing and connecting individual constituents of the electronic device (such as one or more portions of the support structure). By synthesizing the heat pipe structure from its constituents during and/or after formation of the support structure it is possible to implement procedures from microtechnology such as layer deposition (such as plating) and selective material removal (for instance by laser drilling or etching). Hence, it is possible to form a heat pipe structure of extremely small dimension so as to accom- plish efficient cooling and compact construction of the electronic device. By fully surrounding and thereby hermetically sealing fluid in an interior of the heat pipe structure by a fully thermally conductive shell of the heat pipe structure (i.e. a shell forming a closed cover of uninterrupted thermally conductive material), a proper thermal coupling between the heat pipe structure and a heat source is guaranteed . By preventing any portion of the shell to be formed of poorly thermally conductive or even thermally insulating material, thermal weak points around the heat pipe structure may be prevented and proper functioning of the heat pipe structure with high thermal efficien- cy can be ensured . Such thermal weak points may occur when a through hole in the shell used for injecting the fluid in an interior of the shell would be maintained open or would be filled with only poorly thermally conductive material. In particular, the heat pipe structure can be created directly at package level.

In the following, further exemplary embodiments of the electronic device, and the method of manufacturing an electronic device will be explained .

In an embodiment, a material of the thermally conductive material fully surrounding and hermetically sealing the fluid has a value of the thermal conductivity of at least 5 W/m K, in particular at least 50 W/m K, more particularly at least 200 W/m K. Such values of the thermal conductivity are significantly better than the thermal conductivity of conventionally used electrically insulating materials (for instance FR4: «0,3 W/mK) of an electronic devices such as printed circuit boards, which therefore significantly improves the heat removal from the electronic device during operation of the electronic device with heat sources such as electronic components (for instance a microprocessor chip, etc.) mounted thereon. In an embodiment, the thermally conductive shell consists of copper.

In an embodiment, the heat pipe structure is configured as a micro heat pipe structure. Hence, by synthesizing the heat pipe structure during a procedure of patterning layers, connecting layers, forming structures by deposition and connection of material sections, it is possible to form miniature heat pipe structures. For example, a length of such a heat pipe structure may be in a range between 1 mm and 30 mm, in particular between 3 mm and 10 mm. A diameter of such a heat pipe structure may be in a range between 10 pm and 1000 pm, in particular in a range between 50 pm and 500 pm.

In an embodiment, the electronic device is configured as a semiconductor package, in particular a packaged semiconductor chip on which the heat pipe structure is formed, or an arrangement of the heat pipe structure mounted on a semiconductor chip (wherein the heat pipe structure and the semiconductor chip may then be packaged together). A packaged semiconductor chip may be semiconductor chip mounted on an electrically conductive carrier such as a leadframe and at least partially encapsulated within an encapsulation such as a mold compound . A heat pipe structure may then be formed on top of such a packaged semiconductor chip (see for instance Figure 27). Alternatively, it is also possible to monolithically integrate the heat pipe structure with a semiconductor chip and to encapsulate the arrangement of heat pipe structure and chip by an encapsulation such as a mold structure (see for instance Figure 28).

Preferably, the electronic device is however configured as a circuit board (see for instance Figure 26), in particular as one of the group consisting of a printed circuit board, an IC substrate, and an interposer. Other types of circuit boards can be implemented as well.

In the context of the present application, a "printed circuit board" (PCB) may denote a board of an electrically insulating core (in particular made of a compound of glass fibers and resin) covered with electrically conductive material and conventionally serving for mounting thereon one or more electronic members (such as packaged electronic chips, sockets, etc.) to be electrically coupled by the electrically conductive material . More specifically, a PCB may mechanically support and electrically connect electronic components using conductive tracks, pads and other features etched from metal structures such as copper sheets laminated onto an electrically non-conductive substrate. PCBs can be single sided (i.e. may have only one of its main surfaces covered by a, in particular patterned, metal layer), double sided (i.e. may have both of its two opposing main surfaces covered by a, in particular patterned, metal layer) or of multi-layer type (i.e. having also one or more, in particular patterned, metal layers in its interior). Conductors on different layers may be connected to one another with plated-through holes which may be denoted as vias. PCBs may also contain one or more electronic components, such as capacitors, resistors or active devices, embedded in the electrically insulating core.

In the context of the present application, an "interposer" may denote an electrical interface device routing between one connection to another. A purpose of an interposer may be to spread a connection to a wider pitch or to reroute a connection to a different connection. One example of an interposer is an electrical interface between an electronic chip (such as an integrated circuit die) to a ball grid array (BGA).

In the context of the present application, an "IC substrate" (integrated circuit substrate) may denote a physical body, for instance comprising a ceramic and/or glass material, onto which electronic components are to be mounted .

In an embodiment, the electrically insulating material comprises at least one of the group consisting of resin, in particular Bismaleimide-Triazine resin, glass fibers, prepreg material, polyimide, liquid crystal polymer, epoxy-based Build-Up Film, and FR4 material. The resin material may serve as a matrix material having the desired dielectric properties and being cheap and highly appropriate for mass production. The glass fibers may reinforce the circuit board and may stabilize it mechanically. Furthermore, the glass fibers may introduce an anisotropic property of the respective circuit board, if desired. Prepreg is a suitable material for the circuit board, since it is already a mixture of resin and glass fibers which can be further processed (and particular tempered) for converting it into PCB type dielectric material. FR4 is a flame- resistant dielectric material for PCBs which can be suitably used for the packaging concept according to exemplary embodiments.

It should furthermore be said that the electrically conductive material can be made of a metal such as aluminum or copper. Copper is particularly preferred in view of its high electric and thermal conductivity and its compati- bility with PCB technology.

