Connection device

The invention relates to a connection device that allows for efficient heat dissipation.

The electrical losses of semiconductor chips or integrated circuits, due to the Joule effect, lead to a rise of tempera ture of the chips. Requirements to keep the temperature with in bounds for regular operation are usually specified via the so called junction temperature (Tj).

Semiconductors chips are typically characterized by a maximum junction temperature (indicated in the Data Sheet) that make their deployment highly temperature dependent. To guarantee that the semiconductor chip's heat dissipation protects against overheating, it is necessary to keep the device below this temperature.

The junction referred to in the setting of a "maximum junc tion temperature" of a chip is a place directly located in the chip where losses are generated. It is a small area that can easily reach 150 ² C.

Fig. 1 shows an example for a typical thermal model of a sem iconductor deployed as a SMD device (SMD: surface mounted de vice) . A junction 1 forms part of the chip, which is sur rounded by a case 2. By means of a joint 3, the chip is fas tened to a pad 4 which acts as a chip carrier. The pad 4 is soldered (solder layer 5) to a PCB 6 (PBC: printed circuit board) . Typically, the chip's case 2 is mostly surrounded by air, which is referred to as "ambient". On the right side of Fig. 1 thermal resistances are indicated. The junction tem perature is referred to as T _j , the ambient temperature as T _a . Thermal resistances play a role for the removal of heat from the junction. In the top direction, the thermal resistance Rth(j-c) between the junction and the case and the thermal resistance Rth(c-a) between the case and the ambient have to be taken into account. In the bottom direction, the thermal resistance Rth(j-b) between the junction and the PCB and the thermal resistance Rth (b-a) between the PCB and the ambient come into play.

It is particularly important to keep the mechanical joint be tween an encased chip and its case under a maximum tempera ture (if possible by a big margin to provide for safer opera tion) to avoid the thermal fusion of this junction. The high thermal stress in the solder joints of mounted circuit boards is also compromising the robustness of the full circuit de sign .

For keeping the junction temperature under a safe level, the thermal energy generated by the chip must be dissipated. If the heat is not removed, the temperature across the semicon ductor will rise until it exceeds the maximum junction tem perature (Tjmax) , causing serial damage, destruction or loss of performance.

Even when semiconductors are working within a safe operating range, their properties and lifetime are also affected by temperature variations. It is necessary to keep the junction temperature Tj as low as possible to ensure the reliability, service life, and performance of semiconductor device.

The trend to increase power densities is aggravating this problem; therefore a good thermal performance of the full system is more and more a critical characteristic in the de sign of new power electronic systems.

An energy flux is required to dissipate the heat at the junc tion. Therefore, a temperature gradient needs to be estab lished. That is why, for cooling electronic devices, the heat is transferred from the device to another medium that is at a lower temperature and then to the ambient. In this process some kind of conduction 7, convection 8 and radiation 9 is always involved (see Fig. 2 which shows heat dissipation from a SMD semiconductor) . Below, heat dissipation will refer to all different ways of removing heat, including conduction, convection and radiation.

In low power devices, the case surface evacuates sufficient heat to the environment to keep the junction temperature with the safe margin. When device power increases, the case sur face by itself is it not enough to dissipate sufficient heat, i.e. the surface available for heat dissipation must be in creased .

In addition, as the size of IC devices is shrinking, direct heat dissipation from the case to the ambient is complicated necessitating the design of new specific solutions for this new generation of small devices.

Typically, following thee approaches to increase heat dissi pating surface are applied: a) Optimization of PCB stack (fig. 3)

Power semiconductors are normally surface mounted into a cop per surface, which is part of a PCB (generally a compound of two or more copper layers 61) . The heat traverses these lay ers via conduction and evacuates to the environment by con vection. The optimization of these copper layers and the ad dition of thermal vias can reduce the junction temperature Tj significantly by lowering the thermal resistance Rth (b-a) from the board to the ambient (see fig. 3) . This heat spread ing method has the disadvantage that is limited by the elec trical conditions the PCB has to fulfill. b) Heat sinks (fig. 4)

A heat sink is a metal device that conducts the heat generat ed by power semiconductor chips to a medium with a lower tem perature, usually the ambient air, where it is removed by natural or forced convection (see fig. 4) . To increase sur face area in contact with air, the heat sink 10 is provided with multiple fin layers 11. In the heat sink production pro cess, typically the fins are machined or the heat sink is ob tained from an extruded profile of aluminum (many times ano dized to increase the amount of heat dissipated by radia tion) .

