Power semiconductor package

Abstract

A power semiconductor package (Fig. 1: 1000) comprises at least one power module (Fig. 1: 1001-1003) each comprising a plurality of power semiconductor chips (Fig. 4: 41, 42). First and second substra

Description

Technical Field The present disclosure relates to a power semiconductor package and a method for manufacturing the same. More particularly, but not exclusively, the present disclosure relates to a power semiconductor package with double sided cooling while achieving a good thermal performance and high thermal-mechanical reliability. Background Power semiconductor modules have been considered as one of the most delicate components in electric drive systems of electric vehicles (EVs) and hybrid electric vehicles (HEVs). For such applications, there has been an increasing demand for power semiconductor modules with increased power density, improved electrical performance and thermal performance, high reliability and reduced costs. In response to this demand, it is known to provide power semiconductor modules with double-side cooling to directly improve the thermal performance and hence to indirectly improve the power density and electrical performance of the power semiconductor modules. European Patent Application EP2533284A2 discloses a power semiconductor package with a power module and a double-sided heat sink. The double-sided heat sink comprises a top side and a bottom side, which are provided at opposite sides of the power module. Fastening clamps are used to press the top side and bottom side of the heat sink into the power module. The power module is a three-phase inverter module that comprises high side devices and low side devices of three half-bridge switches. The bottom sides of those devices are attached to a direct bonded copper (DBC) substrate and the topsides of those devices are interconnected with conductive clips. The bottom side of the heat sink is directly mounted under the DBC substrate with a layer of thermal coupling material. The top side of the heat sink is placed also with a layer of thermal coupling material over a cap layer attached to the topsides of the conductive clips. United States Patent Application US2010 / 0230800A1 discloses a double-side cooled power module with a power overlay layer, a top substrate, a top channel heat sink assembly, a bottom substrate, and a bottom channel heat sink assembly. A layer of compliant thermal interface material (TIM) is provided between the power overlay layer and the top substrate. Each of the channel heat sink assemblies is bonded to a respective substrate using a high temperature brazing process. United States Patent Application US2011 / 0241198A1 discloses a power semiconductor module with a laminated conductor, which includes two types of metal layers - i.e., aluminium and copper. Straight sections (i.e., the aluminium side) of the laminated conductor are ultrasonically welded on a front side of a semiconductor device. Arch-line protrusions (i.e., the copper side) of the laminated conductor are soldered on a top extraction electrode. The arch-line protrusions can relieve stress applied from the front side of the semiconductor device. A backside of the semiconductor device is soldered on another extraction electrode. The top and bottom surfaces of the power semiconductor module are sandwiched by cooling fins through insulating plates and fixed with a fixing bracket. A plurality of the fixing brackets are disposed so as to uniformly pressurize the double-side cooled module. United States Patent US9941234B2 discloses an integrated double-side cooled power module, which has one or multiple phase legs configuration including one or more planar power packages. Each planar power package has an upper power switch unit and a lower power switch unit directly bonded and interconnected between two insulated power substrates, and further sandwiched between two heat exchangers via direct bonds. A segmented coolant manifold is interposed with the one or more planar power packages and creates a sealed enclosure. The sealed enclosure defines a coolant inlet, a coolant outlet and a coolant flow path between the inlet and the outlet. Chinese Patent CN109755194B discloses a power semiconductor assembly that includes three half-bridge power semiconductor modules to form a three-phase module, a top heat sink, a bottom heat sink and a plastic housing. The top heat sink is pressuremounted onto top surfaces of the three half bridge modules through a layer of TIM. The bottom heat sink is pressure-mounted onto bottom substrates of the three half bridge modules through another layer of TIM. Within the above described power semiconductor packages (such as in EP2533284A2, US2011 / 0241198A1, and CN109755194B) where layer(s) of TIM are applied to mount heat sink(s), the packages still experience relatively high thermal resistances. This is because the layer(s) of TIM applied in those cases have thermal conductivities which are at least one order of magnitude lower than thermal conductivities of other materials in the heat flow paths. Further, the methods of applying pressures to mount the heat sinks in those cases may lead to tensile stresses and poor filling of gaps in the central area of the layer(s) of TIM. In US2010 / 0230800A1 where a layer of compliant TIM is applied between the power overlay layer and the top substrate, some joints between the power overlay layer and the semiconductor chips and hence the power module as a whole may have poor reliability. This is because additional tensile stresses may occur in those joints when the power overlay layer and the bottom substrate are subjected to different levels of thermal expansion and / or contraction, and the pressure applied through the heat sinks is not distributed appropriately among all the joints between the overlay layer and the semiconductor chips. In the case (e.g., US9941234B2) where power chips are directly bonded and interconnected between two insulated power substrates (which are further sandwiched between two heat sinks via direct bonds, e.g.by soldering or sintering), the joints to bond the front sides of the power chips and the top substrate and hence the module as a whole may have poor reliability. This is because the module is subjected to varied temperatures in use, and consequently the two insulated power substrates may bend along opposite directions. Therefore, additional tensile stresses may readily occur in those joints that bond the front sides of the power chips and the top substrate. It is an object of the present disclosure, among others, to provide an improved power semiconductor package, which solves problems associated with known packages / modules, whether identified herein or otherwise. Summary According to a first aspect of the present disclosure, there is provided a power semiconductor package, comprising: at least one power module comprising: a plurality of power semiconductor chips; first and second substrates arranged at opposite sides of the power semiconductor chips, wherein: the first substrate comprises a first insulating layer and a first patterned conductive layer arranged on a surface of the first insulating layer which faces the power semiconductor chips, and the second substrate comprises a second insulating layer and a second patterned conductive layer arranged on a surface of the second insulating layer which faces the power semiconductor chips; and a plurality of first compliant structures arranged between the first and second substrates and electrically connected to the first patterned conductive layer and / or the second patterned conductive layer; first and second heat sinks arranged at opposite sides of the at least one power module, wherein the first heat sink is fixedly bonded to the first substrate; a structural plate configured to press the second heat sink towards the at least one power module such that the second heat sink is pressure-mounted to the second substrate; and a plurality of second compliant structures arranged between the structural plate and the second heat sink. Advantageously, the power semiconductor package is implemented with double-side cooling, and achieves not only good thermal performance but also high thermomechanical reliability. In particular, by arranging a plurality of second compliant structures between the structural plate and the second heat sink, the second compliant structures distribute the pressure applied by the structural plate, and ensure good thermal contact between the second heat sink and the second substrate. Accordingly, the thermal resistance between the at least one power module and the second heat sink is reduced to a relatively low level. Further, the first compliant structures provide compliance between the first and second substrates, and allow the first substrate to follow the thermal bending of the first heat sink to a certain extent without causing any fault in the electrical connections within the power module. This would reduce the thermo-mechanical stress and strain developments between the first heat sink and the power module, and hence improve the reliability of the joint between the first heat sink and the first substrate. In addition, the second compliant structures generate additional compressive stresses. By positioning the second compliant structures suitably with respect to the at least one power module, the additional compressive stresses can be useful for improving the thermo-mechanical reliability of joints within the at least one power module. The power semiconductor package is therefore suitable for use in converter systems with improved power density and electrical / thermal performance for electric drive applications of EVs and HEVs. The term “compliant structure” is intended to mean that the structure has mechanical compliance (e.g., malleability, pliability, deformability, softness, shearability, compressibility, stretchiness, and / or geometric reactiveness to external forces applied to the exterior of the structure). In an example, the structure is able to achieve force and motion transmission through elastic body deformation. The compliant structure may be made of a mechanically compliant material (e.g., silicone rubber), or alternatively may obtain compliance due to its structure (e.g., disk spring, coil spring, etc.). The term “compliant structure” may be used interchangeably with “mechanically compliant structure”. The term “fixedly bonded” used in the present disclosure means that the first heat sink and the first substrate are securely bonded together (e.g., by a soldering, sintering or brazing process) such