Abstract—Power Semiconductor Modules play a key role in Power Electronics Systems. Their inherent advantage of integrating different power chips, circuits and sense, drive and protection functions into one sub-system with electrically insulated cooling has lead to a wide range of products, being different in size, power and function. This paper will provide an overview of today´s power modules and packaging and interconnect technologies. Trends towards next generations of power modules will be highlighted. In the growing market of hybrid and electrical vehicles, products are emerging where power modules are “un-packaged” to arrive at highly integrated, compact sub-systems which are better suited for the harsh environmental conditions and the required power density than the classical power modules.
Thomas Stockmeier
I. INTRODUCTION
A power semiconductor module may be defined as a device which contains more than one semiconductor chip and which (with some exceptions) provides a heat flux path separate from the electric path. The first power semiconductor module was established in the mid seventies of the past century, where, for the first time, two chips (thyristor and/or rectifier) were combined by soldering them together with electrical contacts on metallised ceramic substrates and by putting them in a common plastic housing. Figure 1 shows a photograph of this first power semiconductor module [1]: Gate Contact (Silver) Cathode Contact (“Trimetal” = Cu-Fe/Ni-Cu) Disc (Molybdenum) Anode Conatct (Copper) Base Plate (Copper) (Grey: Solder Layers) Thyristor (Silicon) SEMIPACK DCB-Substrate (Cu-Al2O3- Cu)
Figure 1. Photograph of a 1600V, 90 A module with two antiparallel thyristors, 1975. A schematic cross sections shows the various materials and parts used in this first power module.
These first power modules provided tremendous system advantages, such as mounting many devices on one common, electrically isolated heat sink and connecting them electrically by bus baring. Before, discrete devices had to be clamped or screwed to individual coolers which had to be isolated from each other. Thus, power electronics systems became much more compact, cost efficient and reliable. These requirements for lower cost, lower size, lower weight and more reliable power modules has not ceased over the years.
II. POWER SEMICONDUCTOR MODULE MARKET
The power module market is about 2,100 Mio USD in 2007 [2], with a two digit growth rate. It is quite remarkable, that although technical and commercial progress has been very significant, the very first module (see Fig. 1) is still produced in its original form, fit, and function and still enjoys growing production quantities. The power module market can be split by function as follows: Bipolar modules 270 Mio USD (containing Rectifiers and Thyristors), IGBT Modules 900 Mio USD, Integrated Modules 200 Mio USD (containing rectifier and inverter function), and Intelligent Power Modules 780 Mio USD (IPM, containing gate driver and sensor functions).
From an application point of view, more than half of the market (56 %) is in Motor Drives. In particular in Industrial Drives, it is very important for module manufacturers to cover a wide power range with one module technology platform, as the Industrial Drives also have to serve the market in a modular fashion. Fig. 2 gives an example of such a power module family for Motor Drives [3].
Figure 2. SEMiX Product Family It offers rectifiers and IGBTs for 15 kW to 150 kW in a package outline with standard 17 mm terminal height
The next larger application is traction with 10% market share. For this application, the highest reliability and long term availability through multiple sources are key factors. Fig. 3 shows a typical representative of traction power modules [4].
Figure 3. 1200Amp., 3300V IGBT Module. Photo courtesy of ABB Semiconductors
The consumer market has a share of 9%. Here, intelligent power modules are clearly favoured. In the low horse power range, fully integrated power modules are offered in single-in-line or dual-in-line packages. Such packages are represented in Fig. 4 [5, 6].
Figure 4. Fully integrated intelligent power module in single-in-line and dual-in-line housing. Photos courtesy of Fairchild Semiconductors, International Rectifier, Mitsubishi Electric
Two other market segments shall be highlighted as well. One is power modules for renewable energies, in particular wind power, the other is automotive. The market share of power modules for wind power is only 5%. However, with 25% growth rate it has the strongest growth of all market segments. By 2011, this market is estimated to be 250 Mio USD. Power modules for wind power have similar requirements than modules for traction applications. A very high intermittent operating lifetime, long term availability, high reliability, and suitability for harsh environments are a prerequisite for success. Fig. 5 shows an IPM which is widely used in wind power applications [7], on a custom specific heatsink with integrated gate drive, current, temperature and voltage sensors.
