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Issue 2 2012 Power Electronics Europe www.power-mag.com

With Sinter-TechnologyForward to Higher Reliability ofPower Modules forAutomotive ApplicationsA new kind of applications, the whole automotive market describes more and more a “high end”-powermodule. The unprecedented combinations of thermal, electrical, and reliability performance with a verysmall volume and weight are the challenge today. A consistent further development of sinter technology forpower modules comprises and answers to all these requirements. Sinter technology substitute all solderconnections and also the aluminum bond wires. These are at present the weak points in a standard powermodule. The thermal and reliability results of a 400 A/600 V Dual IGBT power module will be shown in thisarticle. Jürgen Steger, Project-leader, SEMIKRON Elektronik, Nuremberg, Germany

The motivation for a complete newdevelopment of a power electronic moduleis due two main reasons. First is the reliabilityfor electric cars. Secondly, the latestgeneration of power semiconductorswitching devices such as MOSFETs andIGBTs achieve very high power densities sothat conventional thick aluminum wirebonding technology (Figure 1) represents abottleneck for load current capability andreliability. The elimination of bond wires inpower modules has been discussed forseveral years in industry and academia. Mostof the new packaging approaches have beenbased on soldered or welded bumps.Recently, attempts have been made to

improve this technology [1]. However,these attempts lead to comparably largeefforts in the power device metallizationand processing. Environmental conditionssuch as humidity, temperature, industrial

gas atmosphere, and mechanical shockand vibration play an increasinglyimportant role for electric vehicles andother applications. However, not only thesetechnical and environmental challenges

have to be faced, but also power electronicmodules need to be reduced in size andweight for use in vehicles and have to beinterfaced in a simple and reliable manner,even at high load currents.

Chip-to-ceramic substrate sinteringSilver sintering is an established technologywhich has started to replace the solderingof chips to DBC substrates in massproduction. Thanks to its unprecedentedreliability and thermal behavior, this joiningtechnology makes power modules bettersuited for higher temperatures anddemanding applications, such as electricvehicles. However, two issues remainunaddressed: How to replace wire bondingon the chip front side, and how to interfacethe power module to the heat sink.As examples, Figure 2 shows a 5”x7”

DBC card which contains four substrates

Figure 1:Conventional thickaluminium wirebonding technology

Figure 2: 5”x7” DBCcard with foursubstrates (beforelaser cutting) withsintered chips

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(before separation by laser cutting) withsintered chips, as used today in seriesproduction for power modules inautomotive applications. Figure 3 showsone substrate with sintered chips after anextreme mechanical stress test - the sinterjoint are unbreakable. Up to this point, the new technology

follows this well established process.However, rather than continuing in theprocess sequence, which consists of Al-wire wedge bonding, electrical testing, lasercutting, and final automated opticalinspection (AOI) as in conventionaldevices, sintering [3] proceeds differently.

New design possibilitiesFigure 4 shows a schematic drawing of thispackaging technology. The special flex foilwith a metal layer on the bottom with athickness comparable to bond wirediameters serves to connect the chip topside. A thin metal layer on the toprepresents the gate and sensor trackswhich are connected to the power layer byvias. The two metal layers are insulatedfrom each other by polyamide. Also, thetop layer can be used for SMDcomponents, such as temperature sensorsand gate resistors. The second sinter jointconnects the chip back side to a standard

DBC substrate. Figure 5 shows the flex board (top side),

as it is prepared to be positioned andsintered within the next process step. Inareas which are connected to the chipswith the flex board, the power side of theboard is screen or stencil printed with silverpaste. In this particular design, the auxiliarycontacts, such as gate and emitter, arecarried out as tracks on the flex board,reaching out to the left and right to befolded-in later.

Interconnect and coolingThis new sinter technology proceeds in

providing thermal and electrical interfacesby sintering the power terminals and theheat sink to the power device. Figure 6shows the AC and DC power terminals,made from Ag-coated Al or Cu, as well asa pure Al pin-fin heat sink. On top of theheat sink, a defined area is stencil printedwith silver paste where the power part willbe attached.Now the heat sink, the power terminals,

and the power part are assembled into asintering jig and put again into aconventional sinter press, this time formingthe sinter connections from heat sink tosubstrate and substrate to power terminals.Afterwards a simple plastic frame is addedto provide guidance for the auxiliaryterminals and to provide alignment for theuse of this device in the power electronicinverter system. Figure 7 shows how allparts are assembled together to form a400 A, 600 V dual SKiN device. The devicecontains two 200 A IGBTs and one 400 Afreewheeling (FWD) diode for the upperand the lower switch, as well as atemperature sensor chip.Figure 8 shows the finished device as it

is in production. In this specific layout, theauxiliary contacts are formed as flex layertracks, folded in such way that they can beconnected to a printed circuit board (notshown). The auxiliary contacts of the twoswitches come out to the left and the rightof the long side of the heat sink, whereasthe plus, minus and phase terminals comeout on the short sides of the heat sink,opposing each other.

