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DESCRIPTION The present invention relates to the field of production of hybrid electronic circuits and specifically to a process for manufacturing of hybrid circuit modules employing electronic chip devices.

As known, the use in hybrid circuits of chip devices allows the circuits to achieve working frequencies otherwise unachievable using the same devices enclosed in their own cases. In the specific case of hybrid amplifiers the use of transistors on chips also allows production of amplification modules with wider bands. The advantages due to the use of chip devices are especially appreciated in the microwave field where the requirement to continuously raise operating frequencies is greatly felt. The above mentioned advantages are basically based on the absence of the parasite effects introduced by the cases and this brings the ability to provide much smaller networks for impedance adaptation between the device electrodes and the circuit connection points.

The use of chip devices requires in turn that the hybrid circuits be enclosed in sealed cases to protect the chip devices against moisture and possible corrosive agents in the air. These are all factors which could damage the performance of the chips.

It is also known that to reduce the production costs of hybrid circuits it is necessary to provide multiple identical circuits on the same dielectric substrate traversing the different production steps.

In the known processes for manufacturing hybrid circuits employing chip devices there is generally applied of the above mentioned multiple circuit, but it terminates necessarily with cutting of the substrate to separate the individual passive circuits. The subsequent processing steps including also connection to the circuits of the chip devices are completed separately on the individual circuits located in the respective cases. Lastly the cases are sealed. An example of these manufacturing processes, which also illustrates production of the case, is supplied in the article entitled, "Production of miniaturized amplification modules with frequencies over 20 GHz" (REALIZZAZIONE Dl MODULI MINIATURIZZATI Dl AMPLIFICAZIONE A FREQUENZE SUPERIORI Al 20 GHz), by Paolo Bonato and Giorgio Carcano, published in the Proceedings of the Conference promoted by the A.E.I. , namely, "Progress in interconnection and packaging technologies in electronics" (Progressi nelle tecnologie di interconnessione e

packaging nell'elettronica), held in Milan on October 17 and 18, 1988. The above mentioned process makes use of fine-line thin film technology to obtain passive circuits on microstrips of a width less than or equal to 10 μm on an alumina substrate 10 mils (0.25 mm) thick. The grounding connections between front lines and the metallization on the back of the substrate are provided by means of metallized holes. In the article no mention is made of multiple identical passive circuits on the same substrate, but this practice can be reasonably inferred because it is commonly used in thin film technology.

The article includes a chapter in which there is briefly described the type of case used together with its manufacturing steps. This is because the subsequent operations on the individual circuits take place inside the cases after cutting the substrate to separate the passive circuits. The above mentioned operations comprise: a) soldering of the alumina substrate of the passive circuit to the case by means of preformed soldering alloy, b) electrical connection between the passive-circuit lines and the case pins, c) manual soldering of the chip GaAsFET to the substrate, d) electrical connection of the GaAsFET to the microstrip by means of manual therrnocompression welding, and lastly sealing of the case.

Concerning manufacture of the cases it is stated in the article exactly: "The frequencies of use and the dimensions of the components have led to design and production of mechanical cases with dimensional tolerances on the order of 20 μm, indeed mechanical machining swarf even with dimensions of 30 μm to 40 μm in the soldering area of a chip can lead to breakage of the component during the therrnocompression step and possible lack of soldering of the component with resulting electrical problems". In view of the above it is understandable how the manufacturing processes in accordance with the prior art exhibit various shortcomings which are discussed below.

A first shortcoming is due to the need to use specially designed cases with very close manufacturing tolerances and which are accordingly very costly. A second shortcoming is due to the poor parallelism of the processes because it ends with completion of the metallization operations to obtain passive circuits. This means that it is not possible to extend to the finished product the economies of scale initially obtained by provision of multiple passive circuits on the same dielectric substrate. A third shortcoming derives from the fact that, as all the assembly operations of the hybrids must be performed in the cases, automation thereof is difficult.

Accordingly said operations are performed entirely manually or at best semiautomatically. The impossibility of more extensive automation leads to another production cost increase and lack of complete repeatability of the individual operations. There can also be problems of reliability caused by the numerous solderings between the tracks of the hybrids and the pins of the cases.