FIgure 5. Left: 1800 Amp., 1200V Dual Pack IPM, called SKiiP3 mounted on a heatsink. Middle: gate driver w/o cover. Right: explosion drawing of one power module w/ current sensor
The automotive market for power modules is only 4 %, but is growing strongly with about 19% per year. High power automotive applications have a whole set of special requirements which drives the power module requirements. Technically most demanding are high ambient temperatures and a high number of thermal cycles. Fig. 6 shows two representative power modules [8, 9].
Figure 6. SKiM63 (300A, 1200V) and HybridPack1 (400A, 600V) (Photo of HybridPack courtesy Infineon Technologies) are typical sixpack IGBT power modules for automotive applications
III. POWER SEMICONDUCTOR MODULE TECHNOLOGY
A. Electrical Interconnect Technology
As power modules cover a wide range of electrical currents, different technologies are used for connecting the power terminals of the module to the outside world. In the lowest power (typically below 100 Watts), the module will be soldered into a printed circuit board (PCB). For small modules (like IPMs in Consumer Market, see Fig. 4), the modules are typically soldered together with other feedthrough or SMD components such as filters, DC-link caps, and connectors. Therefore the technical requirements for solderability are derived from the printed circuit board manufacturing. The power modules must withstand the thermal budget of solder reflow and solder waves.
As the current increases, the power module gets bigger and heavier. Thus it has to be soldered separately into the PCB, typically by using soldering robots which can solder the module pins to the circuit board one by one. As the module is connected to the PCB, the whole unit has to be mounted to the cooler, afterwards. This can be done by providing special profiles in the cooler where clamps can be fitted which hold the power module to the cooler [10].
In order to make mounting and removal of the power module easy, spring contact systems emerged [11]. Such a power module is shown in Fig. 7. Here the power module is clamped between the cooler and the PCB with one or two screws, enabling electrical and thermal contact at the same time. In case of a defect, the system can be un-screwed to replace the defect component, fast.
Figure 7. MiniSKiiP spring pin contact system. Module cut open to show the position of the spring contacts.
Recently, a press pin configuration has been established, where the terminals of the power module are pressed into the PCB and can be removed again, using special tools [12].
If the current exceeds 100 to 150 Amperes, a direct PCB to module interconnect technology is no longer feasible. Typically, the power terminals will be screwed to busbar metals sheets and / or cables. Whereas on older devices the power terminals where situated on top of the module (comp. Figs. 1, 3), modern power modules (comp. Figs. 2, 5, 6) provide the power terminals on opposite sides of the device, separating the power input from the power output and leaving the space on top for gate drive PCBs. The power module is bolted to the cooler, and the driver PCB is either connected by press-pin, solder pins, or spring contacts. With this construction, compact power electronics systems can be designed. Screw type terminals are used up to highest currents by paralleling connectors.
For very high currents, press pack modules are used. These modules do not have an integrated electrical insulation and may not be counted as power modules for this reason. The top and bottom surfaces of the device serve as the main electrical contacts. Fig. 8 shows a high power press pack IGBT module, used in High Voltage DC transmission [13]. Fig. 9 shows a module with two antiparallel thyristors for soft start applications [14].
Figure 8. 1200A, 2500V Presspack IGBT (236x150x26 LxWxH in mm). Each IGBT and diode chip is contacted individually by a spring
Figure 9. Semistart, a family of non-insulated press pack modules with two antiparallel thyristors
B. Power Semiconductor Module Thermal Management
Power semiconductor chips produce very significant losses during conduction and switching. Therefore, effective cooling of the chips in a power module has to be enabled by mounting the module onto a heat sink. Fig. 10 shows two different technologies in a schematic cross section. For both technologies, the chips are soldered to a double side metallized ceramic substrate which has to provide both, excellent thermal conductivity and electrical insulation. The most commonly used ceramic materials are Alumina (Al2O3) and Aluminum Nitride (AlN). The metallization is typically realized by a thick (300 μm) Copper layer, either connected to the oxide by an eutetic bonding process (direct bonded Copper, DBC) or by an active metal brazing process (AMB). Typical heat conductivity ranges from 20 W/mK for Alumina DCB to 170 W/mK for AMB-AlN. For a base plate module, the chip carrying substrate is soldered to a base plate (3-5 mm thick) which is either made from Copper or a metal-matrix compound material, such as AlSiC. Before mounting this base plate to the heat sink, an interface layer of thermally conducting material has to be applied either to the module or to the heatsink so that no air gaps prevent good thermal contact between module and heat sink. The multitude of technical solutions for this interface layer is a clear sign for the fact that there is no optimal solution, yet. Most common layers are the so called thermal grease which contains ZnO particles in a silicone or non-silicone oil. Thermal grease is quite a poor thermal conductor (1 W/mK) and should be applied as thin as possible to minimize the barrier for the heat transfer from the power chip to the heat sink. The practical thickness for base plate modules is about 100 μm.