Test resultsThe electrical properties of the devicedemonstrate the superiority of acontinuous metal layer on top of the chips,compared to discrete bond wires. Adetailed comparison of electrical testresults of the device with flex board, asshown here, versus a device with the verysame design, but with bond wires insteadof flex board has been published before[4]. The flex board leads to a significantlyhigher surge current capability of the free-

Figure 4: Cross section of a newly designed power module integrated with a pin fin heat sinkexhibiting no solder layers and wire bond interconnects

Figure 5: Flex board(top side) prior to

sintering to thesubstrate

Figure 3: DBC substrate aftermechanical stress test

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Issue 2 2012 Power Electronics Europe www.power-mag.com

wheeling diodes than for wire bondeddiodes.The device has a thermal resistance of

junction to ambient of just 0.44 Kcm2/Wwhich is 35 % lower than that forconventional power modules. Therefore,the current rating can be increasedaccordingly. Figure 9 shows infraredimages of the device under load current,for the IGBT and the FWD, respectively. Asshown, the chip temperature is very

uniform across the chip and from chip tochip. The spacing between the chips isdesigned such that there is almost nothermal overlap between the chips.In power cycling, the device is heated by

the power dissipation during theconduction period and then cooled downwhen the maximum test temperature isreached. The maximum number of cyclescan vary orders of magnitude, dependingon the temperature difference between

the highest and lowest temperature andthe medium temperature. The raise andfall times of the temperature, as well asthe test being carried out under constantmaximum power, constant maximumtemperature, or constant maximum current[5]. Figure 10 shows power cycling results.Passive temperature cycling is

challenging for such an integrated device,as the thermal coefficients of heat sink,substrate, chips, and flex board are alldifferent. Therefore, extensive temperaturecycling tests were carried out, typicallycycling the devices between -50°C and+150°C, with a temperature rise and falltime of 3 K/min. Several hundred cyclesup to a thousand cycles were achieved,

Figure 6: DC and AC power terminals as well as an Al pin-fin cooler with a silver paste area on the top side

Figure 7: Exploded view of a 400 A, 600 V dual IGBT device comprising heat sink, DBC substrate withIGBT and FWD chips, flex board and power terminals - all being connected by silver sinteringtechnology

Figure 8: Seriesmanufactured 400A/600 V dual IGBTdevice in 74 mm x 56mm x 11 mmdimension and 0,095kg weight

before the DBC substrate started todelaminate from the heat sink. In variousexperiments it could be shown thatthickness and morphology of the sinterlayer between substrate and heat sink arekey parameters to achieve a good passivethermal cycling performance.

ConclusionsSKiN, a new packaging technology withoutbond wires and solder layers has beenintroduced. All interconnections to chip topand bottom surface, DBC to heat sink andpower terminals are made by an Ag sinterjoint. The bond wires are replaced by aspecial flex foil which increases the chiptop side attached contact area by a factorof 4. In order to demonstrate the exceptional

Figure 9: Infrared image of the device with DCIGBT load current of 400 A (upper image) andfree-wheeling diodes heated by the same loadcurrent (lower image). The spacing between thechips is optimized to provide no thermal crosstalk

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performance improvements of the overallconstruction and in particular the flex foil acomparison to a benchmark module withtraditional Al bond wires has been made.Due to the elimination of thermalinterface materials and the integration of ahigh performance pin fin heat sink it ispossible to double the power dissipationin comparison to traditional designs. Justthe elimination of the thermal grease layerexhibits an improvement of 25 % of thetotal thermal resistance junction to water.Due to the modified geometry and theincreased chip contact area a 27 %

increase of the diode surge forwardcurrent capability and a 2 nH reduction ofthe total commutation stray inductancehas been achieved. The power cyclingperformance demonstrates animprovement of factor 70 over theindustry standard and an improvement of10 over the single sided sinteredbenchmark module.Further activities are ongoing to exploit

the new possibilities of the two layer flexfoil. These are in particular a furtherimprovement of the thermal resistancedue to dual side cooling and the

integration of passive and activecomponents for gate drive, current andtemperature sensing [6].

Literature[1] F. Hille, F. Umbach, T. Raker:

“Failure mechanism and improvementpotential of IGBT�s short circuitoperation”, Proc. ISPSD 2010, pp. 33,Hiroshima, 2010[2] C. Göbl: “Low Temperature Sinter

Technology Die Attachment for PowerElectronic Applications”, Proc. CIPS2010, pp. 327, Nuremberg, 2010[3] German Patent DE 10 2007 006

706[4] P. Beckedahl, M. Hermann, M.

Kind, M. Knebel, J. Nascimento, A.Wintrich: “Performance comparison oftraditional packaging technologies to anovel bond wire less all sintered powermodule”, Proc. PCIM 2011, Nuremberg,2011[5] U. Scheuermann, S. Schuler:

“Power cycling results for differentcontrol strategies”, MicroelectronicsReliability 50 (2010), 1203-1209[6] T. Stockmeier, P. Beckedahl, C.

Göbl, T. Malzer: “SKiN: Double sidesindering technology for newpackages”, Proc. ISPSD 2011

Figure 10: Powercycling capability ofSKiN as a function of�Tj, compared tostandard andimprovedconventional devices.500,000 cycles can bereached at �Tj =110K

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