Thermo-mechanical Durability Assessment and Microstructural Characterization of 95.5Pb2Sn2.5Ag High Temperature Solder

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There is an increasing need in the avionics, military, oil exploration and automotive industries for high temperature solders that perform reliably in ever-higher temperature applications. In these applications, solders are often used as large area die attaches and due to the high power involved, they need to dissipate large amounts of heat that can further increase the thermal load on the devices. The mechanical, electrical and thermal behavior of the solder must be understood to ensure devices and package reliability. There is an especially urgent need for characterizing constitutive properties and thermo-mechanical durability of high temperature solders.

A partitioned constitutive model consisting of elastic, plastic and creep models was obtained for the 95.5Pb2Sn2.5Ag solder by implementing the direct local measurement technique. The validity of the assumptions used to generate these models have been demonstrated using microstructural characterization.

The thermo-mechanical durability of the 95.5Pb2Sn2.5Ag solder is investigated using thermal cycling tests and finite element modeling. A high reliability package manufacturing technique has been followed. The extensive detailed two-dimensional viscoplastic FE stress and damage analysis is conducted for five different thermal cycling tests of 95.5Pb2Sn2.5Ag solders. The energy-partitioning durability model of the solder is obtained. It is found that 95.5Pb2Sn2.5Ag solder is creep dominant at high temperatures.

The microstructure characterization study on 95.5Pb2Sn2.5Ag solder reveals that it remains primarily a single phase in the range of temperature under study with very few Ag3Sn intermetallics. Fatigue cracks due to thermal cycling have been observed.