Usually, the surface of the heat sink in contact with air is covered with silicon thermal grease to fill any air pocket that can exist. Thus, thermal resistance Rth (Heat sink + Thermal grease) through the metal-air junction is minimized.

A properly designed heat sink often is an economical solution to maintain the junction temperature of the semiconductor de vices well below the maximum operating temperature.

However, the miniaturization of semiconductor chip devices can render the application of heat sinks burdensome. The ma chining of small heat sinks is more expensive and the mount ing on the device can cause serial damage of the soldering due to their weight and the pressure applied. Therefore, al ternatives methods should be considered. c) Surfaced mounted heat sinks (fig. 5)

As an alternative to conventional heat sinks a contactless solution is commercially available. In this case, the heat sink 10 is soldered to a mounting pad 12 on the printed cir cuit board 6. This SMD heat sink 10 includes two side members 101 and 102, a connecting bridge 103 and a base 104, 105 at each side member.

The heat sink 10 can be placed over an SMD electronic device and the base 104, 105 of the heat sink 10 can be soldered to the mounting pad 12 simultaneously with the soldering of the electronic device to the printed circuit 6. Heat dissipated from the surface mount electronic component is conductively transferred to the heat sink body through the solder source pad 12 , printed circuit board 6 and solderable base 104, 105 and finally dissipated to surroundings (see fig . 5 ) .

In order to solder this kind of heat sinks, a predefined size pad is needed which constitutes an additional effort. Also, in some cases the electrical requirements of the system can thwart the location of the heat sink directly over the elec tronic device.

There is a need for additional economical heat dissipation solutions for semiconductor chips that work well for small sizes. The invention's objective is to further such a solu tion.

A connection device is proposed. Shape and material composi tion of the connection device are designed

a) to thermally connect the case of a semiconductor chip mounted on a PCB to a supporting structure, and

b) to dissipate energy to the ambient.

Here and below two objects are called "thermally connecting" if

i) They are physically in touch with each other

ii) A temperature gradient exists across where the ob

jects are in touch.

In particular, the above definition does not require two ob jects to be fastened to each other to be thermally connected.

According to an embodiment, the connection device comprises a surface designed to be fastened to the supporting structure. For positioning on the case of the semiconductor, the connec tion device may comprise another surface, which is essential ly parallel to the surface designed to be fastened to the supporting structure. This other surface is meant to thermal ly connect to the case of a semiconductor chip. It may extend along a length or area that essentially corresponds to a length or an area of the case. Typical values of such a length would be several mm to below 1 mm.

Preferably, the connection device assumes a form that sup ports the dissipation of heat, e.g. an at least approximate Z-shape .

Preferably, the connection device is made at least partially from a conducting material, e.g. sheet-metal. The conducting material may be anodized.

The invention also relates to a structure for an electronic device that comprises a printed circuit board (PCB) , a semi conductor chip with a case (e.g. SMD) and a connection device as defined before. The semiconductor chip is mounted on the printed circuit board and a supporting structure (e.g. anoth er PCB or part of a supporting structure is connected to the connection device) .

According to an embodiment, the support house and the printed circuit board are mounted so as to exert a pressure to the connection device in between.

In addition, a method to manufacture an electronic device ac cording to the invention is proposed, including the steps of i) mounting the semiconductor chip on the printed circuit board,

ii) fixing the connection device to the support structure, and

iii) mounting the printed circuit board with the semiconduc tor-tor chip so that the case thermally connects (pressure) to the connection device.

In step iii) the connection may be assured by carrying through the mounting of the PCB in a way that provides for a pressure to be exerted which holds case and connection device together . Below, embodiments of the invention are described with refer ence to figures.