that they can be handled as a single-piece item. It would be appreciated that elements which form pressure contact with one another (e.g., the second heat sink and the second substrate) are not fixedly bonded. The term “pressure-mounted” used in the present disclosure means that the second heat sink and the second substrate are coupled to one another by pressure only, and that the interface between the second heat sink and the second substrate is a dry interface. A dry interface means that there is no bonding material between two sides of the interface. Each of the first and second insulating layers may be made of an electrically insulating and thermally conductive material. The first heat sink may comprise: a body region which comprises a cooling surface in thermal contact with the first substrate; first and second peripheral regions arranged at a periphery of the body region and being thinner than the body region along a first direction that is perpendicular to the cooling surface. The body region may also comprise a coolant input, a coolant outlet, and a coolant flow path between the coolant input and the coolant outlet. At least one of the first and second peripheral regions may have a non-uniform thickness. At least one of the first and second peripheral regions may comprise a middle portion and two end portions which are arranged along a second direction, the second direction being substantially parallel to the cooling surface. The middle portion may be thicker than the two end portions along the first direction. The middle portion may be about 1 mm to 3mm thicker than each of the two end portions. The term “about” or “approximately” used in the present disclosure indicate a degree of variability (e.g., 20%) in the stated numerical values. The cooling surface may comprises two long sides and two short sides. The first and second peripheral regions may extend along the two long sides, respectively. The plurality of first compliant structures may be electrically conductive. The plurality of first compliant structures may comprise one or more coil springs. A central axis of the one or more coil springs may be parallel to a surface of the first or second substrate. The plurality of first compliant structures may be fixedly bonded to each of the first and second patterned conductive layer. The plurality of first compliant structures may be located closer to an edge of the at least one power module than the plurality of power semiconductor chips. The plurality of second compliant structures may have a combined area which is less than each of: an area of the structural plate, and an area of the second heat sink. One or more of the plurality of second compliant structures may be made of silicone rubber. The plurality of second compliant structures may comprise a set of compliant structure(s) which overlap the at least one power module and occupy a smaller area than the at least one power module. The set of compliant structure(s) may overlap at least some of the plurality of power semiconductor chips. The power semiconductor package may further comprise: a pressure controller configured to adjust a distance between the structural plate and the first heat sink so as to control a pressure applied by the structural plate to the second heat sink. The pressure controller may comprise first and second ends coupled to the structural plate and the first heat sink respectively, and a linkage member extending between the first and second ends. The structural plate, the second heat sink, the at least one power module and the first heat sink may be arranged along a stacking direction. The pressure applied by the structural plate to the second heat sink may be along the stacking direction. The linkage member may extend along the stacking direction. The linkage member may extend through the second heat sink. The second end of the pressure controller may be rotatably coupled to the first heat sink, and a rotation of the second end with respect to the first heat sink is configured to adjust the distance between the structural plate and the first heat sink. The first end of the pressure controller may have a smaller area than the structural plate. The pressure controller may comprise a plurality of screws. The plurality of second compliant structures may comprise a further set of compliant structure(s) which at least partially surround the linkage member. In each of the at least one power module, the second substrate may comprise a plurality of separated substrates. The first substrate may comprise a single continuous substrate. The at least one power module may further comprise at least one conductive shim which is electrically connected in series with one or more of the plurality of power semiconductor chips between the first patterned conductive layer and the second patterned conductive layer. The conductive shim is useful for replacing wire bonds as the topside interconnection of the semiconductor chips, and have an improved ability of carrying higher current. The power semiconductor package may further comprise thermal interface material (TIM) sheets between the second heat sink and the second substrate. Each of the at least one power module may be a half-bridge switch. According to a second aspect of the present disclosure, there is provided an electric drive system for an electric vehicle or a hybrid electric vehicle, comprising a power semiconductor package of the first aspect. According to a third aspect of the present disclosure, there is provided an electric vehicle comprising an electric drive system according to the second aspect. According to a fourth aspect of the present disclosure, there is provided a hybrid electric vehicle comprising an electric drive system according to the second aspect. According to a fifth aspect of the present disclosure, there is provided a method of manufacturing a power semiconductor package, comprising: bonding a plurality of first compliant structures between first and second substrates of at least one power module which further comprises a plurality of power semiconductor chips, wherein: the first and second substrates are arranged at opposite sides of the power semiconductor chips; the first substrate comprises a first insulating layer and a first patterned conductive layer arranged on a surface of the first insulating layer which faces the power semiconductor chips, and the second substrate comprises a second insulating layer and a second patterned conductive layer arranged on a surface of the second insulating layer which faces the power semiconductor chips, and wherein the plurality of first compliant structures are electrically connected to the first patterned conductive layer and / or the second patterned conductive layer; fixedly bonding a first heat sink to the first substrate; and pressing, by a structural plate, a second heat sink towards the at least one power module such that the second heat sink is pressure-mounted to the second substrate, wherein a plurality of second compliant structures are arranged between the structural plate and the second heat sink. Where appropriate any of the features described above in relation to one aspect of the present disclosure may be applied to any other aspect of the disclosure. It would also be understood that the terms “first” and “second” are simply used in the present disclosure to label the relevant elements for the ease of description, and do not imply any limitations to the sequence or locations of the relevant elements. Brief Description of the Drawings In order that the disclosure may be more fully understood, a number of embodiments of the disclosure will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a circuit topology of a power semiconductor package according to an embodiment of the present disclosure; Figure 2 is a top perspective view of a power semiconductor package which implements the circuit topology of Figure 1; Figure 3 is a bottom perspective view of the power semiconductor package of Figure 1; Figure 4 is a top perspective view of a bottom part of each power semiconductor module within the power semiconductor package of Figure 2, with the bottom part of the power semiconductor module including a bottom substrate and power semiconductor chips attached thereto; Figure 5 is a bottom perspective view of the bottom part of each power semiconductor module as shown in Figure 4; Figure 6 is a bottom perspective view of a top part of each power semiconductor module within the power semiconductor package of Figure 2, with the top part of the power semiconductor module including top substrates and conductive structures attached thereto; Figure 7 is a top perspective view of the top part of each power semiconductor module as shown in Figure 6; Figure 8 is a top plan view of part of the power semiconductor package of Figure 2, where the bottom substrates of the power semiconductor modules are attached to a bottom heat sink, and power semiconductor chips and bonding materials are placed on the bottom substrates; Figure 9 is a side view of part of the power semiconductor package of Figure 2, where the power semiconductor modules are bonded to the bottom heat sink of the power semiconductor package. Figures 10 and 11 are perspective views of components of the bottom heat sink before the bottom heat sink is assembled; Figures 12 and 13 are bottom and top perspective views of the bottom heat sink; Figure 14 is a side view of the bottom heat sink when viewed along an arrow A in Figure 13; Figure 15 is a side view of the bottom heat sink when viewed along an arrow B in Figure 13; Figure 16 is a top perspective view of part of the power semiconductor package of Figure 2, where the power and signal / control terminals are further bonded to the power semiconductor modules; Figure 17 is a top perspective view of part of the power semiconductor package of Figure 2, where a top heat sink is further mounted; Figure 18 is an exploded view of the part of the power semiconductor package as shown in Figure 17; Figure 19 schematically illustrates a spatial relationship between second compliant structures and power semiconductor chips; Figure 20 shows process steps of a method for manufacturing a power semiconductor package according to an embodiment of the present disclosure. In the figures, like parts are denoted by like reference numerals. It will be appreciated that the drawings are for illustration purposes only and are not drawn to scale. Detailed Description of the Preferred Embodiments Figure 1 shows a high-level circuit topology of a power semiconductor package (referred to as “package” below for