In an embodiment, the heat pipe structure is configured and electrically connected to the electrically conductive material so that the heat pipe structure provides an electric function. Hence, the heat pipe structure may also serve as part of the electrical circuit of the electronic device (in particular PCB), since it may also be made of copper. Thus, it may function as an electronic component or in cooperation with another electronic component. For example, it may serve as an electric trace for carrying signals or energy, as a resistor, a capacitor, an inductance, etc. The heat pipe structure can be made of electrically conductive material such as copper and, for that reason, can also be used as electrical connector in the electric circuit of the PCB (for example as ground).

In an embodiment, the fluid (such as water) enclosed in an interior of the shell (for instance made of copper) is configured to be evaporated from a liquid phase into a gas phase (which may include a vapor phase) in the presence of heat at a portion of the shell, wherein the heat pipe structure comprises a guiding structure (which may comprise one or more narrow channels through which the fluid may flow under the influence of a capillary action) in an interior of the shell configured for guiding the evaporated gas phase from said portion to another cooler portion of the shell to thereby condense the fluid from the gas phase into the liquid phase, and for guiding back the condensed fluid towards said portion. Thus, the described hermetically sealed structure may allow for a highly efficient heat transfer while being able to be operated in a closed cycle manner. Such a heat pipe structure can be manufactured during the circuit board formation procedure. The fluid insertion may be carried out during the circuit board formation procedure as well, wherein an access hole in the shell can be closed with thermally conductive material before completing the procedure of formi ng/bui ld ing the heat pipe structure in the electronic device, for instance in the support structure. Advantageously, the entire shell surrounding and hermetically sealing the fluid may be formed of a single homogeneous thermally conductive material such as copper.

In an embodiment, the guiding structure is configured as an arrangement of microstructures provided along an inner surface of the shell, in particular being integrally formed with the shell. The guiding structure may be realized by a sinter body, grooves, microprotrusions and/or microrecesses, channels, and/or a surface roughness at an interior surface of the shell.

In an embodiment, the guiding structure is configured as at least one of the group consisting of a plurality of V-shaped narrow corners at an interior of the shell, and at least one, in particular a plurality of, (in a cross-sectional view) rectangular recesses (such as grooves) arranged along at least one interior side of the shell. In particular when the heat pipe structure is formed as a micro heat pipe structure, i.e. with very small dimensions and in particu- lar with smaller dimensions than those of conventionally used prefabricated heat pipes, corners of a channel or a cavity delimited in an interior of the shell or even a surface roughness at the interior surface of the shell may be small enough that the fluid in its liquid phase may accumulate in or at such micro structures due to capillary effects or the like.

In an embodiment, the heat pipe structure is constituted of patterned interconnected layer structures integrated within an interior of the electronic device. In particular, a part of the walls of the for instance cuboid or substantially cuboid shell may be formed by one or more (in particular planar and/or horizontal) layers of thermally conductive material (for instance forming part of the support structure or being arranged below or on top of the support structure) which are patterned to define dimensions of the heat pipe structure. Furthermore, another part of the walls of the for instance cuboid or substantially cuboid shell may be formed by one or more (in particular planar and/or vertical) trenches which are filled with thermally conductive material . More particularly, four vertical trenches filled with thermally conductive material together with two patterned horizontal layers of thermally conductive material may be interconnected, i.e. may be arranged with direct contact to one another and free of gaps in between so as to delimit together a hermetically sealed (for instance cuboid) channel within which the fluid can be accommo- dated .

In an embodiment, the heat pipe structure is monolithically integrated within the support structure. More particularly, the heat pipe structure and the support structure may be constituted as a common single body which is entirely formed of material of the electrically insulating material (more particu- larly prepreg, FR4, or the like) and the electrically conductive material (more particularly copper), and the fluid (preferably water).

In an embodiment, the heat pipe structure is entirely embedded within an interior of the support structure. In this embodiment, the entire exterior surface of the heat pipe structure is covered with material of the support structure. It is even possible that portions of the electronic device simultaneously serve as part of the support structure and as part of the heat pipe structure.

In an embodiment, the electronic device comprises a heat source cou- pling structure made of thermally conductive material and being configured for thermally coupling a portion of the heat pipe structure to a heat source mounting position of the electronic device at which heat source mounting position at least one heat generating electronic component is to be mounted. Such a heat source coupling structure may comprise one or more vias as vertical interconnections and/or may comprise patterned thermally conductive structures of the plurality of connected layer structures. Advantageously, the heat source coupling structure may be made of copper which is anyway present in PCB technology due to its high electric conductivity, but which simultaneously has also a very high thermal conductivity which renders this material particularly appropriate as heat source coupling structure. The heat source mounting position may be on a main surface of the plurality of connected layer structures of the support structure so that the heat source can be mounted on the circuit board as a surface mounted device (SMD). It is however also possible that the heat source mounting position is within an interior of the plurality of connected layer structures, for instance when the heat source is an embedded component. The heat source may be an electronic components generating heat during operation, which heat is to be removed using the heat pipe structure for cooling purposes.

Such a heat source, when being embodied as electronic component, may particularly denote any active electronic component (such as an electronic chip, in particular a semiconductor chip) or any passive electronic component (such as a capacitor). Examples of the embedded components are a data storage memory such as a DRAM (or any other memory), a microprocessor, a filter (which may for instance be configured as a high pass filter, a low pass filter or a bandpass filter, and which may for instance serve for frequency filtering), an integrated circuit (such as a logic IC), a signal processing component (such as a microprocessor), a power management component, an optical electrically interfacing member (for instance an optoelectronic member), a voltage converter (such as a DC/DC converter or an AC/DC converter), a cryptographic component, a capacitor, an inductance, a switch (for instance a transistor-based switch) and a combination of these and other functional electronic members.

