Influence of Microstructure and Cryogenic Temperature on Fatigue Failure of Indium Solder Joint

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This thesis aims to develop a fundamental understanding of the underlying mechanisms that govern indium attach degradation in applications requiring repeated excursions and extended long time dwells at temperatures below -55oC. This work was prompted by original effort of developing low temperature SiGe BiCMOS modules for Martine and Lunar exploration.

Current exploration vehicles use a "warm electronic box (WEB)" to maintain the electronics in an earth-like temperature environment. This results in increasing system complexity and weight. Warm boxes also consume power and are not practical for the ~330 hour lunar night. Furthermore, intelligent nodes in a distributed system must operate in an ambient environment to monitor the health and performance of a space craft or rover, to sense the environment for scientific exploration and to act on the environment, such as to use a drill to obtain a soil sample for analysis. Nevertheless, the reliability and life span of electronic devices systems without WEB can be significantly degraded by thermal fatigue damage as a result of wide daily temperature swings during their space exploration, when cryogenic temperatures (below -55oC) can be encountered. Attachment layer, such as die attach, solder joint and substrate attach are most inclined to fatigue damage due to the global CTE mismatch between packaging materials and their material properties at extreme cold temperatures.

With the aim of enhancing the reliability of cryogenic electronic package, indium was selected as the attachment material due to its excellent wetting capability, greater ductility and high electrical conductivity, with respect to standard PbSn solders at cryogenic temperatures. However, information on the reliability of indium attach is sparse and only concerns isothermal fatigue conditions at room temperature. No investigation has been reported on its thermal fatigue ranging from cryogenic temperature to high homologous temperatures (above room temperature), or on its isothermal fatigue behavior at cryogenic temperatures, or of the effect of microstructure evolution, in terms of intermetallics, under isothermal fatigue conditions on joint lifetime. Current lack of these fundamental understanding inhibits the assessment of the reliability of indium attach.

In this thesis, an efficient and systematic assessment was conducted to evaluate the reliability of indium attach. Constitutive properties of indium solder joint at extended low temperature were measured and the Anand constitutive model was validated for an extended temperature range, -150oC to 140oC, including extreme cold temperature. This was used to assess thermal fatigue life of indium attach. The effect of intermetallics and surface finishes on the reliability of indium attach subjected to mechanical fatigue has also been investigated. In addition, fatigue failure site, modes and mechanisms in indium attach at low temperature were identified and correlated with microstructure evolution. A fatigue model was also calibrated for indium attach at cryogenic temperatures.