brevity) 1000 according to an embodiment of the present disclosure. The power semiconductor package 1000 integrates three power semiconductor modules (referred to as “power modules” below) 1001-1003 each of which being a half-bridge switch. A negative temperature coefficient (NTC) thermistor T1, T2 or T3 is provided for each power module as a temperature sensor. The three power modules 1001-1003 have identical topologies, and only one power module 1001 is described herein for brevity. The power module 1001 includes a high side device S1 and a low side device S2 connected in series between a DC positive power terminal P1 and a DC negative power terminal N1, and an output terminal II between the high side device S1 and the low side device S2. Figure 1 shows that each of the high side and low side devices S1, S2 comprises a power transistor chip in antiparallel connection with a diode chip. It would be appreciated that depending on power and current ratings, one or each of the high side and low side devices S1, S2 may be formed by more than one power transistor chips connected in parallel with one another, and / or more than one diode chips connected in parallel with one another. In an example, the power transistor chips are insulated gate bipolar transistor (IGBT) chips, and the diode chips are fast recovery diode (FRD) chips. Alternatively, the power transistor chips and diode chips may be wide-bandgap power semiconductor chips, such as silicon carbide (SiC) metal-oxide-semiconductor field-effect transistor (MOSFET), and Schottky diode chips. Furthermore, the body diodes of the SiC MOSFETs may be used such that the Schottky diode chips may be eliminated. The power module 1001 includes three power terminals P1, N1 and II, and seven signal / control terminals C1, G1, E1, C2 / E1’, G2, E2, E2’ which control the operations of the devices S1, S2. The NTC thermistor T1 has two output terminals T11C and T12C. It would be understood that the above description in relation to the power module 1001 similarly applies to the other two power modules 1002, 1003. The physical structure of the package 1000 that implements the topology of Figure 1 is shown in Figures 2 and 3. Figures 2 and 3 are top and bottom perspective views of the package 1000. As shown in Figures 2 and 3, the package 1000 comprises a bottom heat sink 70, a plastic frame 91 (which may also be referred to as a plastic housing) and a plastic lid 92. These components are exposed to an exterior of the package 1000. The package 1000 is an integrated double-side cooled power semiconductor package. As described below, the package 1000 also includes a top heat sink which is covered by the plastic lid 92 and is therefore invisible in Figures 2 and 3. By covering the top heat sink, driving circuits and control panels can be readily placed on top of the plastic lid 92 in use, and the top heat sink is unlikely to cause any accidental electric short in the driving circuits and the controlling panels. As shown in Figure 2, the package 1000 includes three sets of terminals 1 to 12, and each set of terminals belong to a respective one of the power modules 1001 to 1003. In particular, Terminals 1 to 3 correspond to terminals P1, N1 and II, or terminals P2, N2 and V, or terminals P3, N3 and W. Terminals 4 to 10 correspond to the seven signal / control terminals of each module. Terminals 11 and 12 correspond to the output terminals of the NTC thermistor T1, T2 or T3. As shown in Figure 3, a coolant inlet 701 and a coolant outlet 702 are formed on a bottom surface of the bottom heat sink 70. The inlet and outlets 701, 702 are connected to an external supply of coolant in use. As described below, the top heat sink is in fluid communication with the bottom heat sink, such that the coolant entering the inlet 701 can flow through the interior of each of the top heat sink and the bottom heat sink 70, before exiting the package 1000 at the outlet 702. With future reference to Figure 3, screws 703 are used to attach the plastic frame 91 to the bottom heat sink 70. There are also through holes 704 formed at peripheral areas of the bottom heat sink 70. The through holes 704 facilitates the installation of the external supply of coolant. Figures 4 to 19 illustrate the internal structure of the package 1000 through an exemplary sequence of assembling the package 1000. In this example, each of power modules 1001-1003 comprises a bottom substrate and two top substrates, and electrical components (e.g., high side and low side devices, NTC thermistor) and internal electrical connections of each power module are fixedly bonded between the bottom substrate and the top substrates. The bottom substrate may be referred to as a “first substrate”, and the two top substrates may be collectively referred to as a “second substrate”. Figures 4 and 5 show the bottom substrate of each power module. The bottom substrate comprises an insulating layer 201, a patterned electrically conductive layer arranged on an inner surface of the insulating layer 201, and another electrically conductive layer 221 arranged on an outer surface of the insulating layer 201. The “inner surface” in the present disclosure refers to a surface which faces the electrical components of the respective power module, and the “outer surface” refers to an opposite surface which faces away from the electrical components. The patterned electrically conductive layer of the bottom substrate forms conductive tracks 211 to 219 on the inner surface of the insulating layer 201. The conductive tracks 211 to 219 are separated, thus electrically isolated, from one another. Each of the conductive tracks 211 to 219 is a single continuous (i.e., unbroken) track. With further reference to Figure 4, four IGBT chips 41, four FRD chips 42 and one NTC thermistor 43 are attached to the conductive tracks, 211, 213, 218 and 219, with joints 401, 402 and 403. The joints 401,402 and 403 may be formed by soldering or sintering technologies. Wire bonds 404 are used to connect the gate pads of the IGBT chips 41 to the conductive tracks 214 and 216. In this example, the two IGBT chips 41 and two FRD chips 42 directly bonded to the conductive track 211 form the high side device S1, S3 or S5 of each power module. In particular, the collector pads of those IGBT chips and cathode pads of those FRD chips are securely bonded to the conductive track 211. Although it is not shown in the figures, the conductive track 211 is further connected to Terminal 1 of Figure 2 (i.e., P1 / P2 / P3 terminal of Figure 1). The remaining two IGBT chips 41 and two FRD chips 42 directly bonded to the conductive track 213 form the low side device S2, S4 or S6 of the same power module. In particular, the collector pads of those IGBT chips and the cathode pads of those FRD chips are attached to the conductive track 213. Although it is not shown in the figures, the conductive track 213 is further connected to Terminal 3 of Figure 2 (i.e., ll / V / W terminal of Figure 1). The remaining interconnections required by the circuit diagram of Figure 1 are achieved by the top substrates. Figures 6 and 7 show the two top substrates of each power module. A first top substrate (at the left side in Figures 6 and 7) comprises an insulating layer 301, a patterned electrically conductive layer (which forms conductive tracks 311 to 313) arranged on an inner surface of the insulating layer 301, and another electrically conductive layer 321 arranged on an outer surface of the insulating layer 301. A second top substrate (at the right side in Figures 6 and 7) comprises an insulating layer 302, a patterned electrically conductive layer (which forms conductive tracks 314 and 315) arranged on an inner surface of the insulating layer 302, and another electrically conductive layer 322 arranged on an outer surface of the insulating layer 302. Four solid conductive shims 44, four solid conductive shims 45, and six compliant conductive structures 46-1 to 46-4 (referred to as 46 collectively) are bonded to the conductive tracks 311 and 315, with joints 404, 405 and 406. These joints 404, 405 and 406 may be formed by soldering or sintering technologies. The compliant conductive structures 46 may be referred to as “first compliant structures” of the package 1000. To assemble each power module, the top substrates shown in Figure 6 is flipped over with the conductive tracks 311 to 315 facing the bottom substrate as shown in Figure 4. The conductive shims 44, 45 as well as the compliant conductive structures 46 are then bonded to the chips 41,42 and the conductive tracks of the bottom substrate. In general, the second top substrate (at the right side in Figures 6 and 7) is connected to the high-side device of the same power module, and the first top substrate on the left is connected to the low-side device of the same power module. More specifically, when the two top substrates (Figures 6 and 7) are bonded to the bottom substrate (Figures 4 and 6), the compliant conductive structure 46-2 is directly bonded between the conductive track 215 and the conductive track 315, and the compliant conductive structures 46-1 are directly bonded between the conductive track 213 and the conductive track 315. Further, the compliant conductive structures 46-3 are directly bonded between the conductive track 212 and the conductive track 311, and the compliant conductive structure 46-4 is directly bonded between the conductive track 217 and the conductive track 311. When the two top substrates (Figures 6 and 7) are bonded to the bottom substrate (Figures 4 and 6), the solid conductive shims 44 are directly bonded between the topside bond pads (i.e., power emitter pads) of the IGBT chips 41 and the conductive track 311 or 315. Similarly, the solid conductive shims 45 are directly bonded between the topside bond pads (i.e., anode pads) of the FRD chips 42 and the conductive track 311 or 315. It would be understood that a height of each of the compliant conductive structures is substantially the same as a sum of a height of each chip 41,42 and a height of each of the conductive shims 44, 45, when measured along a stacking direction Z of the top and bottom substrates which is perpendicular to surface(s) of the top / bottom substrates as shown in the figures. In this way, the top substrates of an assembled power module are generally parallel to the bottom substrate. Figure 8 shows that the bottom substrates of all the three power modules 1001 to 1003 are attached to a main surface (which is a cooling surface) of the bottom heat sink 70. The three bottom substrates are arranged along an X direction, which is defined, by a