Accordingly the purpose of the present invention is to overcome the above mentioned shortcomings and indicate a process for manufacturing hybrid circuit modules including electronic chip devices.

To achieve these purposes the present invention has for its subject matter a process for the manufacture of hybrid circuit modules including electronic chip devices consisting of the steps: a) provision of a plurality of identical passive circuits on microstrip by means of deposit of metallic films on the front of the same dielectric substrate and an extended metallization on the back having the function of ground plane, said plane being electrically connected by means of metallized holes to grounding points of the front circuits; b) connection of one or more chip devices to each of said passive circuits to obtain as many hybrid circuits; c) sealing of said metallized holes; d) superimposition of respective capsules on said hybrid circuits and sealing thereof upon contact with the substrate, the dimensions of the capsules being such that externally thereto emerge some of said microstrips; e) cutting of the substrate along the edge of the circuits to separate the individual hybrid circuit modules, as better explained in claim 1.

The process which is the object of the present invention possesses a number of advantages with respect to the prior art. A first advantage is that of not requiring particularly costly cases and indeed the capsules do not necessitate the strict dimensional tolerances of the previous packages because no assembly operation is performed on the circuits inside the capsules.

A second advantage is that of fully utilizing economies of scale, thanks to the high degree of process parallelism maintained until completion of the finished product. A third advantage is the drastic production cost reduction which is the result of fuller automation introduced in the operations of connection of the chip devices to the circuits, and the fact that the connections made in the prior art between the tracks of the hybrids and the cases are no longer necessary.

Lastly, the process which is the object of the present invention allows greater repeatability and reliability of the individual operations. Further purposes and advantages of the present invention are clarified in the detailed description of an embodiment thereof given below by way of non-limiting

example with reference to the annexed drawings wherein:

FIG. 1 shows a flat substrate on which are deposited identical passive circuits in planar form,

FIG. 2 shows hybrid circuits obtained from the passive circuits of FIG. 1 , FIG. 3 shows capsules adhering to the substrate and including the hybrid circuits of FIG. 2, and

FIG. 4 shows hybrid circuit modules corresponding to the encapsulated hybrid circuits of FIG. 3 after their separation by cutting of the substrate.

With reference to FIG. 1 there is seen a flat substrate 1 of dielectric material on whose front side have been deposited six identical metallizations 2 (layout) corresponding to as many passive circuits. The circuit geometry shown in the figure is only a diagram of the actual circuits. On the back of the substrate 1 is deposited a metallization (not shown in the figures) extending over the entire surface and acting as an electrostatic ground. At some points of the substrate 1 there are metallized holes (not shown in the figures) for grounding of points of the metallizations 2 designed for this purpose. These metallizations 2 comprise microstrips and pads 3 for connection of as many chip devices.

With reference to FIG. 2, in which the elements in common with FIG. 1 are indicated by the same symbols, it can be seen that at each pad 3 there is a chip device 4. The chips 4 are electrically connected to the microstrips 2 by means of gold wires and altogether form six identical hybrid circuits 5 on the substrate 1.

With reference to FIG. 3, in which the elements in common with FIG. 2 are indicated by the same symbols, it can be seen that on the hybrid circuits 5 are superimposed as many capsules 6 which adhere to the substrate 1 and that some microstrips 2 extend beyond the edge of the respective capsules 6 for a short length 2'.

With' reference to FIG. 4, in which the elements in common with FIG. 3 are indicated by the same symbols, it can be seen that the substrate 1 has been cut all around each capsule 6, keeping an excess margin 7 with respect to the edge thereof where there are shown the lengths 2'. Cutting of the substrate 1 separates the six hybrid circuits 5 from each other complete with their respective capsules 6, creating as many hybrid modules 8.

With reference to the above FIGS, there is now explained the manufacturing process of the hybrid modules 8. This process is performed observing a very precise sequence of steps each consisting of several elementary operations. As the process which is the object of the present invention is largely independent of the particular

type of the manufactured circuits and considering that use of the chip devices also involves raising the operating frequencies thereof, it is necessary to adopt in the manufacturing process all those technical arrangements which allow achievement of the greater working frequencies and the best operating performances. These arrangements consist essentially of reduction of the microstrip width and the thickness of the dielectric substrate, and of the provision of ground connections between front and back of the substrate by means of metallized holes. Basically these are the same arrangements suggested in the above mentioned article of P. Bonato and G. Carcano which explains the production of hybrid thin-film amplifiers operating at frequencies higher than 20 GHz.