Figure 10. Module Construction with and without base plate
For a power module without base plate, the chip carrying substrate is directly pressed to the heat sink. Again, a layer of thermal grease is applied between substrate and heat sink. However, the layer can be much thinner (typically 20 μm), as the substrate is more flexible than a base plate. This module construction is advantageous, as is saves quite some material and weight. And it provides better thermal cycling capability as the large area solder layer between substrate and base plate is omitted. However, modules without base plate have a reduced thermal capacitance and therefore an inferior thermal impedance. Fig. 11 shows a populated dual IGBT substrate of such a power module (compare Fig. 5) without base plate.
Figure 11. 600 Amp. 1200V SKiiP3 power module substrate (140mm x 53 mm), populated with IGBT chips and diodes.
One key issue in a power module is it´s passive thermal and it´s active power cycling capability [15, 16]. The outside temperature of a power module may change from typically -40°C to +90°C during day to day operation. Thus, solder layers which connect chip to substrate and substrate to base plate may fatigue over time, as the thermal expansion coefficients of the materials in contact are quite different. If one considers one such cycle per day in a 20 years life of a module, the requirement will be that the power module can withstand more than 7,000 of such thermal cycles.
As for the power cycling, the junction temperature of chips will change during operation, as the load in the devices changes. Therefore, the module will constantly go up and down in temperature, due to its own operation, aging the solder layers inside by mechanical stress. The rate of temperature change can be very different, depending on the load. In real applications, this so called mission profile is quite complex and will vary substantially for different applications. Thus the requirement for power modules is that they withstand millions of power cycles with a temperature swing from +25°C to +125°C, if the heat is applied and removed within seconds, and that the power modules withstand tens of thousands of power cycles in the same temperature range, when the frequency is in the range of minutes (i.e. 20 sec heating, 40 sec. cooling) [17].
Fig. 12 shows how the failure rate of power devices will change over time, as the device fatigues and thus looses it´s strength, whereas the load profile remains constant.
Figure 12. Reliability function over time. Graph courtesy of Vestas A/S.
IV. POWER SEMICONDUCTOR MODULE ASSEMBLY
The key technologies inside a power module are soldering, ultrasonic bonding and silica gel filling.
The soldering technologies for power modules shall provide large area, void-free and lead-free solder layers. Typically, chips and terminals are soldered to substrates, and substrates are soldered to base plates. Most commonly used solder techniques are solder reflow under vacuum, using pre-applied solder pastes which contain no-clean or water-solvable flux, or use formic acid during the solder process. The second technology is soldering with preforms in vacuum ovens or conveyor belt furnaces, where oxides are removed by a hydrogen containing atmosphere. Soldering alloy, thickness and thickness uniformity of the solder as well as the solder process kinetics all play an important role to determine the resulting thermal and power cycling capability of the device.
Ultrasonic wedge bonding of thick aluminium wires (300 – 500 μm diameter) is the commonly used interconnect technology to connect the upper side of chips to substrates and to connect substrates to terminals. Today, highly automated wire bonders provide an economic and reliable process. Fatigue of the ultrasonically welded contact of the aluminum wire to the chip is one of the major limitations for the intermittent operating life of power modules due to the mismatch in thermal expansion coefficients of the contact patners. Once the module is ready mounted into a plastic housing, a silica gel is finally poured into the module, because in air the distances inside the module would be insufficient to provide save electrical insulation. Additionally, the silica gel provides mechanical protection and protection against contamination. Air bubbles have to be avoided in the gel fill process. Therefore the silica gel is mostly filled in under vacuum or the module is evacuated after gel filling. It is worthwhile to mention that silica gel provides little protection against humidity [18]. In a matter of a few hours, the module will have the same relative humidity inside as the outside environment.