In an embodiment, the electronic device comprises a heat sink coupling structure made of thermally conductive material and being configured for thermally coupling a portion of the heat pipe structure to a heat sink mounting position of the electronic device at which heat sink mounting position a heat sink is to be mounted. Such a heat sink coupling structure may comprise one or more vias as vertical interconnections and/or may comprise patterned thermally conductive structures of the plurality of connected layer structures. Advantageously, the heat sink coupling structure may be made of copper which is anyway present in PCB technology due to its high electric conductivity, but which simultaneously has also a very high thermal conductivity which renders this material particularly appropriate as heat sink coupling structure. Examples for suitable heat sinks are cooling bodies having cooling fins, a copper block, an aluminum block, a housing of the circuit board, any other cooling body or the like. A heat sink may also be a fluid-based (for instance water-based) cooling member attached to the electronic device.

In an embodiment, the electronic device comprises the heat sink mounted at the heat sink mounting position, in particular being arranged at a main surface of the electronic device and/or at a lateral surface of the electronic device. Mounting the heat sink on the main surface allows to use also large dimensioned heat sinks which efficiently remove a large amount of heat from the circuit board . Mounting the heat sink on a (usually non-used) lateral surface of a sheet-shaped or plate-shaped circuit board may however allow to use the main surface(s) of the circuit board very efficiently for mounting electronic components thereon, thereby rendering the circuit board highly efficient.

In an embodiment, the support structure comprises or consists of a laminated layer structure. Thus, the support structure may be formed by pressing together various layers of electrically conductive material and electrically insulating material. Such layers may be full continuous layers or may be patterned layers. The support structures may also comprise one or more vias as vertical interconnects. In an embodiment, the method further comprises forming a first recess in a first thermally conductive structure, forming a second recess in a second thermally conductive structure, and connecting the first thermally conductive structure and the second thermally conductive structure to one another so that the first recess and the second recess form a common cavity of the heat pipe structure, the fluid being located in the cavity. By forming a cavity by aligning two recesses in two thermally conductive structures, it is possible to form a relatively large cavity with a relatively large interior microstructured surface and therefore having a very efficient heat removal capacity.

In an embodiment, the method further comprises forming a recess in a first thermally conductive structure, providing a (for example at least partially planar) second thermally conductive structure, and connecting the first thermally conductive structure and the second thermally conductive structure to one another so that the first recess and a surface portion of the second thermally conductive structure form a common cavity of the heat pipe structure, the fluid being located in the cavity. For instance, the second thermally conductive structure may function as a simple cover. In such an embodiment, a very simple manufacturing is possible, since only one of two thermally conductive structures needs to be provided with a recess such as an oblong groove. Apart from the reduced effort for forming only one recess, also efforts in terms of aligning two recesses become dispensable in such an embodiment. Furthermore, such an embodiment allows to manufacture extremely small sized heat pipes.

In an embodiment, the cavity has a shape selected from a group con- sisting of an oblong shape and a two-dimensional shape. A cavity with an oblong shape has an inverse stripe shape and therefore has a substantially one-dimensional design. A cavity with a sheet-like shape has a larger extension in two perpendicular directions.

In an embodiment, the method further comprises forming the heat pipe structure by forming a recess in the support structure, and applying thermally conductive material onto exposed walls of the recess to thereby form an annular structure of thermally conductive material. Thus, a very simple procedure of forming a circumferentially closed shell of the heat pipe structure can be provided by merely depositing material onto exposed surface portions of the recess. When designing the recess with a suitable shape, the material deposition will directly result in the formation of an annularly closed thermally conductive portion of the shell (see Figure 2).

In an embodiment, the recess is formed in the support structure with a shape which is narrower closer at a surface of the support structure than deeper within the support structure. In other words, the recess may be designed in such a manner that a cross sectional area of the recess close to a main surface of the support structure is smaller than a cross sectional area of the recess further away from the main surface.

In an embodiment, the recess is formed by milling or laser drilling. In particular, it is possible to form a shield structure having a through-hole on the support structure, and to form the recess by laser drilling to thereby remove material of the support structure selectively through the through-hole. However, alternative recess formation procedures also possible, for instance by chemical etching. Other possible technologies for forming a recess are embossing, imprinting, etc.

In an embodiment, the method comprises providing a stop structure defining a stop of abrasion of material when forming the recess in the support structure. A stop structure may be a structure having different physical and/or chemical properties than the other material of the support structure above the stop structure so that the material removal procedure will automatically stop when it reaches the stop structure, thereby defining a precise end of the recess. For example, when the removal of material is performed by chemical etching, the stop structure may be made of a material which is not etchable by an etchant which etches the support structure. When the removal of material is performed by laser drilling, the stop structure may be made of a material which is not removable by a laser beam which removes material of the support structure.

In an embodiment, the recess is formed with substantially trapezoidal shape and/or the applied thermally conductive material forms a substantially triangular structure in a cross-sectional view. For example, such a kind of recess is also denoted as an elephant foot (compare Figure 1) which is conventionally undesired in terms of processing layer structures, but which can be advantageously used by exemplary embodiments of the invention for the formation of the heat pipe structure.

In an embodiment, the thermally conductive material is applied galvani- cally. In other words, the arrangement with the support structure and the recess (for instance having an elephant foot shape) only needs to made subject of a copper deposition procedure (for instance, galvanic deposition) in which thermally conductive (and usually electrically conductive) material is applied to automatically form the shell of the heat pipe structure along an exposed surface of the elephant foot shaped recess.

In an embodiment, the method further comprises forming the heat pipe structure by forming at least one recess in a first body of the support structure and forming microstructures within the at least one recess, forming at least one further recess in a further body of the support structure and forming further microstructures within the at least one further recess, and connecting the first body and the second body to one another so that the at least one recess and the at least one further recess with their microstructures form at least one closed channel. In such an embodiment, two material removal procedures may be combined . The first one removes material of the support structure (or a preform thereof) for forming the large recess in which, in a second material removal procedure, a large plurality of microstructures are formed. The latter formation may be performed with a laser beam having a very small diameter or a chemical treatment resulting in the formation of pores in the exposed material of the recessed support structure, to thereby form a porous guiding structure.

