Authored By:
Tyler Richmond, Hongwen Zhang, Francis Mutuku,
Jie Geng, Huaguang Wang, Kyle Aserian
Indium Corporation
NY, USA
Summary
The usage of high power devices in electrical vehicles or high temperature electronics requires the interconnection of components to maintain long-term joint integrity under junction temperatures up to 175 °C. Ag- or Cu-sintering materials exhibit excellent thermal and electrical performance, and improved efficiency compared to solder materials. Due to their favorable properties and high melting temperatures, they have been used for die-attach in power inverters/converters. However, the high material and processing costs limit their general usage in the power industry. Alternatively, traditional lead-free solder materials may present challenges for these high power or high temperature devices although they are widely used in electronics interconnections.
The high junction temperature and/or service temperature (175 °C or above) in the device leads to homologous temperatures of as high as 0.91. This close to the critical melting point, atomic diffusion is exacerbated and creep resistance is significantly weakened. This has adverse effects on joint reliability as the thermal migration under elevated temperatures promotes grain coarsening, excessive intermetallic compound (IMC) growth, and phase segregation. Electro-migration can also occur if an electrical current is involved. This current study focused on the thermal fatigue performance and the microstructural evolution of five lead-free solders: Sn3.5Ag, Sn5Sb, SnAgCu5.5Sb, SnSbCuAg, and BiAgX® under harsh service conditions.
The solder joints were subjected to Temperature Cycling Tests (TCT) of -40 to 175 °C for up to 2000 cycles and 210 °C aging up to 240 hours. 5x5mm2 Si die (Ti/Ni/Ag metallization) on direct-bonded-Cu (DBC) packages were used for TCT. Aging tests were performed with a Cu die on a ceramic substrate. The ceramic substrate had two different metallization types: (1) electrolytic Ni-plated Cu and (2) electroless nickel immersion gold (ENIG) surface metallization. The joint morphology and the evolution of the joint microstructure were studied with X-ray inspection and scanning electron microscopy (SEM). We find heterogeneous responses were found across solders to TCT and aging tests.
Partial cracking at the joint corner and inside IMC layers were observed in Sn5Sb after TCT, while SnAgCu5.5Sb, SnSbCuAg, and BiAgX® sustained less damage. However, the excessive IMC growth in the Sn-rich solder under 210 °C aging was the primary cause of joint degradation and cracking. During the aging test, electrolytic Ni metallization was mostly consumed and the material beneath the Cu layer was partially consumed. ENIG metallization samples resisted degradation for the same aging test. These results may indicate that lead-free solders are only suitable for power application under mild conditions.
Conclusions
Traditional lead-free solders are not suitable for applications involving operating temperatures of 175 °C or higher. Due to excessive IMC growth, grain coarsening, and phase segregation, the tested lead-free solders are not suitable for these high temperature or power applications. After 240 hours at 210 °C, all the solders experienced joint-encompassing IMC growth, grain coarsening, phase separation, and two of the solders experienced continued crack propagation.
TCT after 2000 cycles at -40 to 175 °C for Sn5Sb, SnAgCu5.5Sb, BiAgX®, and SnSbCuAg resulted in notable IMC growth for all samples and corner cracking within the bulk solder for all except SnSbCuAg. SnSbCuAg did, however, experience vertical cracking toward the center of the joint at 2000 cycles. Additionally, the TCT resulted in micro-crack propagation within the IMC attaching to the DBC for Sn5Sb, SnAgCu5.5Sb, and BiAgX®. These results indicate that current lead-free solders are only suitable for power applications under mild testing and operating conditions.
Initially Published in the SMTA Proceedings
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