The process steps are:

(a) drilling of the substrate 1 , deposit of the appropriate metallizations 2 on the front (FIG. 1) and of the ground plane on the back, and metallization of the hole walls;

(b) connection of the chip devices 4 to the pads 3 (FIG. 2); (c) electrical connection of the chip devices 4 to the metallic lines 2 to obtain the hybrid circuits 5;

(d) sealing of the metallized holes;

(e) encapsulation of the hybrid circuits 5 (FIG. 3); and

(f) cutting of the substrate 1 to separate the hybrid modules 8 (FIG. 4). The individual operations performed in step (a) are know to those skilled in the art and illustrated in detail in the numerous publications dealing with the technology of thin-film circuits and to which reference is made. Among these is included the article of P. Bonato and G. Carcano which includes the bibliography mentioned. It should be specified that the choice of the material of the substrate 1 depends on the power to be dissipated to the surface in contact with the chip devices. The material chosen should avoid excessive overheating of the chip devices which could affect operation thereof. The materials most used in practice are alumina and diamond, which are selected depending on the density of thermal power to be dissipated. Help in this choice is given by knowledge of the thermal conductivity of the two materials which for alumina is 21 W/m.°K and for diamond 2300W/m.°K.

The holes are made with a carbon dioxide laser and metallized with gold to obtain good electric continuity between the two sides of the substrate, as better described in European patent application EP-94117039.1 , with Italian priority dated 22 December 1993 in the name of this same applicant. General notions concerning in particular steps (b) and (c) are contained in the volume entitled, "MICROELECTRONICS PACKAGING HANDBOOK", by Rao R.

Tummala and Eugene J. Rymaszewsky, published by VAN NOSTRAND REINHOLD, New York, 1989. The individual operations performed in the above mentioned steps are discussed fully below.

In step (b), attachment of the chip device 4 to the pads 3 (die attach) is performed by means of soldering, using a gold and tin alloy with eutectic composition of 80% Au and 20% Sn, with melting point of 280°C. Soldering is made possible by the fact that on the back of the chip device 4 a metallization is provided for that purpose.

An alternative solution is that of cementing the chip devices 4 to the substrate 1 using epoxy resins made conductive. The choice between the two solutions depends on the power dissipated by the individual chip devices and remembering that the resins have thermal conductivity lower than that of the commonest soldering alloys. Both solutions are described below, starting with the soldering operations, for which is required a specially dedicated semiautomatic machine (die bonder).

In the case of soldering, at the beginning of step (b) the dielectric substrate 1 is positioned on a hot plate with which the machine is equipped. The hot plate consists of a thin graphite lamina connected electrically to a power supply capable of supplying current pulses controllable both as to amplitude and duration. The machine has a work table on which are arranged appropriate supports on which are positioned the chip devices 4 to be soldered and soldering alloy preforms having the same surface dimensions as the chips 4 and thickness of 25 μm.

By a semiautomatic process the alloy preform is taken, using for this purpose a tool located on the machine head, and then positioned and released on the pad 3 of a first passive circuit 2. On the head of the machine is also available a vacuum tool used for picking the devices to be soldered. The form and dimensions of the vacuum tool depend on the particular chip device used and in any case it exhibits a through hole along its own longitudinal axis which communicates with a vacuum pump. Resting the second tool on the chip 4 and operating the pump there is created a vacuum in the axial hole allowing picking up of the chip 4 and positioning it over the alloy soldering preform previously positioned. At this point the controlled supply applies to the graphite lamina a current pulse causing rapid temperature rise of the lamina up to 310°C, and upon cessation of the pulse follows an equally rapid cooling of the lamina to the previous temperature of 250°C. The thermal cycle lasts approximately 20 seconds during which the preform melts and again solidifies to solder the metallization on the back of the chip 4 to the metallic pad 3 present on the

substrate 1. For all this time there is applied locally at the soldering point a flow of hot inert gas, typically nitrogen and hydrogen in molecular form and mixed together, which preserves the soldering alloy from oxidation during the time it is in liquid state.