In an embodiment, the method further comprises applying a layer

(preferably of uniform thickness) of thermally conductive material on each of the at least one recess with the microstructures of the first body and the at least one further recess with the microstructures of the further body so that the layers have corresponding microstructures at their exposed surfaces.

Therefore, the recess with the microstructures may serve as a screen or master structure on which a layer of thermally conductive material with uniform thickness may be deposited, so that the pattern of microstructures at the exposed surface of the recess translates into a pattern of corresponding microstructures in exposed surfaces of the deposited layer of thermally conductive material . This layer of thermally conductive material (for instance copper) may then serve as shell and guiding structure of the heat pipe structure.

The aspects defined above and further aspects of the invention are ap- parent from the examples of embodiment to be described hereinafter and are explained with reference to these examples of embodiment.

The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited .

Figure 1 to Figure 4 illustrate different views of structures obtained during carrying out a method of manufacturing an electronic device according to an exemplary embodiment of the invention.

Figure 5 to Figure 9 illustrate different cross-sectional views of structures obtained during carrying out a method of manufacturing an electronic device according to another exemplary embodiment of the invention.

Figure 10 to Figure 13 show images taken of a microchannel structures manufactured during carrying out a method of manufacturing an electronic device according to an exemplary embodiment of the invention.

Figure 14 to Figure 18 illustrate different plan views and cross-sectional views of structures obtained during carrying out a method of manufacturing an electronic device according to yet another exemplary embodiment of the invention.

Figure 19 to Figure 26 show cross-sectional views of electronic devices (and preforms thereof) embodied as printed circuit boards according to exemplary embodiments of the invention

Figure 27 shows a cross-sectional view of an electronic device embodied as packaged electronic chip with a heat pipe structure manufactured integrally therewith according to an exemplary embodiment of the invention .

Figure 28 shows a cross-sectional view of an electronic device embodied as electronic chip with heat pipe structure manufactured integrally therewith, wherein the electronic chip-heat pipe structure arrangement is encapsulated by an encapsulant according to an exemplary embodiment of the invention.

Figure 29 illustrates a cross sectional view of a monolithically integrated heat pipe structure forming part of correspondingly processed and

interconnected layer structures of a circuit board according to another exemplary embodiment of the invention. The illustrations in the drawings are schematical. In different drawings, similar or identical elements are provided with the same reference signs.

Before exemplary embodiments will be described in further detail referring to the figures, some general considerations of the present inventors will be presented based on which exemplary embodiments have been developed.

In power electronics (especially when considering wide-bandgap semiconductors such as GaN, SiC, diamond, etc.), a limiting factor is generally temperature rather than the traditional current. With junction temperatures well above 150°C, packaging polymers tend to age too quickly, causing premature failures through delaminations. Additionally, increased junction temperature leads to decreased efficiency and semiconductor failure, which can be particularly critical in e-mobility applications. To mitigate those effects, improved thermal-management strategies need to be developed and implemented, without dramatically increasing size, weight, upfront or in-use costs.

According to one exemplary embodiment of the invention, built in (par- ticularly galvanically deposited copper) micro heat pipe structures are provided (see for example the embodiment of Figure 1 to Figure 4). Such an embodiment provides a method of and a structure for cooling hot spots in printed circuit boards (PCBs). More particularly, it relates to micro heat pipes formed in the core of a PCB, a method of cooling devices embedded in or installed on the PCB using micro heat pipes, and a method of fanning the micro heat pipes. This embodiment is based on the creation of micro trenches with a narrow opening and a broad base in the core material of the PCB. Sharp corners in the interior of the pipe created by the shape of the trench or channels and the natural or enhanced (for instance via etching) copper surface roughness may work as wick for the micro heat pipes. The trench can be formed, for example, via laser, mechanical drilling, plasma etch, photolithographic methods etc. The shape of the trench shall be engineered in such a way that the interior of the copper heat pipe structure has very sharp edges. It can assume, for example, triangular shape. The trench shape can be engineered depending on the method employed, for example, the drill/cutter geometry in case of a mechanical process or the laser pulse shape and power in case of laser drilling . These trenches can be filled with chemical copper and/or galvanic copper. Since the opening of the trench is narrow the trench will be closed before being com- pletely filled with copper. This process will form a micro copper pipe embedded in the core material of the PCB. A typical trench width and a typical deepness may be approximately lOOprn, for instance may be each in a range between 30 prn and 300 prn. The heat pipe structure shall be designed in a way that at least one aperture is left at the end of the galvanic process. One possible way to do it to leave a large opening in one end of the heat pipe structure. The open aperture can be used to let the liquids used in the galvanic process out. Afterwards, the pipe can be filled with liquid (such as water, methanol, etc.) and closed with a thermally conductive material. The closing method can vary. It can be done, for example, via a copper coin which can be cold welded to the open end of the micro heat pipe and serve also as heat sink. Other possible ways to close the micro heat pipe can be used (for example inkjet printing methods, dispensed silicon, etc.).

