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Evaluation of Reliability of Lead-free Solders in Silver-free Hybrids


 Silver and silver-filled materials are widely used for hybrid substrate conductors, component terminals and conductive attachment materials. However, some concerns exist about using silver, especially for high-voltage hybrid assemblies that must operate in harsh non-hermetic or semi-hermetic applications.
 This paper discusses the reliability testing of silver-free components attachment materials. The target application temperature for this research was 185°C. Target duration at temperature was 500 hours. The evaluation of Sn100C solder was performed in comparison to solders of Sn96 and Au80Sn20. Two types of Ag-free thick-film substrates were tested with different type of solders to mount surface-mounted technology (SMT) components. The assembled test samples were subjected to thermal aging and thermal cycle tests. At each test interval, shear tests were performed to evaluate the mechanical performance. Cross sections of the solder joint and thick-film interface were used to understand the intermetallic compounds (IMC) formation and failure mechanisms under thermal stress. Tests for conductive filament growth was performed to evaluate the behavior of Sn-based solders in powered assemblies under target environmental conditions. Test results were discussed and suggestions for achieving a reliable, silver-free thick-film hybrid were made.