V. POWER SEMICONDUCTOR MODULE TRENDS
A. Ceramic Substrates
The trend is to make the ceramic layer thinner and the Copper layer thicker in order to improve the heat transport. The thickness of Alumina has been reduced from 0.63 mm to 0.38mm. Recently, Zirconium doped Alumina has been established with a thickness of 0.32mm [19]. An alternative material is silicon nitride (0.32 mm) which has a better thermal conductivity (70 W/mK) and which has a much higher mechanical strength than Alumina [20]. Therefore it seems feasible to increase the Copper thickness from 0.3 mm to 0.6 mm, thus providing a better thermal spreading and less ohmic losses in the metal layer and enhance the thermal capacitance.
B. New Interconnect Technologies
Fig. 13 shows the replacement of thick wire bonds by ribbon bonding [21]. The thickness and width of the ribbon in Fig. 13 is 0.2 mm x 1.5 mm.
Figure 13. Al thick wire (300 μm) bonding compared to ribbon bonding (0,2mm x 1,2 mm) on a 24 x 24 mm² thyristor chip
Another new interconnect technology for the replacement of wire bonds is welding of Copper ribbons between power terminals and DBC substrates. This connection can carry higher currents and has better cycling capability (compared to wire bonding).
C. Solderless Power Module
It was explained that solder fatigue is one of the predominant power module lifetime limitations. Thus a greatly enhanced cycling capability should be achieved when solder layers are omitted. Fig. 14 shows the first power module which does not contain any soldering interconnect [8]. The power chips and sensors are sintered to a DBC substrate, and the power and auxiliary contacts are pressed to this substrate. The module does not have a base plate. The substrate is in direct contact with the heat sink.
Figure 14. SKiM 63: A 300A, 1200V sixpack IGBT without any solder layers
In the sintering process, the chips are first placed into a silver paste layer which is pre-applied by stencil printing. Under very high pressure (30 MPa) and moderate temperatures (250°C), the silver paste transforms into a solid layer of silver (melting point 961 °C). Fig. 15 shows a photograph of a 5”x 7” DBC card where four substrates, each containing 12 IGBTs and 6 diodes are sintered in one step. To demonstrate the excellent adhesion of the chip to the substrate, one substrate was rolled up: The chips do not lift from the substrate.
Figure 15. 5” x 7” DBC card, containing four substrates with sintered chips. One substrate was rolled up to demonstrate the adhesion of the chips.
Fig. 16 shows the comparison of soldering and sintering technology in an intermittent operating life test of an IGBT module with base plate. Remarkable differences in performance and failure mode can be observed. The junction temperature in the soldered module starts to increase after a few thousand cycles. This is a clear sign that the solder layer between chip and substrate delaminates with the progressing number of cycles, decreasing the cooled area. Thus the junction temperature increases, provided the test is carried out under constant load current. This in turn leads to higher temperature excursions, accelerating the delaminating process and consequently, the stress between bond wires and chip. In the end the IGBT exhibits a junction temperature of 250 °C and the wire bonds lift from the chip metallisation.
The power module with sintered chips behaves quite different. The junction temperature and thus the thermal resitance remains at 150 °C and does not change with the progressing number of cycles. The module fails at about three times the number of cycles as a soldered module. Fig. 17 shows photographs of opened modules with solder and sinter contacts. When opening the failed solder module, one can see that the bond wires are detached from the chip contact. However, in the sintered module (right side in the figure), the bond wires have not lifted from the chip metallization, but show cracks in the heel, instead.
Figure 16. Change of thermal resistance and Vce(sat) during power cycling for sintered and soldered Semitrans modules
The reason for this is a pull force from the wire during heating, due to it´s higher thermal expansion than the chip. Therefore, it seems feasible to increase the cycling capability of sintered standard modules by modifying the bond wire loop and bond tools.
.
Figure 17. Failed modules after intermittent operating life tests In the soldered module (left side) the bond wire has lifted from the chip metallization. In the sintered module (right side), the bond wire has been ripped apart at the heel of the bond.