The use of the micro heat pipe can be diverse. For example, it is possible to attach a device on one or many micro heat pipes and use the welded coin as heat sink. Another possibility is to attach the device to be cooled on the middle of one or many micro heat pipes and attach heat sinks at each of the endings of the micro heat pipes. Besides that, thermal copper vias perpendicular to the plane where the micro heat pipe structures lie can be directly attached to the micro heat pipe structure. Regular electrical connections inherent of PCBs can be built in the same plane (core) of the heat pipe structure. Clearly, the micro heat pipe structures grown in the core of the PCB allows great freedom of design. By creating the micro heat pipe in the core of the PCB, the micro heat pipe can be created at package level only in the relevant areas where efficient heat transport is needed . The small dimensions also allow the heat pipe to be designed for mobile devices and any other devices with severe spatial restrictions. The effective thermal conductivity varies with heat pipe length, and can approach 100000 W/mK for long heat pipes, in comparison with approximately 400 W/mK for the same copper volume. The trench where the pipe is built can be realized in any direction, assume any format and be formed in the same plane of the electrical contacts of the PCB, which means total freedom of design. Due to the small diameter of the heat pipe structures, they might also be applied in applications with reduced size capabilities such as mobile devices. According to exemplary embodiments of the invention, a heat pipe structure can be formed as a whole part, so that no registration problems occur. The micro heat pipe structures can be manufactured economically in terms of material consumption. Since the heat pipe structures can be configured as hollow copper structures, micro heat pipe structures drive to reduction of weight of the PCB. Advantageously, no extraordinary PCB manufacturing technology is needed to produce the micro heat pipe structures, and there is no need of special machinery.

Possible applications of exemplary embodiments of the invention are automotive applications, lighting, mobile devices, power electronics, etc.

According to another exemplary embodiment of the invention, heat pipe structures (such as micro copper heat pipe structures) may be embedded on thermal formable flexible polyimide sheets (see for instance the embodiment of Figure 5 to Figure 13). Such an embodiment relates to a method of and a structure for cooling hot spots in printed circuit boards. More particularly, it relates to micro heat pipes fanned on thermal polyimide sheets that can be integrated to any PCB, a method of cooling devices embedded in or installed on the PCB using micro heat pipes, and a method of forming the micro heat pipes. This embodiment is based on the creation of micro trenches via laser ablation methods on thermal formable polymers, for example, polyimide flexible material or LCP, PEN, PET, etc. Alternatively to the laser ablation process, the trenches can be generated by hot embossing, transfer molding processes or printing of liquid materials or photoimageable build-up materials. Commercial polyimide sheets are for example up to 125 prn thick. The depth of the trenches depends on the thickness of the polyimide sheet under work and the design of the heat pipe structure. For example, trenches as deep as 85 prn in a 125 pm polyimide sheet are possible. Extra grooves in the interior of the trench are created after a second laser ablation process. These grooves and the natural copper surface roughness will work as wick for the micro heat pipe structure. This fact does not hinder the feasibility of the micro heat pipe structure. The trenches can be filled with sputtered copper (about 100 nm). The copper deposited on the top of the polyimide sheet outside of the trenches can be removed from the surface. This can be done via laser blow-off ablation or a Flash etch process. Afterwards the remaining metal coating in the trenches may be reinforced with electroless (PTH process) or electroplated copper deposition (up to lOpm). Alternatively, the metallisation can be via a hard mask. For obtaining a closed structure, the structure as previously described can be closed by a lid which is produced in the same way as described beforehand . The lid is attached to the base via a lamination process for polyimide with pressure under temperatures up to 300°C. Afterwards, the heat pipe structure can be filled with liquid (such as water, methanol, etc.) and closed. This can be done via apertures designed for the filling process. There are industrialized processes available which can be adapted for the specific case of the micro heat pipe structure. The filling degree can be controlled over the immersion time and depth. A closing method for obtaining hermetically closed shells of the micro heat pipe structures can vary (for example inkjet printing methods, dispensed silicon, laser ablation, etc. can be used).

There are different possibilities of using polyimide micro heat pipe structures. For example:

1. applied directly on top of the PCB as an outlay.

2. as heat spreading layer in the PCB.

3. as flexible part of the PCB having also electrical connections connecting two PCB parts or to an external heat sink.

Clearly, the polyimide micro heat pipe structures grown in the core of the PCB allow great freedom of design. By creating the micro heat pipe directly in polyimide, the micro heat pipe can be created at package level only in the relevant areas where efficient heat transport is needed. The small dimensions and the polyimide flexibility allow the heat pipe to be designed for mobile devices and any other devices with severe spatial restrictions. The trench where the heat pipe structure is built can be realized in any direction, assume any format and be formed in the same plane of the electrical contacts of the PCB, which means total freedom of design. Due to the small diameter of the heat pipe structures, they may also be applied in applications with reduced size capabilities such as mobile devices. These heat pipe structures can be manufactured economically in terms of material consumption. Since the heat pipe structures can be manufactured as hollow copper structures, their implementation results in a reduction of weight of the PCB. The flexibility of such heat pipe structures allows direct connection to heat sink so that no extra part needed . Also for the described embodiment, no extraordinary PCB manufacturing technology is needed to produce the micro heat pipe structure, and is also no need of special machinery.

According to yet another exemplary embodiment of the invention, a microelectronics package with integrated heat pipe may be provided . The described embodiment builds up on standard ECP processes. In such an embodiment, one or more of the copper foils are replaced with pre-structured micro-pipe copper sheets. Those elements are formed by creating (for instance by etching, plating, depositing, etc.) a specific pattern on a first sheet, to which a second one (which may be patterned or non-patterned) is bonded, (for instance friction) welded, (for instance diffusion) soldered . The embedded PCB may be produced as usual, with the component connected through soldering, microvia, conductive adhesive, sintering past, etc. Once the product is finished, channel filling may occur (for instance at panel, strip or single- component level). A thermally conductive but electrically insulating material (such as 3M Novec) may be poured within the piping, which may then be soldered, bonded or welded shut. The fluid can be replaced with a solid (for example wax) depending on application. An alternative process flow can involve placing the fluid/solid in the pipe before joining both sheets. Handling would be more complicated, but further processing (in particular plating) may be simplified. A phase-change material can yield a very efficient heat transfer, but a simple fluid can also be used . A proper performance can be accomplished by the assembly of a heatsink on the electronic device.