long side of the main surface of the bottom heat sink 70. The short side of the main surface of the bottom heat sink defines a Y direction, and the three power modules are coincident with one another along the Y direction. The bottom heat sink 70 has a thickness along the stacking direction Z that is perpendicular to each of the X and Y directions. With reference to Figures 6 and 8, in each of the power modules, the solid conductive shims 44, 45 (which have been bonded under the top substrates) are bonded with joints 407 and 408, on the topsides of the chips 41 and 42 (which have been attached to the bottom substrate). The compliant conductive structures 46 (which have been bonded under the top substrates) are bonded with joints 409 on the conductive tracks 212, 213, 215 and 217 of the bottom substrate. The joints 407, 408, and 409 may be formed by soldering or sintering technologies. In conventional power semiconductor modules, wire bonds are commonly used as the interconnections between the topsides of the power semiconductor chips and the conductive tracks of the substrates. In contrast, each of the power modules 1001 to 1003 replaces such wire bonds with the solid conductive shims 44, 45 and compliant conductive structures 46. The solid conductive shims 44, 45 are bonded between the top substrates and the power semiconductor chips 41, 42 which are attached to the bottom substrate to facilitate the double side cooling of the chips 41, 42. The solid conductive shims 44, 45 have an improved ability of carrying higher current than wire bonds. The solid conductive shims 44 and 45 may be made of pure metal (such as pure copper and pure aluminium), alloy (such as copper-molybdenum alloy and copper-tungsten alloy), metal-matrix composite (such as copper-graphite composite and aluminiumcarbon fibre composite), or other materials with high thermal conductivity and high electrical conductivity. The solid conductive shims 44, 45 are preferably made of materials with coefficients of thermal expansion (CTEs) matched or close to those of the substrates and / or the power semiconductor chips 41, 42. Matching CTEs are useful for mitigating the stress and strain developments in the joints 407, 408 between the conductive shims and the power semiconductor chips 41, 42 and in the joints 404, 405 between the conductive shims and the respective substrate. Accordingly, the thermomechanical reliability of the joints 404, 405, 407, 408 is improved. Figure 9 shows a side view of part of the package 1000 when the three assembled power modules are attached to the bottom heat sink 70. More specifically, the back conductive layer 221 of each bottom substrate is securely bonded to a top surface of the bottom heat sink with a joint 50. The joints 50 may be formed by soldering or sintering technologies. The joints 407, 408, 409 and 50 may be formed by a single processing step or multiple processing steps. In the example shown in Figures 6 and 9, each of the compliant conductive structures 46 is in the shape of a coil spring. The coil spring is bonded to a top / bottom substrate in a way such that the axis of the spring is parallel to a surface of the top / bottom substrate (which is parallel to the X-Y plane). In other words, each of the compliant conductive structures 46 provides a number of wires that interconnect a conductive track of the top substrate and a conductive track of the bottom substrate, and the number of wires are connected in parallel to one another. Due to the parallel connection of the wires, the compliant conductive structures 46 would not introduce noticeable electrical resistances and inductances between the top substrates and the bottom substrate. The compliant conductive structures 46 provides compliance along the Z direction (i.e., a direction extending from the top substrates to the bottom substrate of each power module). More specifically, the compliant conductive structures 46 are elastically deformable (to a certain extent) along the Z direction. In other words, the compliant conductive structures 46 are compressible or stretchable along the Z direction, depending upon the pressure applied across the power modules in the Z direction. As a result, the compliant conductive structures 46 are able to compensate for small variations in the distance between the top substrates and the bottom substrate of each power module. The compliant conductive structures 46 are preferably made of metals or alloys such as pure copper, pure aluminium, copper alloy or aluminium alloy with low resistivity, sufficient strength and good ductility. It would be understood that the compliant conductive structures 46 are made of an electrically conductive material, so as to electrically connect conductive tracks of the top substrates to the bottom substrate. It would be appreciated that the compliant conductive structures 46 may take any suitable form other than coil springs. For example, one or more of the compliant conductive structures 46 may comprise a plurality of wires which are electrically connected in parallel with one another between a top substrate and a bottom substrate, and each wire may form a complete loop, part of a loop, or any other suitable shape which allows the wire to be deformable along the Z direction. In an alternative example, one or more of the compliant conductive structures 46 may be a disk spring, and the central axis of the disk spring may be parallel to the Z direction. The bottom heat sink 70 is described below with reference to Figures 10 to 15. In this example, the bottom heat sink 70 is an integrated heat sink with pin fins, coolant channels, two-side cold plates, and inlet / outlet connectors. The bottom heat sink 70 comprises a first cold plate 71 (Figure 10) and a second cold plate 72 (Figure 11). The first cold plate 71 comprises a coolant chamber 78 (which has an opening 710) and two peripheral regions 75, 76. The second cold plate 72 has a size and shape matching those of the opening 710. When the second cold plate 72 is bonded to the first cold plate 71 (as shown in Figures 12 and 13), the assembled bottom heat sink 70 comprises a body region 79 which encloses the coolant chamber 78. The body region 79 comprises two opposite cooling surfaces 73, 77 which are provided by the second cold plate 72 and the first cold plate 71, respectively. The cooling surfaces 73, 77 are generally parallel to one another in the assembled heat sink 70. The two peripheral regions 75, 76 are arranged at a periphery of the body region 79. With further reference to Figure 13, the first cold plate 71 comprises two connection regions 74 extending upwards from the cooling surface 77 (i.e., the top side of the heat sink 70). The two connection regions 74 are arranged at opposite ends of the first cold plate 71 (or the cooling surface 77) along the X direction. In each connection region 74, there are two blind holes 741, 742 (which does not extend through the first cold plate 71), a through hole 743 (which extend through the first cold plate 71) and a cut-out (or recess) 744 which surround the through hole 743. On the cooling surface 77, there are also provided several blind holes 745 at the periphery of the cooling surface 77. The blind holes 741, 742 and 745 have threads cut into the inside surfaces thereof, and are engageable with fasteners 84, 85 (Figures 17 and 18). With reference to Figure 10, the first cold plate 71 further comprises pin fins 706 on its bottom side (i.e., opposite to the cooling surface 77) within the coolant chamber 78. The pin fins 706 are cooling-enhanced features, and the gaps between the pin fins 706 are cooling channels. It would be understood that the cooling-enhanced features may take a different form than those illustrated in Figure 10. The through hole 743 of each connection regions 74 is in fluid communication with the cooling channels. The cut-out 744 is made to hold a sealing ring (not shown in the figures) which provides sealing between the top heat sink 80 and the bottom heat sink 70. With reference to Figure 11, the second cold plate 72 has two through holes 701, 702 which act as the coolant inlet and the coolant outlet of the package 1000, respectively. In use, coolant can flow into the bottom heat sink 70 via the coolant inlet 701, through the cooling channels between the pin fins 706, and exit the bottom heat sink 71 at the coolant outlet 702. Due to the through holes 743, the coolant which has entered the bottom heat sink 71 can also flow into the top heat sink 80 via the through hole 743 of one connection region 74, through the cooling channels inside the top heat sink 80, and return from the top heat sink 80 to the bottom heat sink 70 via the through hole 743 of the other connection region 74. In this way, the coolant flow paths through the top heat sink 80 and the bottom heat sink 70 are parallel to one another. The first cold plate 71 and the second cold plate 72 are preferably made of pure aluminium or aluminium alloy, so as to have light weight and low costs (due to low costs of both the raw materials and the manufacturing processes). Alternatively, they can be made of other pure metal (such as pure copper), alloy (such as copper-molybdenum alloy and copper-tungsten alloy), metal-matrix composite (such as copper-graphite composite, aluminium-silicon carbide composite and aluminium-carbon fibre composite), or other materials with high thermal conductivity. The first cold plate 71 and the second cold plate 72 may be manufactured by using casting, machining or other metalworking processes. The first cold plate 71 and the second cold plate 72 are bonded together to form the integrated bottom heat sink 70 by using, for example, welding, brazing or other bonding technologies. With reference to Figures 12 and 13, the body region 79 (or each of the cooling surfaces 73, 77) comprises two long sides (which extend along the X direction) and two short sides (which extend along the Y direction), and the two peripheral regions 75, 76 extend along the two long sides of the body region 79. With reference to Figure 10, the through holes 704 which facilitates the installation of the external supply of coolant are formed in the two peripheral regions 75, 76. The peripheral regions 75, 76 also comprises through holes 703’ for engaging with the screws 703 (Figure 3) which attach the plastic frame 91 to the bottom heat sink 70. The through holes 703 may have helical threads cut into the inside surfaces thereof. The arrangement of the two peripheral regions 75, 76 allow the bottom heat sink 70 to have a reduced total stiffness, and a lower level of thermal induced bending