All the operations just described are repeated identically for all the chip devices to be soldered.

When step (b) calls for cementing chips to the substrate, step (b) will include the operations: deposit of the conductive epoxy resin, positioning of the chip devices, and - performance of a thermal cycle for curing the resin.

Deposit of the resin is done with an automatic dispensing machine which allows good accuracy and repeatability. By means of these machines there are placed drops of resin over all the pads 3 of the substrate 1. The deposited drops have diameter of 300 μm and thickness of 50 μm, which are dimensions agreeing quite well with those of the most common GaAsFET on chips which are typically 500x400x100 μm.

Positioning of the chip devices 4 is performed by automatic machines termed 'pick and place', similar to those used for the soldering. Each chip is taken by a sucking tool and positioned on the respective pad 3 where it is pressed with a force going from 0.2 N to 0.3 N depending on the type of device used. After placing of all the devices there is performed a thermal cycle for curing the resin, an operation which depends on the type of resin used. For epoxy resins the cycle consists of heating the substrate 1 to a temperature of 150°C which is held for approximately 15 minutes followed by cooling to the room temperature. At the end of the cycle the resin is hard. The heating can be performed, depending on convenience, by means of belt or static ovens or hot plates. At the end of cooling the chip devices 4 are attached to the substrate 1 in the correct positions and it is now possible to perform the following step (c) of the process.

It this step are performed the electrical connections between the metallic pads belonging to the chip devices 4 and the contact pads provided in the respective metallizations 2, or those between two different chip devices. The above mentioned connections are performed by means of automatic or semiautomatic machines (wire bonders) using for this purpose gold wires or ribbons applied by means of the prior art of 'wire bonding'. This technique can be applied by two different procedures of which the first, termed 'wedge bonding', is recommended for electrically connecting the active devices such as the GaAsFETs while a second, termed 'ball bonding', is

preferable in the connection of passive components, e.g. chip capacitors. The 'wedge' wire-bonding technique in turn, depending on how the chip devices are constrained to the substrate, achieves the connections by means of therrnocompression or thermosonically. Specifically, in the case of soldering of the chips 4 to the pads 3 of the substrate, the electrical connections are made by means of therrnocompression of the gold wires at the contact points while in the case of cementing the chips with epoxy resins it is preferable to use the thermosonic method. The above mentioned wire bonding techniques are described below observing the following order. - Therrnocompression wedge bonding.

Thermosonic wedge bonding. Ball bonding.

In the case of therrnocompression the substrate 1 is positioned on a hot plate equipping the machine, whose temperature is held constantly at 250°C. For the connections is used a gold wire 20 μm in diameter which emerges from an axial hole made in the tip (wedge) of a wire dispenser which positions the wire over a first of two points to be connected. A compression force less than or equal to 0.3 N is then exerted by the tool point on the gold wire in contact with the underlying metallization. The connection consists of interpenetration of the surface layers of the two metallic parts in mutual contact and results from the combined effect of temperature and compression. When the connection is completed at the second point also, the wire is cut and the operation repeated for all the remaining connections of the chip 4 and for all the chips present on the substrate 1. Therrnocompression allows making of very short connections between the chip devices and the microstrips and this is an essential requisite for correct operation of microwave circuits.

In thermosonic wedge bonding the temperature of the hot plate supporting the substrate 1 is held constantly at 150°C. In addition, the tip of the wire dispenser is connected to a transducer which subjects it to a mechanical stress at ultrasonic frequency which it transmits to the gold wire at the contact point. In ball bonding the temperature of the hot plate supporting the substrate 1 is held constantly at 150°C. The wire emerging from the tip of the dispenser forms a small ball which is positioned at the contact point. The tip communicates to the ball both an ultrasonic-frequency mechanical stress and compression. Once the electrical connections of the chip 4 are finished the manufacturing process enters step (d) in which all the metallized holes in the substrate 1 are sealed. This is necessary because it is the substrate itself which closes the capsules 6 after their

application over the hybrid circuits 5. Closing of the holes is done by filling them with a material made to adhere firmly to the cylindrical walls of the substrate body surrounding the holes. The material used for sealing the holes can be either a soldering alloy or a resin. In the first case soldering will be done and in the second cementing.