By combining copper structuring and embedding, the heat pipe structure can be created directly at package level, thereby greatly enhancing thermal performance and reliability. As it is possible that the heat pipe struc- ture is created only in the relevant area, overall systems costs can be reduced compared with traditional methods. Moreover, thermal management can be directly integrated in the package, produced at panel level.

By taking these measures, the following advantages may be obtained : The production costs can be reduced through large-scale processing . Design and system-level costs may be optimized . The manufactured electronic device can be formed with a reduced weight. Furthermore, improved testability compared with COB (Chip-an-Board) architectures can be achieved.

Exemplary embodiments of the invention may be implemented in power electronics applications, in particular when trying to create QFN like packages to replace standard COB architectures.

Figure 1 to Figure 4 illustrate different views of structures obtained during carrying out a method of manufacturing an electronic device 200 according to an exemplary embodiment of the invention.

In order to obtain a layer structure 150 as shown in Figure 1, a connected or laminated stack of layers 152 of electrically conductive material (here embodied as copper foils) and one or more layers 154 of electrically insulating material (here embodied as hardened prepreg or FR4) is provided, forming for instance a printed circuit board (PCB). By this design of the layer structure 150, a support structure 100 is formed which comprises electrically insulating material and electrically conductive material.

This layer structure 150 is then made subject of a material removal procedure : Firstly, a through hole 108 is formed in the upper one of the electrically conductive layers 152 to thereby form a shield structure 106.

Subsequently, material of the electrically insulating layer 154 is removed through the through hole 108 (serving as a mask) for instance by laser drilling or mechanically drilling through the through hole 108. In both cases, the copper material of the lower electrically conductive layer 152 serves as stop layer 110 for stopping the material removal procedure. As a result, a recess 102 with a so-called elephant foot shape is obtained, i.e. having a smaller cross-section or diameter Dl at the upper surface of the layer structure 150 as compared to a larger cross-section or diameter Dl' deep or within the layer structure 150, i.e. at a larger distance from the through hole 108. The recess 102 or laser trench may be formed with high power intensity or by applying the laser with different incident angles to obtain the elephant foot shape of the recess 102, or the recess 102 can be designed mechanically. The formation of the recess 102 in the support structure 100 forms part of the manufacture of a heat pipe structure 202 synthesized during processing and integrated within the layer structure 150, as will be described in further detail in Figure 2. In order to obtain the electronic device 200 shown in Figure 2, the recess 102 in the support structure 100 is filled with a circumferentially closed layer of electrically conductive and thermally conductive material by applying this material onto exposed walls 104 of the recess 102 to thereby form an annular structure of thermally conductive material. This circumferentially closed layer of electrically conductive and thermally conductive material may be deposited galvanically and is preferably copper material . When a ratio D2/D1 (D2 if the thickness of the electrically insulating layer 154) is selected appropriately (for instance in a range between 1 and 2, in particular

approximately 1.4) trench galvanization results in an annularly closed thermally conductive shell 204, as shown in Figure 2.

Before hermetically closing both ends of the thermally conductive shell 204, a fluid 206 such as water may be inserted into a central cavity delimited by the thermally conductive shell 204 (for instance using a syringe or a pipette). At least part of this fluid 206 can accumulate in microstructures of a guiding structure 208 at an inner surface of the thermally conductive shell 204 as a result of capillary forces. As can be taken from Figure 2, the guiding structure 208 is partly formed as small beak-shaped or V-shaped corners of the thermally conductive shell 204. Another part of the guiding structure 208 is contributed by microstructures due to surface roughness at the interior surface of the thermally conductive shell 204, see detail 250 in Figure 2.

Hence, the guiding structure 208 is configured as an arrangement of

microstructures provided along an inner surface of the shell 204 and being integrally formed with the shell 204, in combination with the V-shaped narrow corners.

Although not shown in Figure 1 and Figure 2, two opposing open ends of the annularly closed galvanically formed copper structure may be, after the supply of the fluid 206 into the thermally conductive shell 204, closed by forming trenches extending into the upper electrically conductive layer 152, the electrically insulating layer 154 and the thermally conductive shell 204 close to said ends thereof, and by filling these trenches with thermally conductive material (preferably also copper) so as to obtain a hermetically sealed thermally conductive shell 204. Corresponding vertical thermally conductive structures resulting from the filling of the trenches with copper material are denoted with reference numeral 2900 in Figure 29. Consequently, the whole circumferentially and hermetically closed thermally conductive shell 204 consists entirely of thermally conductive and electrically conductive copper, i.e. does not have any thermal weak point.

Thus, Figure 1 and Figure 2 illustrate a method of manufacturing the electronic device 200 as printed circuit board with monolithically synthesized heat pipe structure 202. The heat pipe structure 202 is synthesized by and during processing of the support structure 100 and actually forms part thereof. As a consequence, the synthesized heat pipe structure 202 is integrated and embedded in the support structure 100. The heat pipe structure 202 is composed of the thermally conductive shell 204, the guiding structure 208 and the fluid 206, wherein the shell 204 fully surrounds and thereby hermetically seals the fluid 206 in an interior of the shell 204 with regard to surrounding material of the support structure 100. The

monolithically integrated heat pipe structure 202 is intrinsically constituted and synthesized during processing of the support structure 100 and consists of the hermetically sealing shell 204, the fluid 206 enclosed in an interior of the shell 204, and the guiding structure 208 which forces the fluid 206 in its liquid phase to accumulate in small channels thereof. The aqueous fluid 206 is configured to be evaporated from a liquid phase into a gas phase in the presence of heat at a portion of the shell 204, in accordance with the general working principle of heat pipes. The guiding structure 208 in an interior of the shell 204 is configured for guiding the evaporated gas phase (or vapor phase) from said portion to another cooler portion of the shell 204 to thereby condense the fluid 206 back from the gas phase into the liquid phase, and for guiding back the condensed fluid 206 towards said portion. The heat pipe structure 202 is fully embedded within an interior of the electronic device 200.