after the bottom substrates of the three power modules 1001 to 1003 are bonded to the bottom heat sink 70. This is described below in more detail. In particular, during the process of bonding the bottom substrates of the three power modules 1001 to 1003 to the bottom heat sink 70, the stacked structure as shown in Figure 9 is subjected to a varied temperature. Similarly, in use of the package 1000 after full assembly, the package 1000 is typically subjected to varied temperatures. The varied temperatures may cause thermo-mechanical stress and strain to develop within and around the power modules 1001 to 1003. The thermo-mechanical stress and strain developments are partly caused by different CTEs of different layers or parts of the package 1000. Different CTEs cause the different layers or part to experience different levels of thermal-induced expansion and contraction. The thermo-mechanical stress and strain developments are also related to stiffness of the different layers or parts of the package 1000. The stiffness of a layer or part is determined by both the Young's modulus and the thickness / volume of the layer or part. With reference to Figure 9, it can be seen that the bottom heat sink 70 is the thickest and has the stiffness which is significantly larger than the stiffness of other layers / parts bonded to the bottom heat sink 70. In addition, the CTE of the bottom heat sink 70 is in generally larger than those of the substrates and the semiconductor chips within the power modules 1001 to 1003. As a result, the other layers / parts would be against but have to follow the thermal-induced deformation / bending of the bottom heat sink 70 once they are bonded together during the assembling processes or in the service environments. For example, the thermal-induced deformation / bending of the bottom heat sink 70 may cause the left and right ends of the bottom heat sink 70 to move downwards in Figure 9. With reference to Figures 14 and 15, the two peripheral regions 75, 76 have a reduced thickness along the Z direction as compared to the body region 79 which is between the peripheral regions 75, 76 and provides the cooling surfaces 73, 77. The reduced thickness of the peripheral regions 75, 76 may be achieved by cutting, milling or any other suitable material removal process. By removing materials from the two long sides of the bottom heat sink 70 to form the thin peripheral regions 75, 76, the total stiffness of the bottom heat sink 70 is reduced. Consequently, the moment of the other layers / parts which would be against the thermal-induced deformation / bending of the bottom heat sink 70 is also reduced. As a result, the thermo-mechanical stress and strain developments in the joint 50 which bonds the bottom substrates to the bottom heat sink 70 can be reduce and the reliability of the joint 50 is improved. With further reference to Figure 14, each of the two peripheral regions 75, 76 is thinner at the two end portions (e.g., 752, 753) than at the middle portion (e.g., 751) along the long side (e.g., X direction) of the cooling surfaces 73, 77. The two peripheral regions 75, 76 are symmetric across the body region. Therefore, only the peripheral region 75 is described for brevity. In particular, the top surface of the peripheral region 75 is substantially flat and is lower than the top cooling surface 77 by a distance ‘a’. The bottom surface of the peripheral region 75 is curved, with the middle portion 751 being above the bottom cooling surface 73 by a distance ‘c’ and with the two end portions 752, 753 being above the bottom cooling surface 73 by a distance ‘b’. The distance ‘b’ is greater than the distance ‘c’. The difference between ‘b’ and ‘c’ may be between about 1 mm to about 3 mm. For instance, the overall thickness (i.e., the distance between the two cooling surfaces 73, 77 along the Z direction) of the bottom heat sink 70 may be 8 to 12 mm. The width ‘w’ of each peripheral region 75 or 76 along the Y direction (Figure 15) may be 10 to 13 mm. The total thickness of the cut off (i.e., ‘a’ + ‘b’) at the thinnest end portions (e.g., 752, 753) of the peripheral regions 75, 76 may be 5 to 9 mm, while the total thickness of the cut off (i.e., ‘a’ + ‘c’) at the thickest middle portion (e.g., 751) of the peripheral regions 75, 76 may be 3 to 7 mm. It has been found that the non-uniform thickness of the peripheral regions 75, 76 along the X direction is useful for reducing the bending of the bottom heat sink 70 (in particular at the edge of the bottom heat sink 70) after bonding the heat sink 70 under the bottom substrates of the power modules 1001 to 1003. The top substrates of the power modules 1001 to 1003 generally have relatively low CTEs and thus could restrict the power semiconductor chips 41, 42, the solid conductive shims 44, 45 and the compliant conductive structures 46 from following the bending of the bottom heat sink 70 and the bottom substrates. Therefore, low bending of the bottom heat sink 70 is useful for reducing the thermo-mechanical stress and strain developments, thereby improving the reliability of the joints (e.g., the joints 401, 402, 404, 405, 406, 407 to 409) which attach the power semiconductor chips 41,42, and bond the solid conductive shims 44, 45 and the compliant conductive structures 46. It would be understood that the bending of the bottom heat sink 70 would cause variations in the distances between the top substrates and the bottom substrate of the power modules, and the distance tends to be increase from the middle to the periphery of the bottom heat sink 70. With reference to Figures 8, the compliant conductive structures 46 are bonded with joints 409 on the conductive tracks 212, 213, 215 and 217 of the bottom substrate, and therefore the locations of the joints 409 represent the locations of the compliant conductive structures 46 along the X-Y plane. Similarly, the locations of the joints 407, 408 represent the locations of the power semiconductor chips 41, 42. It can be seen that the compliant conductive structures 46 are placed closer to the periphery regions 76, 75 of the bottom heat sink 70 than the power semiconductor chips 41, 42. In this way, the compliant conductive structures 46 can effectively compensate for the distance variations between the top substrates and the bottom substrate, by providing compliance along the Z direction. Consequently, the compliant conductive structures 46 improves the reliability of the electrical connections between the top substrates and the bottom substrate of the power modules. Following up from Figure 9, after the bottom substrates of the three power modules are bonded to the top cooling surface 77 of the bottom heat sink 70, power terminals 1 to 3 are then bonded to the conductive tracks 211 to 213 of each bottom substrate (Figures 4 and 8), and signal / control terminals 4 to 12 are bonded to the conductive tracks 211 to 219 of each bottom substrate (Figures 4 and 8). The power and signal / control terminals 1 to 12 may have been moulded in the plastic frame 91, and be bonded on the conductive tracks with ultrasonic welding technology to achieve high reliability. In that case, the power and signal / control terminals 1 to 12 are preferably made of metals or alloys such as pure copper, pure aluminium, copper alloy or aluminium alloy with sufficient strength and good ductility. Figure 16 is a top perspective view of part of the package 1000 after bonding all the power and signal / control terminals 1 to 12 to the power modules and after attaching the plastic frame 91 to the bottom heat sink 70. As described above, the power terminals 1 to 3 of Figure 16 are Pi, Ni, ll / V / W of Figure 1, with i=1, 2, 3. Further, the signal / control terminals 4 to 10 are Ei, Gi, Ci, Ej’, Ej, Gj, Cj of Figure 1, with i=1, 3, 5, and j= 2,4, 6. The signal / control terminals 11 and 12 are the terminals of the NTC thermistors. Alternatively, the power and signal / control terminals 1 to 12 may be bonded on the corresponding conductive tracks with soldering or sintering technologies, which are also used to bond the power semiconductor chips 41, 42, the solid conductive shims 44, 45, and the compliant conductive structures 46. In that case, the power and signal / control terminals 1 to 12 may be made of materials similar to those of the conductive shims with high thermal conductivity and high electrical conductivity. Further, the power and signal / control terminals 1 to 12 may be bonded on the conductive tracks of the bottom substrates during the same processing step for bonding the back conductive layers 221 of the bottom substrates on the top cooling surface 77 of the bottom heat sink 70. If the plastic frame 91 is not able to withstand the temperatures during the bonding process, the power and signal / control terminals may not be moulded on the plastic frame 91, and the plastic frame 91 may be installed on the bottom heat sink 70 after bonding the power and signal / control terminals 1 to 12. The bonding materials for the above mentioned soldering technologies to attach the power semiconductor chips 41, 42, and bond the solid conductive shims 44, 45, the compliant conductive structures 46, the back conductive layers 221 of the bottom substrates and the power and signal / control terminals 1 to 12 can be lead-free solder alloys (such as tin-silver, tin-copper, tin-silver-copper, tin-antimony or bismuth-silver solder alloys). On the other hand, the bonding materials of the above mentioned sintering technologies may be pastes or films of silver and copper particles and / or nanoparticles. It should be noted that the same solder alloy or several solder alloys with different melting points, or combined sintering materials and solder alloys can be employed to attach the power semiconductor chips 41, 42, and bond the solid conductive shims 44, 45, the compliant conductive structures 46, the back conductive layers 221 of the bottom substrates and the power and signal / control terminals 1 to 12. The lead-free solder joints are preferred for low cost applications where the junction temperatures of the semiconductor chips are in general lower than 150 °C. The sintered silver or copper joints are preferred for high temperature and high reliability applications where the junction temperatures of the power semiconductor chips may be higher than 150 °C. As shown in Figures 17 to 19, the top heat sink 80 are then pressure mounted on the top conductive plates 321 and 322 of the top substrates of the three power modules 1001-1003. The pressure mounting of the top heat sink 80 involves the use of a structural plate 83, a pressure