List of figures:

Fig. 1: Typical thermal model of a SMD device

Fig. 2: Heat dissipation from a SMD semiconductor

Fig. 3: Thermal model of a semiconductor mounted on a 4-layer PCB with thermal vias

Fig. 4: Thermal model of a semiconductor with a classical heat sink attached

Fig. 5: Thermal model of a semiconductor with a SMD heat sink attached

Fig. 6: Thermal model of a semiconductor connected to a spring heat sink

Fig. 7: Different shapes of a connection device according to the invention

Fig. 8: Picture of a spring heat sink according to the inven tion mounted on a PCB

The embodiment of fig. 6 of the invention features a bent Z- shape sheet 20 that acts not only as a heat sink, but also as a bridge or connection device between the semiconductor pack age and a bigger conductive surface 16 (e.g. a bigger PCB or the housing) .

The heat sink 20 according to the invention draws on two ef fects for removing heat from junction 1: a) Heat sink 20 transports heat from the junction 1 to the surface 16 from which it is dissipated.

b) Heat is dissipated to the ambient in the vicinity of the heat sink 20.

As a consequence, heat is removed very efficiently. In addi tion heat sink 20 adds mechanical stability by providing a connection between case 2 and surface 16.

Heat sink 20 may be considered to be a combination between a classical heat sink and a SMD heat sink: on the one hand it provides for a direct contact with the semiconductor, which allows for removing heat by conduction like in the classical approach, but on the other hand it is basically a bent sheet- metal part that, taking advantage of it shape and orienta tion, dissipates heat by means of convection (see fig. 6) .

This heat sink or heat spreader 20 is made from a conductive and, eventually, solderable material plate bent into a Z- shape . The anodizing of this material should be also consid ered to improve the thermal radiation properties of the in vention .

Another form than the Z-shape is possible, as long as the form enables a connection between case 2 and surface 16 and lends itself to dissipation of energy to the ambient. In ad dition to the z-form two alternative forms are shown in fig. 7. These forms feature surfaces 21 and 22 to contact case 2 and surface 16, respectively. Between the surfaces 21 and 22 a connection part 23 is provided that assumes a form suitable for dissipating heat to the ambient.

As shown by help of fig. 8, the connection between heat sink 20 and case 2 can be put into effect just by pressure, i.e. any kind of glue, solder, etc. is not used for the installa tion. The installation is carried through as follows: - First, one of the horizontal legs of the Z-shape is fixed to the bigger surface 16 that is going to work as heat spreader (for this propose it can be integrated in the design or it can be soldered to the surface, e.g. to a PCB) .

- Then, PCB 6 (the one where the power semiconductor is sol dered) is mounted at a certain distance in parallel to this surface 16, causing, therefore, a pressure on the Z- shape (that acts as a spring) , producing as result the con tact of the free horizontal leg with the device.

The heat is, therefore, transported from the packaging sur face 6 to a much bigger surface 16 by conduction. During this process convection takes also an important role, since the Z- profile of the "spring heat sink" increases the amount of surface that dissipates the heat. This incurs a big advantage especially when the heat is dissipated by means of forced convection and the "spring heat sink" is correctly oriented. Like all the SMD electronic components, the spring heat sink can be placed onto the printed circuit board manually or automatically .

The use of the "spring heat-sink" on a SMD device has the following advantages:

1. Contact improvement.

2. Reduction of problems with pressure and weight during the installation / no electrical dependency.

3. Reduction of machining cost / easily applicable to small devices .

4. Combination of heat dissipation by conduction and by con vection .

More specifically, these advantages are brought about as fol lows : 1. Contact: The spring system guarantees the right contact between the semiconductor and the heat-sink, no glue is nec essary to keep them together.

2. Reduction of problems during installation: Since the spring is not attached to the semiconductor no extra load is added to it / the lack of contact with the PCB (the one that hold the semiconductors) make possible to remove the heat al most directly from the junction without any electrical re striction .

3. No precision machining cost/ adaptable to small devices: Typically, the heat sinks are made of extruded metals or by milling, while the spring heat sink allows a much simple and cheaper method (meta) sheet bending) .

4. Conduction and convection heat dissipation: The spring heat sink combines the advantages of the traditional heat sinks, removing the heat by conduction to a bigger surface and the ones of the SMD heat sinks, due to the profile shape that removes heat by convection.

Above, the invention was disclosed with reference to a SMD device. However, it is not limited to this technology and can be considered for employment for other devices, e.g. THT mounted devices.