D. Silica Gel Coating
As mentioned before, silica gel is poured into power modules to provide electrical and mechanical protection. In order to reduce the amount of silica gel and to enable shorter processing time, silica gel coating has been established. This technology is demonstrated in Fig. 18. In this fully automated process, a substrate edge protection silica gel is first applied through a jet valve. Secondly, a thin gel layer is applied by a shower head across the entire substrate. UV light starts the polymerisation of the gel afterwards. While the gel is still not cured, the substrate is flipped over such that the gel flows along the bond wires. This will ensure that the bond wires are coated by the force of gravity. Finally the substrate is automatically fitted into the housing and the gel cures completely.
Figure 18. Schematic cross section of gel coating vs. gel filling and a photograph of a gel coated chip on a substrate.
E. SiC Devices
SiC based chips, such Schottky diodes and MOSFETs are emerging for the 600 V to 1700V range [22]. To use the inherent advantages of SiC devices, power modules are required with low parasitic inductivity and capability for high junction temperature. Fig. 11 gives an example of such an advanced power module. In this particular case, 14 IGBT chips and 6 SiC Schottky diodes are connected in parallel per switch (the substrate contains a dual switch). Three substrates are then used to form a three phase inverter (corresponding to 36 SiC chips per inverter). By the use of SiC diodes the current could be incerased by about 30%, compared to silicon diodes. However, considerably ringing was observed when using SiC diodes in parallel, as seen in Fig. 19.
Figure 19. IGBT Turn-on waveforms in clamped inductive switching of a SKiiP3 device with SiC Schottky diodes in parallel per switch.
F. Intelligent Power Modules
With emerging control-ICs based on SOI-technology [23], it is feasible to place the control ICs (which do not latch even at 200 °C) directly onto the substrate, thus enabling the design of very compact intelligent power modules. Fig.20 shows the substrate of a fully integrated Converter- Inverter-Brake (CIB) IPM. The naked IC chip is soldered to the fine pitch substrate and connected by 25 μm aluminium wedge bonding. A silicone glop protects the IC in the further assembly. All drive and protection functions, including high voltage insulation of the primary to the secondary side drive electronics have been integrated in the
IC. Only spacious bootstrap capacitors have to be furnished elsewhere.
Figure 20. Photo of a substrate for a 600V, 50 Amp , CIB IPM MiniSKiiP. The SOI driver IC with full electrical insulation has been integrated.
G. SKiN Technology
Figs. 21 and 22 show the cross section of a SKiN IPM and photos of the populated SKiN layer. Bond wires have been replaced by welding the chips with an ultrasonic flip chip bond process to a sandwich layer composed of Aluminium, Polyimide and Copper.
Figure 21. Schematic cross-section of a fully integrated SKiN IPM.
The aluminum side provides the load and gate tracks, the copper side can be designed to carry the drive and sense electronics. Vias through the polyimide layer enable contacts from the upper metal layer to the gates and sensors by thin wire bonding.
Figure 22. Left side photo shows the layer with welded power chips. The right side photo shows the upper side with driver IC and SMD components attached.
Once the double sided populated layer is finished, it is folded and fitted into a high temperature compatible plastic housing. The result is a SMD mountable sixpack or CIB IPM.
H. Power modules for High Ambient Temperatures
A lot has been discussed about so called high temperature electroncis [24, 25, 26]. Today´s silicon based 1200V IGBTs and freewheeling diodes are suitable for junction temperatures of 175 °C. In the near future 200 °C will be feasible, as is already the case for MOSFETs and 600V IGBTs and FWDs. Rectifiers and thyristor will be available for 150 °C maximum junction. High temperature chips can be used in two different ways: One is to leave the heatsink temperature at today´s values (thus not imposing health risks and risks for all other electronic components around the power module) and to increase the power handling capability of the power module by allowing a higher temperature swing of the junction. A 25°C higher temperature swing will provide 30% more power. However, the power cycling capability will be reduced at least by a factor of 3 to 5, depending on the packaging technology. Therefore new technologies, such as sintering have to be established before the higher junction temperatures of the chip can be harvested.