controller comprising fasteners 84, 85, compliant pads 81, 82 and TIM sheets 61,62. The compliant pads 81, 82 may also be referred to as “second compliant structures” of the package 1000. Similar to the bottom heat sink 70, the top heat sink 80 is an integrated heat sink with a pin fin cold plate and a coolant manifold. In particular, the top heat sink 80 has cooling channel(s) (along which a coolant flows) formed therein, as well as a coolant inlet and a coolant outlet for coupling with the two through holes 743 of the bottom heat sink 70. The coolant inlet and outlet of the top heat sink 80 are formed on the bottom cooling surface of the top heat sink 80, and therefore are invisible in Figure 18. The structural plate 83 is placed over the top heat sink 80. The sealing between the top and bottom heat sinks 70, 80 is secured when the top heat sink 80 is pressed against the bottom heat sink by the structural plate 83 along a direction (Z- direction) that is opposite to the Z direction. Therefore, the Z direction of the package 1000 may also be referred to as a stacking direction of the top and bottom heat sinks 70, 80. The top heat sink 80 may be made of pure metal such as pure copper and pure aluminium, or alloy such as coppermolybdenum alloy and copper-tungsten alloy, or metal-matrix composite such as copper-graphite composite, aluminium-silicon carbide composite and aluminium-carbon fibre composite, or other materials with high thermal conductivity. Preferably, the top heat sink 80 is made of the pure aluminium or aluminium alloy with light weight and low cost. The structural plate 83 may be deemed as part of a supporting frame of the package 1000. It would be understood that the structural plate 83 would have sufficient mechanical strength in order to pressurise the heat sinks 70, 80 without experiencing noticeable deformation in itself. In an example, the structural plate 83 are made of structural steel or other materials with similar mechanical properties and thermal stability, and may be 1 to 3 mm in thickness. While the surface of the structural plate 83 is shown as flat in Figures 17 and 18, it would be understood that reinforcing features (e.g., reinforcing ribs, protrusions, and contours) may be incorporated into the surface(s) of the plate. The pressure applied by the structural plate 83 to the heat sinks 70, 80 is controlled by the pressure controller. In the example of Figures 17 and 18, the pressure controller comprises two sets of fasteners 84, 85. Each fastener 84 includes a first end 84A coupled to the structural plate 83, a second end 84B which can be coupled to the blind holes 741 or 742 (Figure 13) of the bottom heat sink 70, and a linkage member 84C which extend along the Z direction between the first end 84A and the second end 84B. While it is not shown in Figure 18, it would be understood that helical threads are cut into the circumferential outer surface of the second ends 84B, and are engageable with the helical threads formed within the blind holes 741, 742. The top heat sink 80 comprises four through holes 87 at its four corners. The linkage members 84C of the first set of fasteners 84 also extend through the through holes 87. In this way, the first set of fasteners 84 not only bind the structural plate 83 to the bottom heat sink 70, but also align the top heat sink 80 with respect to the structural plate 83 and the bottom heat sink 80. Similarly, each fastener 85 includes a first end 85A coupled to the structural plate 83, a second end 85B with helical threads (not shown) which are engageable with the helical threads formed within the blind holes 745 (Figure 13) of the bottom heat sink 70, and a linkage member 85C which extend along the Z direction between the first end 85A and the second end 85B. As shown in Figure 18, the linkage members 85C of the second set of fasteners 85 bypass the top heat sink 80 and do not extend through the top heat sink 80. To assemble the package 1000, the fasteners 84, 85 are rotated so as to couple the second ends 84B, 85B with the blind holes 741, 742, 745 of the bottom heat sink 70. The amount of rotation of the fasteners 84, 85 determines the distance between the structural plate 83 and the bottom heat sink 70. The distance in turn controls the amount of pressure applied by the structural plate 83 to the heat sinks 80, 70 along the Z direction. In the particular example illustrated by Figures 17 and 18, the fasteners 84, 85 resemble bolts, and the first ends 84A, 85A are in contact with the top surface of the structural plate 83. It would be appreciated that the pressure controller may incorporate a different mechanism for controlling the pressure applied by the structural plate 83. For example, the fasteners 84, 85 may be screws, or may be integrally formed with the structural plate 83. Alternatively, the bottom heat sink 70 may comprise through holes at the locations of the blind holes 741, 742, 745, and the linkage members 84C, 85C may extend through such through holes and additional nuts may be coupled to the second ends 84B, 85B at the bottom side of the bottom heat sink 70. In general, the first ends 84A, 85A are expected to have a much smaller area (e.g., less than one tenth) than the structural plate 83. In other words, it is the structural plate 83 that applies the pressure to the heat sinks 70, 80 and the first ends 84A, 85A merely regulate the pressure applied. The pressure controller comprising the fasteners 84, 85 effectively binds the structural plate 83, the top heat sink 80, and the bottom heat sink 70 together. Therefore, the pressure controller may also be referred to as a pressure adjuster or a binding structure. TIM sheets 61,62 are inserted between the bottom cooling surface of the top heat sink 80 and the top conductive layers 321, 322 of the top substrates of the power modules 1001 to 1003, thereby thermally coupling the top heat sink 80 to the conductive layers 321, 322. The dimensions of the TIM sheets 61, 62 preferably match the dimensions of the top conductive layers 321, 322. The TIM sheets 61, 62 may be 0.2 to 1 mm in thickness and made of compliant TIM, e.g. graphite sheets, or other TIM sheets or films with high thermal conductivity and thermal stability. Compliant pads 81, 82 are inserted between the structural plate 83 and the top heat sink 80. When viewed along the stacking direction Z, the compliant pads 81, 82 have a combined area that is much smaller than (e.g., less than a third of) the area of the structural plate 83, and is also much smaller than (e.g., less than a third of) the cooling area of the top heat sink 80. The complaint pads 81 are designed with specific sizes and layout corresponding to those of the IGBT chips 41 and / or the conductive shims 44 which are bonded to the topsides of the IGBT chips 41 in each of the power modules 1001 to 1003. Figure 19 is the top plan view of the package 1000 where the plastic lid 92, the structural plate 83, the fasteners 84, 85, the top heat sink 80, the TIM sheets 61, 62 and the top substrates of the power modules 1001 to 1003 are invisible. In this way, Figure 19 shows the sizes and layout of the compliant pads 81 in comparison with the IGBT chips 41 and / or the conductive shims 44 which are bonded to the topsides of the IGBT chips 41. It can be seen that, when viewed along the Z direction, the compliant pads 81 overlap the IGBT chips 41 and the conductive shims 44. Due to the thickness of the compliant pads 81, when the structural plate 83 applies pressure to the heat sinks 70, 80, the compliant pads 81 generate additional compressive stresses in the Z direction on the joints (e.g., the joints 404, 407,) between the IGBT chips 41, the conductive shims 44 and the respective conductive tracks 211, 213, 311 and 315 of the top and bottom substrates of the power modules 1001 to 1003. The additional compressive stresses are useful for improving the reliability of the joints. It would be appreciated that the compliant pads 81 may be modified so that they also overlap the FRD chips 42 and the conductive shims 45 which are bonded to the topsides of the FRD chips 42. In this way, the compliant pads 81 would also be useful for improving the reliability of joints associated with the FRD chips 42 and the conductive shims 45. Further, the additional compressive stresses are also useful for improving the thermal contact between the TIM layers 61, 62 and the top heat sink 80, and the thermal contact between the TIM layers 61,62 and the conductive layer 321 and 322 of the top substrates of the power modules 1001 to 1003. In other words, the compliant pads 81 ensure that the pressure applied by the structural plate 83 is distributed to achieve intimate contact between the power modules 1001 to 1003, TIM layers 61, 62 and the top heat sink 80. Therefore, the use of the compliant pads 81 effectively reduces the thermal resistance between the power modules 1001 to 1003 and the top heat sink 80. With further reference to Figures 18 and 19, it can be seen that the compliant pads 82 are ring-shaped and surround the linkage members 84C. The compliant pads 82 generate additional compressive stresses around the through holes 87 through which the linkage members 84C extend. The complaint pads 82 are useful for achieving tight connection between the top heat sink 80 and the bottom heat sink 70 with pressure applied by the structural plate 83. The compliant pads 81, 82 may be attached to the bottom surface of the structural plate 83, and may be made of a mechanically highly compliant and thermally stable material (e.g. silicone rubber or other materials with similar mechanical properties and thermal stability). The thickness of the compliant pads 81, 82 may be 0.2 to 2 mm. Alternatively, the compliant pads 81, 82 may obtain mechanical compliance due to their structures, and may take the form of springs (e.g., disk springs or Belleville washers). It would be understood that the compliant pads 81, 82 may take any other suitable forms as long as they are able to achieve force and motion transmission through elastic body deformation. After mounting the top heat sink 80 and the structural plate 83 as shown in Figure 17, silicone gel may be injected as encapsulant to fill gaps between the bottom substrates and the top substrates of the power modules 1001 to 1003, gaps between the power modules 1001 to 1003 and the heat sinks 70, 80, and gaps between the heat sinks 70, 80 and the plastic frame 91. Alternatively, moulding compound may be used as the encapsulant to fill the gaps. The moulding compound may be epoxy, other polymer based materials, or inorganic materials with