If sealing is done by soldering, the soldering alloy used is again that specified in the preceding step (b) in which the chips 4 are soldered to the pads 3 of the substrate, i.e. Au/Sn with eutectic composition and melting point of 280°C. The soldering machine used during accomplishment of step (d) is again that of step (b) and the temperature of the plate on which the substrate is placed is again 250°C. After centering and alignment of the substrate 1 on the hot plate, a special suction device is operated to take the small spheres of soldering alloy from a case located on the work table. The spheres are then positioned on the holes to be sealed and released there. At this point is activated a hot flow of inert gas, typically nitrogen and hydrogen in molecular form mixed together appropriately, which forms a sort of hood over the substrate 1. Then the controlled power supply with which the machine is equipped applies to the hot plate a current pulse which causes rapid rise of the temperature thereof up to 310°C. Upon cessation of the pulse there follows an equally rapid cooling of the plate down to the previous temperature of 250°C. The thermal cycle lasts approximately 15 seconds during which the alloy spheres melt and solidify to cause sealing of the holes in an inert atmosphere protecting the soldering alloy from oxidation while it is in the liquid state.

If the holes are sealed by cementing, either electrically conductive or insulating resins may be used. Application of the resin is done using special dispensers comprising a head on which are mounted syringes containing the resin to be deposited. Controlled quantities of resin are deposited in all the metallized holes of the substrate 1. Then there is performed a thermal resin-curing cycle which depends on the type of resin used. For epoxy resins the cycle consists of heating the substrate 1 up to a temperature of 150°C, holding this temperature for 15 minutes, and then cooling to the room temperature. Heating can be performed, depending on convenience, by means of belt ovens, static ovens, or a hot plate. After cooling the holes are sealed.

It is now possible to proceed with step (e) of encapsulation of the hybrid circuits 5. The capsules 6 are of ceramic material and have a hollow parallelepiped form. The encapsulation operations are done in an environment kept at humidity controlled by means of nitrogen flow constantly below 30 ppm. Along the entire edge

of the capsules 6 there has been spread in advance an electrically insulating epoxy resin which has undergone a thermal pre-curing treatment. The capsules 6 are perfectly centered over the hybrid circuits 5, using for the purpose a special template and subsequently subjected to a pressure approximately equivalent to atmospheric pressure, that is 1.01,10 ^s Pa. Without interrupting the compression, the resin is subjected to a thermal curing cycle consisting of heating to 150°C for 90 minutes. After cooling the capsules 6 appear cemented hermetically to the substrate 1.

In the final step (f) there is performed separation of the individual modules 8 by means of a series of right angle cuts of the substrate 1 along mutually orthogonal directions (axes X, Y). For this purpose there is used a diamond disk saw of the type usually employed for cutting substrates of alumina, glass, silicon, etc.. The saw in question is a numerically controlled machine whose table has many small holes in which is created a vacuum to hold the substrates immobile during cutting. The initial centering is performed by means of an optical system with which the machine is equipped, after which the cutting sequence starts and terminates only when all the modules have been separated. The cutting is done leaving a margin of substrate beyond the edge of each capsule 6. This margin comprises the metallic lines 2' for electrical connection of the hybrid modules 8 to external devices and/or circuits.

The use of ceramic capsules does not constitute a limitation for the process which is the object of the present invention and capsules of other materials lending themselves to cementing by means of resins can also be used. Regardless of the type of material selected, if the encapsulated circuit is an amplifier the capsule must not cause the arise of resonance therein. To achieve this, the dimensions of the capsule must be such that the modes of propagation of the electromagnetic field (radiated by the microstrips) are sufficiently attenuated along the direction where there is gain variation, preventing undesired feedback capable of deforming the trend of the gain, until to the limit of the firing. If it is not possible to choose the most appropriate dimensions for the capsules, it is necessary to apply to the internal walls thereof appropriate electromagnetic radiation absorbers. When the encapsulated hybrid circuit is an oscillator, the capsule could also be used as a resonant cavity.

In the case of metallic capsules, if they are connected to the ground plane, they also act as an electromagnetic shield for the hybrid circuit inside them.