Solder joint fatigue study under low temperature Martian conditions

Electronics, without requiring heater power or enclosure in a centralized "warm electronics box," will need to survive mean surface temperatures of -120degC to +20degC for an extended (>2 years) Martian mission and an operational temperature up to 85degC. These surface temperatures range over one Martian day or typical cycle. Since the electronics will need to survive extended thermal cycles (3times life of 2010 cycles) under these conditions, thermally induced fatigue is a significant concern. The solder joint reliability of connectors with Sn60Pb40 finish and In80Pbl5Ag5 solder on a printed wiring board was investigated. Between 638 to 1431 cycles from -120degC to +85degC, a small number of leads failed on a printed wiring board with a silicone conformal coating. Scanning electronic microscopy analysis revealed crack initiation at the lead finish and In80Pbl5Ag5 solder interface in at least one case with martensitic phase transformation of tin/lead solder and the loss of ductility in the tin phase at low temperatures as the failure modes. Another failure region was composed of several intermetallic compounds with intermetallic embrittlement as a failure mode. This fatigue study determines the corresponding failure modes of the thermally stressed components at each failed interface