high insulating strength. Preferably, a polymer-based moulding compound may contain fillers such as silica, aluminium nitride or boron nitride fillers having high thermal conductivity and low CTE to improve the thermal conductivity and constrain the CTE of the moulding compound. The curing temperature of the moulding compound may be at least 20°C lower than the lowest melting point of the solder alloys used to attach the semiconductor chips 41, 42, and to bond the solid conductive shims 44, 45, the terminals 1 to 12, the compliant conductive structures 46, and the back conductive layers 221 of the bottom substrates. Finally, the plastic lid, 92 is installed over of the structural plate 83 and a fully assembled power semiconductor package 1000 as shown in Figures 2 and 3 is obtained. The package 1000 is implemented with double-side cooling, and achieves not only good thermal performance but also high thermo-mechanical reliability. In particular, the bottom heat sink 70 is fixedly (or securely) bonded under the bottom substrates of the power modules 1001 to 1003 while the top heat sink 80 is pressuremounted on the top substrates of the power modules 1001 to 1003 with TIM sheets 61, 62 therebetween. The joint between the bottom heat sink 70 and the power modules typically provides a relatively low thermal resistance. However, the thermal resistance between a pressure-mounted heat sink and a power module is typically much higher. To reduce the thermal resistance between the top heat sink 80 and the power modules, compliant pads 81 with specified size and layout (e.g., overlapping with the power semiconductor chips 41, 42) are inserted between the structural plate 83 and the top heat sink 80 to distribute the pressure applied by the structural plate 83. Such an arrangement to mount the top heat sink 80 ensures good thermal contact both between the TIM sheets 61,62 and the top heat sink 80 and between the TIM sheets 61, 62 and the top substrates of the power modules 1001 to 1003, and hence achieves low thermal resistance and good thermal performance of the power modules 1001 to 1003. The compliant pads 81 also generate additional compressive stresses on the joints between the power semiconductor chips 41, 42, the conductive shims 44, 45 and the conductive tracks of the top and bottom substrates of the power modules 1001 to 1003. In this way, the additional compressive stresses improve the thermo-mechanical reliability of these joints. Within each of the power modules 1001 to 1003, compliant conductive structures 46, rather than solid conductive shims (e.g., 44, 45), are bonded between the conductive tracks of the top substrates and the bottom substrate to achieve the electrical interconnections therebetween. The compliant conductive structures 46 provides compliance between the top substrates and the bottom substrates, and allows the bottom substrates to follow the thermal bending of the bottom heat sink 70 to a certain extent without causing any failure to the electrical connections within the power modules. This would reduce the thermo-mechanical stress and strain developments between the bottom heat sink 70 and the power modules, and hence improve the reliability of the joint 50 between the bottom heat sink 70 and the bottom substrates. The use of the compliant conductive structures 46 would also improves the reliability of the electrical connections between the top substrates and the bottom substrates of the power modules. Further, the bottom heat sink 70 has two peripheral regions 75, 76 which extend along long sides of the bottom heat sink 70 with a reduced thickness as compared to the body region 79 of the bottom heat sink 70. The peripheral regions 75, 76 reduce the total stiffness of the bottom heat sink 70 and thus reduce the extent of bending experienced by the bottom heat sink 70 after securely bonding the heat sink under the bottom substrates. This would reduce the thermo-mechanical stress and strain developments between the bottom heat sink 70 and the power modules, and hence improve the reliability of the joint 50 between the bottom heat sink 70 and the bottom substrates. Low bending of the bottom heat sink 70 also reduces the thermo-mechanical stress and strain developments within the power modules (because the bottom substrates of the power modules tend to follow the bending of the bottom heat sink 70). Accordingly, the reliability of joints 401-409 which bond the power semiconductor chips 41, 42, the conductive shims 44, 45, and the compliant conductive structures 46 is also improved. It would be appreciated that the solid conductive shims 44, 45 may be replaced with compliant conductive structures similar to the structures 46. In that case, the thermo mechanical stress and strain developments within each power module can be further reduced, and thus the reliability of the joints 401, 402, 404, 405, 407, 408 for attaching the power semiconductor chips 41,42 can be further improved. In each of the power modules 1001 to 1003, a larger bottom substrate is used together with two smaller top substrates. In other words, the top substrate has been split into two (or more) smaller substrates. By splitting one substrate into two or more smaller substrates, the smaller substrates are allowed to experience different degrees of deformations without causing substantial thermal stress to the power module. Therefore, the deformation of the power module as a whole tends to be more elastic and less plastic. In this way, the thermo-mechanical reliability of each power module (in particular, the reliability of joints within the power module) is improved. It would be appreciated that the bottom substrate may also be split into two or more smaller substrates. One or more of the bottom substrate and the top substrates of each power module may be chosen from direct bonded copper (DBG), directed bonded aluminium (DBA) or active brazed metal (ABM) substrates. In that case, one or more of the insulating layers 201, 301 and 302 may comprise 0.2 mm to 1 mm thick alumina, aluminium nitride or silicon nitride ceramic tiles. The conductive layers on both sides of the insulating layers may comprise 0.1 mm to 1 mm thick pure copper, pure aluminium, copper-molybdenum alloy, copper-tungsten alloy, or other pure metals or alloys with similar thermal, electrical and thermo-mechanical properties. The conductive layer on one side of the top substrate may be patterned (e.g., by dry or wet etching) to form the conductive tracks 311 to 315. The conductive layer on one side of the bottom substrate may be patterned (e.g., by dry or wet etching) to form the conductive tracks 211 to 219. Of the above-mentioned DBG, DBA, ABM substrates, silicon nitride based ABM substrates can be used for high performance and high reliability applications. This is because silicon nitride based ABM substrates have high mechanical strength and can withstand high mechanical stresses for sealing or mounting the double side heat sinks and have good thermo-mechanical reliability. In the example described above, the package 1000 encloses three power modules 1001 to 1003 and each power module has a half-bridge switch. It would be appreciated that the package 1000 may include any number of power module(s), and each power module(s) may contain any suitable power circuitry which is not limited to a single halfbridge switch. Accordingly, the numbers and / shapes of the solid conductive shims 44, 45, and the location of the compliant conductive structures 46 as well as the layouts of conductive tracks of both the bottom and top substrates may be modified accordingly based upon the particular power circuitry contained within the power module. Figure 20 schematically illustrates processing steps of a method for manufacturing a power semiconductor package (e.g., the package 1000). At step S1, a plurality of first compliant structures (e.g., the compliant structures 46) are bonded between first and second substrates of at least one power module (e.g., one or more of the power modules 1001 to 1003). The at least one power module further comprises a plurality of power semiconductor chips (e.g., the chips 41, 42). The first and second substrates are arranged at opposite sides of the power semiconductor chips. The first substrate comprises a first insulating layer (e.g., the insulating layer 201) and a first patterned conductive layer (e.g., the conductive tracks 211-219) arranged on a surface of the first insulating layer which faces the power semiconductor chips. The second substrate comprises a second insulating layer (e.g., the insulating layers 301, 302) and a second patterned conductive layer (e.g., the conductive tracks 311-315) arranged on a surface of the second insulating layer which faces the power semiconductor chips. The plurality of first compliant structures (e.g., the compliant structures 46) are electrically connected to the first patterned conductive layer (e.g., the conductive tracks 211-219) and / or the second patterned conductive layer (e.g., the conductive tracks 311-315). The plurality of first compliant structures may be fixedly bonded between the first and second substrates of the at least one power module. At step S2, a first heat sink (e.g., the bottom heat sink 70) is fixedly bonded to the first substrate. At step S3, a second heat sink (e.g., the top heat sink 80) is pressed by a structural plate (e.g., the structural plate 83) towards the at least one power module (e.g., one or more of the power modules 1001 to 1003) such that the second heatsink is pressure-mounted to the second substrate. A plurality of second compliant structures (e.g., the compliant pads 81, 82) are arranged between the structural plate and the second heat sink. It would be appreciated that the steps S1 to S3 may be performed in any suitable temporal order that is different from the order of description. The terms “having”, “containing”, “including”, “comprising” and the like are open and the terms indicate the presence of stated structures, elements or features but not preclude the presence of additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise. The skilled person will understand that in the preceding description and appended claims, positional terms such as ‘top’, ‘bottom’, ‘left’, ‘right’ etc. are made with reference to conceptual illustrations of a power module, such as those shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to a power module when in an orientation as shown in the accompanying drawings. Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the disclosure, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein. 25 06 25

