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Evaluation of Low Cost, High Temperature Die and Substrate Attach Materials for Silicon Carbide (SiC) Power Modules
Keywords: Die Attach Materials, Substract Attach Materials, SiC Power Modules
The performance of SiC power modules have been demonstrated to exceed that of conventional Silicon (Si) modules. One of the major advantages of SiC is the ability to perform at higher device junction temperatures. Another benefit of using SiC technology is to enable a higher power density. SiC modules are capable of processing significant levels of power within much smaller volumes compared with its Si counterparts. These high thermal loads present a formidable challenge in integrating SiC devices in power modules. Known-good materials and processes for silicon power modules are not rated at the aggressive operating conditions associated with SiC devices. Two of the most critical interfaces in a power electronics module are the die-attach and substrate-attach. A degradation in these interfaces often results in potentially catastrophic electrical and thermal failure. Therefore, it is very important to thoroughly evaluate die-attach materials before implementing them in SiC power modules. There is a significant amount of recent research activity targeted toward the development of novel die attach materials for harsh environments. These materials have promising properties, but their behavior under specific application conditions need to be understood. This paper presents the methodology for the evaluation of die attach materials for SiC power modules. Preforms of a solder material were used to perform a die and substrate attach process on a conventional power module platform. The initial soldering quality was inspected using non-destructive methods like acoustic microscopy and X-ray scanning. Die attach and substrate attach voiding of < 5% was obtained indicating a very good soldering quality. Cross-sectioning techniques were used to validate the inspection methods. The initial soldering strength was measured using pull tests and shear tests. The measurements were repeated at the rated temperature of the module to ensure that the properties did not degrade excessively at the service temperature. At the rated module temperature of 175 °C, the die bonding strength was found to be ~ 75 kg. This was only 25% lower than the strength at room temperature. The contact pull strength was measured to be > 90 kg at 175 °C, which was 25% lower than the value measured at room temperature. The effect of power cycling and thermal cycling on the quality and strength of the solder joints was also investigated. A minimal change in the die and substrate attach quality was observed after rigorous reliability testing. The results of these analyses have been detailed in this paper.
Sayan Seal, Electronics Design Engineer
WolfSpeed, a CREE Company
Fayetteville, AR

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