The other is to use power modules in high temperature environments, such as an automobile. Here, an ambient temperature of 130°C and a liquid coolant temperature of 105°C and higher are required. In this case, all conventional packaging materials have to be reviewed: New plastic housing materials, such as new PPAs have to be considered. Also the temperature stability of silica gel needs to be looked at.
One way to overcome the limitations of package materials at higher temperatures is to simply leave them out. Instead of producing power modules in the classical fashion, populated DBC substrates may be used to build power electronics systems “from scratch” by mechanical integration. This stripping of most of the classical package components, such has housings, terminals, and base plates shall be named “Un-Packaging”.
VI. POWER SEMICONDUCTOR MODULE UNPACKAGING
Fig. 23 shows in an exploxed view how a power electronics system (in this case a 360 Arms, 48 V, 3 phase motor drive) is build without using power modules [27]. The total volume of the complete unit is only 2,3 liter which is very important to fit this unit in various compartments within electric vehicles. In the first step, substrates are manufactured which contain power MOSFETs (soldered or preferably sintered) in a dual pack configuration, temperature sensors and filter capacitors. Fig. 24 shows a photograph of such a substrate which contains seven top and seven bottom switches, one RC filter per switch and a temperature sensor. Three of such substrates are mounted on a customized heatsink, with a 20 μm thick layer of thermal grease applied by screen printing. Secondly, a frame (made of high temperature compatible plastic material) with already moulded-in screw type terminals is mounted. In a third step, a DC and AC busbar system with integrated DC-link capacitor is placed.
A so called pressure part is mounted on top of the DC link by screwing it to the heatsink. This pressure part which contains pre-assembled springs for auxiliary contacts, presses the power terminals to the substrate, thus enabling thermal and electrical contact at the same time. A silicone foam layer between pressure element and terminals assures evenly distributed mechanical pressure across the entire device. Now, a PCB can be mounted which contains gate drivers, current-, voltage-, and temperature-sense electronics, as well as the controller PCB. Finally, a metal or a plastic hood is placed over the assembly, providing environmental protection to the required class, i.e. IP 54.
Figure 23. Explosion drawing of a 48 V, 360Arms motor drive for electric vehicles
Figure 24. Substrate with seven 100A, 100V MOSFETs, RC filters, and a temperature sensor
Therefore a complete DC to AC motor drive inverter has been build in less than 10 production steps with a minimum amount of materials and technologies, resulting in a power electronics system which exhibits a benchmark power density, withstands 20 g vibration and 100g shock, shows – in absence of any solder contacts - a very high thermal and power cycling capability, and can therefore fully utilize the 200 °C junction temperature capability of the most advanced power MOSFETs. Due to it´s compact design and the integrated filter capacitors it exhibits very little conducted and radiated emissions.
Figure 25. 1200V, 300Arms three phase inverter system with integrated liquid cooling. The size is 400x215x100 (LxWxH in mm). The inverter has a power density of 30 kVA / dm³.
With the same un-packaging technology, high voltage systems can be build, as well. Fig. 25 shows a 1200V, 300 Arms IGBT inverter for 150 kW with a power density of 30kVA/dm³ and 20kVA/kg, build on the same technology platform as described above [28]. Such systems are extremly suitable for vehicle applications. The outer form can be tooled to fit into almost any specific compartment, it´s mechanical and environmental ruggedness and simple interconnect technology enable free choice of placement in the vehicle and the modular substrate approach inside allows to scale power and to adopt the circuit, to such as brake choppers and charging circuit.
VII. SUMMARY
Power modules play an important technical and economical role in power electronics. Saving energy in the industrial and in the mobile world, as well as generating and distributing energy from renewable sources fuels a very high growth rate for the power module business. Progress in power chips and ever increasing requirements for lower cost, higher quality and reliability and reduced size drive the technical evolution. Trends are to replace solder contacts by sintering for higher temperature and more reliable devices, to replace wire bonds by welded contacts, and to mechanically integrate more functions, even in higher power. An entirely new package concept, called SKiN, provides a platform to embrace these trends into one new class of power modules. In order to fulfill the requirements of high temperature electronics, power modules can be un-packed. Highly integrated power electronics systems are constructed without the use of power modules, starting the assembly from mounting substrates to a heat sink and to stack power and control layers on top.
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