Figure 3 shows a plan view of electronic device 200 and particularly shows that the heat pipe structure 202 can be arranged with a high design freedom along substantially any desired path of the electronic device 200. A free end of the heat pipe structure 202 is denoted with reference numeral 300.

Figure 4 shows a detail of the free end 300. It can be closed by a copper filled trench, see reference numeral 302, to obtain a hermetically sealed heat pipe structure 202. A larger trench aperture is possible to keep the end of the wire open after the galvanic process.

Figure 5 to Figure 9 illustrate different cross-sectional views of structures obtained during carrying out a method of manufacturing an electronic device 200 according to another exemplary embodiment of the invention .

Figure 5 shows the starting point of the manufacturing procedure which, in the present embodiment, is a sheet-shaped or plate-shaped first body 500, for instance thermal polyimide upilex, forming part of a support structure 100. For instance, a thickness, D, of the first body 500 may be in a range between 50 pm and 300 pm, for example 125 pm .

For forming multiple heat pipe structures 202 (see Figure 9)

simultaneously, multiple recesses 600 each having a rectangular cross-section are formed in the first body 500, as can be taken from Figure 6. The formation of the recesses 600 can be performed by a first laser ablation procedure. A depth, d, of the recesses 600 into the first body 500 may for example be 40 pm to 280 pm, in particular 85 pm to 100 pm .

As can be taken from Figure 7, a plurality of microstructures 700 are formed within each of the recesses 600. These microstructures 700 or wick channels may be formed by a second laser ablation procedure. The

microstructures 700 should be formed with a sufficiently small dimension (for example in the order of magnitude of 1 pm to 10 pm) so that a pronounced capillary effect occurs in the presence of fluid 206 in the recesses 600 forcing the fluid 206 into the microstructures 700. By excimer laser projection mask ablation, traces may be recessed into a surface of a dielectric substrate so that lines of 3 pm to 10 pm may be easily manufactured with high positional accuracy and uniformity.

Referring to Figure 8, the method further comprises applying a preferably uniform (i .e. having a constant thickness) layer 800 of thermally conductive material (copper in the shown embodiment) on exposed walls of each of the recesses 600 with the microstructures 700 of the first body 500. Hence, a copper metallization procedure is carried out, for instance a plating procedure. Although not shown in the figures, the procedure described referring to Figure 5 to Figure 8 can then be repeated so as to obtain a further body 500 with further microstructures 700 within further recesses 600 and with a further layer 800 of thermally conductive material . In other words, two identical or similar structures of the type shown in Figure 8 are formed, which later together form support structure 100 with integrated heat pipe structures 202.

In order to obtain the electronic device 200 with multiple integrated heat pipe structures 202 synthesized during processing of the support struc- tures 100 shown in Figure 9, the first body 500 processed according to Figure 8 and the further body 500 processed accordingly are connected to one another (for instance are pressed together, wherein adhesive may be used to accomplish bonding) so that the recesses 600 of the first body 500 and the further recesses 600 of the further body 500 with their microstructures 700 are aligned so as to form closed cavities or channels 900. Before connecting the two bodies 500, 500 to one another, fluid 206 (not shown in Figure 9) may be filled in recesses 600 of any of these bodies 500, 500. The electronic device 200 embodied as a PCB may then be further processed, for instance by forming one or more additional patterned electrically conductive layers 152.

Hence, multiple integrated parallel aligned to one another heat pipe structures 202 are obtained forming part of the support structure 100. Although not shown in the figures, it is possible to connect the arrangement of Figure 9 with one or more further electrically insulating layers, and to mount one or more electronic components thereon and/or therein. Heat generated by such electronic components during operation can then be efficiently removed from the respective electronic component by the heat pipe structures 202.

However, it should be mentioned that the further body 500 (i.e. the upper part of the structure shown in Figure 9) does not necessarily have to be identical to the first body 500 (i.e. the lower part of the structure shown in Figure 9). Alternatively, the further body 500 can for instance also be any other type of lid, for example a plain cover.

Furthermore, it should also be mentioned that with the method of producing the first body 500 shown in Figure 8 and/or the further body 500 it is also possible to produce two-dimensional (in particular non-straight and/or non-planar) heat pipe structures 202 as well.

Figure 10, Figure 11, Figure 12, and Figure 13 show images taken of a microchannel structure manufactured during carrying out a method of manufacturing an electronic device 200 according to an exemplary

embodiment of the invention. The dimensions of the various microchannel structures shown in Figure 10 to Figure 13 show the high reliability and reproducibility.

Figure 14 to Figure 18 illustrate different plan views and cross-sectional views of structures obtained during carrying out a method of manufacturing an electronic device 200 according to yet another exemplary embodiment of the invention.

As can be taken from Figure 14, substantially U-shaped first recesses 1400 are formed as grooves in a first thermally conductive structure 1402 such as a copper foil or sheet. Figure 15 shows an alternative, in which first recesses 1400 in first thermally conductive structures 1402 have a substantially square-shape.

As can be taken from Figure 16 and Figure 17, a planar second thermally conductive structure 1700, such as a non-recessed further copper foil or sheet is connected to the first thermally conductive structure 1402 processed according to Figure 14 or Figure 15. As a result, a planar surface portion of the second thermally conductive structure 1700 and a connected recessed surface portion of the first thermally conductive structure 1402 form a common cavity of the heat pipe structure 202 to be formed, the fluid 206 being located in the cavity. In one embodiment, the fluid 206 may be inserted into the recesses 1400 before the connection with the second thermally conductive structure 1700. In other embodiments, the fluid 206 may be inserted into the cavity after the connection, but a supply opening for supplying the fluid by a syringe or pipette may be subsequently closed with a fluid-tight sealing plug of thermally conductive material, preferably copper material.