Claims

1. A power semiconductor package, comprising:at least one power module comprising:5 a plurality of power semiconductor chips;first and second substrates arranged at opposite sides of the power semiconductor chips, wherein: the first substrate comprises a first insulating layer and a first patterned conductive layer arranged on a surface of the first insulating layer which faces the power semiconductor chips, and the second substrate10 comprises a second insulating layer and a second patterned conductive layerarranged on a surface of the second insulating layer which faces the power semiconductor chips; anda plurality of first compliant structures arranged between the first and second substrates and electrically connected to the first patterned conductive15 layer and / or the second patterned conductive layer;first and second heat sinks arranged at opposite sides of the at least one power module, wherein the first heat sink is fixedly bonded to the first substrate;a structural plate configured to press the second heat sink towards the at least one power module such that the second heat sink is pressure-mounted to the second20 substrate; anda plurality of second compliant structures arranged between the structural plate and the second heat sink.

2. A power semiconductor package according to claim 1, wherein the first heat sink25 comprises:a body region which comprises a cooling surface in thermal contact with the first substrate;first and second peripheral regions arranged at a periphery of the body region and being thinner than the body region along a first direction that is perpendicular to the30 cooling surface.

3. A power semiconductor package according to claim 2, wherein at least one of the first and second peripheral regions has a non-uniform thickness.35 4. A power semiconductor package according to claim 3,25 06 25wherein the at least one of the first and second peripheral regions comprises a middle portion and two end portions which are arranged along a second direction, the second direction being substantially parallel to the cooling surface, andwherein the middle portion is thicker than the two end portions along the first 5 direction.

5. A power semiconductor package according to claim 4, wherein the middle portion is about 1mm to 3mm thicker than each of the two end portions.10 6. A power semiconductor package according to any one of claims 2 to 5, whereinthe cooling surface comprises two long sides and two short sides, and the first and second peripheral regions extend along the two long sides, respectively.

7. A power semiconductor package according to any preceding claim, wherein the 15 plurality of first compliant structures are electrically conductive.

8. A power semiconductor package according to any preceding claim, wherein the plurality of first compliant structures comprise one or more coil springs.20 9. A power semiconductor package according to claim 8, wherein a central axis ofthe one or more coil springs is parallel to a surface of the first or second substrate.

10. A power semiconductor package according to any preceding claim, wherein the plurality of first compliant structures are fixedly bonded to each of the first and second 25 patterned conductive layer.

11. A power semiconductor package according to any preceding claim, wherein the plurality of first compliant structures are located closer to an edge of the at least one power module than the plurality of power semiconductor chips.3012. A power semiconductor package according to any preceding claim, wherein the plurality of second compliant structures have a combined area which is less than each of: an area of the structural plate, and an area of the second heat sink.25 06 2513. A power semiconductor package according to any preceding claim, wherein one or more of the plurality of second compliant structures is made of silicone rubber.

14. A power semiconductor package according to any preceding claim, wherein the 5 plurality of second compliant structures comprises a set of compliant structure(s) which overlap the at least one power module and occupy a smaller area than the at least one power module.

15. A power semiconductor package according to claim 14, wherein the set of 10 compliant structure(s) overlap at least some of the plurality of power semiconductor chips.

16. A power semiconductor package according to any preceding claim, further comprising:15 a pressure controller configured to adjust a distance between the structural plateand the first heat sink so as to control a pressure applied by the structural plate to the second heat sink.

17. A power semiconductor package according to claim 16, wherein the pressure 20 controller comprises first and second ends coupled to the structural plate and the first heat sink respectively, and a linkage member extending between the first and second ends.

18. A power semiconductor package according to claim 17, wherein the second end 25 of the pressure controller is rotatably coupled to the first heat sink, and a rotation of the second end with respect to the first heat sink is configured to adjust the distance between the structural plate and the first heat sink.

19. A power semiconductor package according to claim 17, wherein the plurality of 30 second compliant structures comprises a further set of compliant structure(s) which at least partially surround the linkage member.

20. A power semiconductor package according to any preceding claim, wherein in each of the at least one power module, the second substrate comprises a plurality of 35 separated substrates.25 06 2521. An electric drive system for an electric vehicle or a hybrid electric vehicle, comprising a power semiconductor package according to any preceding claim.5 22. An electric vehicle comprising an electric drive system according to claim 21.

23. A hybrid electric vehicle comprising an electric drive system according to claim 21.10 24. A method of manufacturing a power semiconductor package, comprising:bonding a plurality of first compliant structures between first and second substrates of at least one power module which further comprises a plurality of power semiconductor chips, wherein: the first and second substrates are arranged at opposite sides of the power semiconductor chips; the first substrate comprises a first insulating15 layer and a first patterned conductive layer arranged on a surface of the first insulating layer which faces the power semiconductor chips, and the second substrate comprises a second insulating layer and a second patterned conductive layer arranged on a surface of the second insulating layer which faces the power semiconductor chips, and wherein the plurality of first compliant structures are electrically connected to the first patterned20 conductive layer and / or the second patterned conductive layer;fixedly bonding a first heat sink to the first substrate; andpressing, by a structural plate, a second heat sink towards the at least one power module such that the second heat sink is pressure-mounted to the second substrate, wherein a plurality of second compliant structures are arranged between the structural25 plate and the second heat sink.

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