Packaging Techniques for Compact SiC Power Modules Operable in an Extended Tj Range

Abstract

Packaging technology applicable to SiC power devices operated in an extended junction temperature range (Tjmax > 200°C) must be developed in order to create much more compact and cost-effective SiC power modules. This paper describes some of the technical challenges involved in improving the reliability of the critical package components—die attachment system, Al wire bonds and encapsulation—in direct contact with SiC devices inside the power module. Two numerical targets, (I) 3000 hours for a storage test at 250°C and (II) 3000 cycles for thermal cycling between -40°C and 250°C, were achieved through optimization and various improvements.

High Junction Temperature and Low Parasitic Inductance Power Module Technology for Compact Power Conversion Systems

“…The wire bond life determined the power cycle lifetime of the test samples (approximately 35 000 cycles). The PCT results revealed that, within acceptable experimental error, there was little decline in die shear strength until the test ended at 37 421 cycles [38]. A X-SEM of a Au-Ge die attachment after 37 421 PCT cycles showed that there were no signs of thermomechanical fatigue, such as microcracks and folding, in the solder layers.…”

“…5, bond pull strength declined with increasing thermal cycles, albeit the rate of decrease lessened. However, after 3000 cycles, it was at a level of 127 gf, still satisfying the IEC criterion [37] by a sufficient margin [38]. The breakage mode of the wire bond samples subjected to the bond pull test was either beam breakage or heel breakage.…”

“…Fig. 4 shows the relationship between the average bond pull strength (gf: gram-force) and T j (Al wire temperature) [38]. Fracture (breaking) occurred either in the beam (bulk) or at the heel, not at the interface between the chip and the wire.…”

“…A 250°C storage test for 3000 h was conducted on the Al wire bonds [38]. The test results showed that the bond pull strength dropped quickly within 50 h but afterward stayed at a level approximately 2.5 times higher than the IEC criterion [37].…”

“…The PCT tests of Al wire bonds were also implemented under the conditions of T j max = 200°C and T j = 165°C [36], [38]. Test samples were picked out one by one according to a planned schedule and their wire bond pull strength was measured.…”