In an alternative to the procedure shown in Figure 16 and Figure 17, a second recess 1800 is formed in the second thermally conductive structure 1700, compare Figure 18. Subsequently, the recessed first thermally conductive structure 1402 and the recessed second thermally conductive structure 1700 are connected to one another so that the first recesses 1400 and the second recesses 1800 are aligned and form a common cavity of the heat pipe structure 202, the fluid 206 being located in the cavity.

Figure 19 to Figure 25 show cross-sectional views of electronic devices 200 (and preforms thereof, respectively) embodied as printed circuit boards according to exemplary embodiments of the invention. The structures shown in Figure 19 to Figure 25 may implement the structures manufactured according to Figure 14 to Figure 18.

Figure 19 shows an embedded stack with a single sided heat pipe 202. The stack shown in Figure 19 comprises an embedded die as electronic component 1902 being a heat source during operation. The embedded stack furthermore comprises a heat source coupling structure 1900 made of thermally conductive material and being configured for thermally coupling a portion of the later heat pipe structure 202 (see Figure 21) to a heat source mounting position at which the heat generating electronic component 1902 is mounted, here as embedded component. The later heat pipe structure 202, in its preform of Figure 19, still has an open portion in form of access hole 1910 and is not yet filled with fluid 206.

Figure 20 shows the embedded stack of Figure 19 with single sided heat pipe structure 202 after filling a cavity thereof with fluid 206 through the access hole 1910, which is still open with regard to the environment.

Figure 21 shows electronic device 200 formed based on the embedded stack of Figure 20 after closing the access hole 1910 with a thermally conductive plug 2100, for instance a drop of metal which hermetically seals the access hole 1910 and provides thermal conductivity.

Figure 22 shows an embedded stack with a double sided heat pipe structure 202 having access hole 1910 at a lower main surface of the embedded stack and additionally having a further access hole 2210 at an upper main surface of the embedded stack.

Figure 23 shows the embedded stack of Figure 22 after having closed the access hole 1910 by thermally conductive plug 2100, while the access hole 2210 is still open.

Figure 24 shows the embedded stack of Figure 23 after having filled the cavity of the heat pipe structure 202 with liquid 206.

Figure 25 shows electronic device 200 obtained based on the embedded stack of Figure 24 after closing also the further access hole 2210 by a further thermally conductive plug 2510 (for instance a copper plug).

The double sided heat pipe configuration according to Figure 22 to Figure 25 has the advantage of a particularly efficient heat removal from the electronic component 1902, since the double sided heat pipe structure 202 transports generated heat from the electronic component 1902 via the heat source coupling structure 1900 towards a first main surface (in the shown embodiment the lower main surface) of the electronic device 200 to provide for a first cooling effect and further towards a second main surface (in the shown embodiment the upper main surface) of the electronic device 200 to provide for a second cooling effect at another location.

Electronic device 200 according to another exemplary embodiment as shown in Figure 26 is again embodied as a PCB with monolithically integrated and intrinsically constituted heat pipe structure 202. The electronic device 200 according to Figure 26 comprises a heat sink coupling structure 2600 made of thermally conductive material, embodied as a via, and being configured for thermally coupling a portion of the heat pipe structure 202 to a heat sink mounting position of the electronic device 200 at which heat sink mounting position a heat sink 2602 is mounted . The heat sink 2602 can for instance be a copper block, an aluminum block, a thermal coupling to a casing (not shown), or a cooling body having fins. Both the heat source as electronic component 1902 and the heat sink 2602 are mounted on a respective one of the opposing main surfaces 2604. However, they could alternatively also be mounted on a lateral surface 2606 of the electronic component 200.

Figure 27 shows a cross-sectional view of an electronic device 200 embodied as an electronic component in form of a packaged electronic chip 2700 with a heat pipe structure 202 manufactured integrally therewith according to an exemplary embodiment of the invention. More specifically, the heat source embodied as electronic chip 2700, for instance a semiconductor chip, is mounted on a copper lead frame 2750 and is encapsulated in an encapsulation 2702 such as a mold compound. A heat source coupling structure 1900 embodied as a via thermally connects the packaged electronic chip 2700 to the integrated heat pipe structure 202 which, in turn, forwards the transported heat via a heat sink coupling structure 2600 to heat sink 2602.

Figure 28 shows a cross-sectional view of an electronic device 200 embodied as electronic chip 2700 with heat pipe structure 202 manufactured integrally therewith, wherein the electronic chip-heat pipe structure

arrangement is packaged altogether, according to an exemplary embodiment of the invention. In this embodiment, both the electronic chip 2700 and the heat pipe structure 202 together with a heat source coupling structure 1900 are encapsulated within the encapsulation 2702.

Figure 29 illustrates a cross sectional view of a monolithically

integrated heat pipe structure 202 for integration into PCB-based layer structures of a circuit board according to another exemplary embodiment of the invention.

As can be taken from Figure 29, upper and lower walls 2910 of the shell 204 of the heat pipe structure 202 are patterned horizontal layers (in particu- lar copper layers) in the support structure 100, whereas sidewalls 2900 are vertical trenches filled with thermally conductive material (in particular copper) so as to obtain a hermetically sealed shell 204.

It should be noted that the term "comprising" does not exclude other elements or steps and the "a" or "an" does not exclude a plurality. Also ele- ments described in association with different embodiments may be combined .

It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.

Implementation of the invention is not limited to the preferred embodiments shown in the figures and described above. Instead, a multiplicity of variants are possible which use the solutions shown and the principle according to the invention even in the case of fundamentally different embodiments.