Accelerated Aging of Sn-Pb and Pb-Free Solder Joints on Hybrid Microcircuit Assemblies

The implementation of alternative Pb-free solders has required careful consideration of their impact on the long-term reliability of electronic systems used in military, space, and satellite applications.  One particularly unique case in the high-reliability electronics industry is the use of ceramic substrates – these are the so-called hybrid microcircuit (HMC) assemblies.  In these products, ceramic substrates provide better performance under extreme temperature conditions or they are preferred because of the use of high frequency signals within the system. Ceramic substrates include traditional oxide ceramics (e.g., alumina) or one of the newer glass-ceramic substrates (e.g., low-temperature co-fired ceramic, LTCC).  Regardless of the particular product, the conductor to which are made the solder joints is often a fired-on thick film layer (10 – 20 µm).  Because thick film layers are often comprised of precious metals (Au, Pt, Pd, Ag, either as single elements or as two- and three-component alloys) to be compatible with the firing processes, they are susceptible to leaching and solid-state reactions with Sn-based solders.  An extensive study was undertaken to investigate the long-term reliability of Sn-Pb and Sn-Ag-Cu solder joints made to Au-Pt-Pd and Pd-Ag thick film layers on an alumina HMC test vehicle. The test vehicles contained two area-array packages as well as several peripherally leaded devices.  The accelerated aging conditions included isothermal aging (85C and 205C forup to 5000 hours), thermal cycling (-55C/125C for up to 5000 cycles), as well as thermal shock exposures (-55/125C, liquid-to-liquid for up to 500 cycles).  The survivability of the soldered interconnections was assessed using the metrics of shear strength and microstructural analyses.  It was observed that all four variants of solder alloy and thick film system combinations exhibited solder joints having satisfactory strength even after the most severe exposures.  The metallographic cross sections determined that the interface bond between the thick film and alumina substrate had retained an adequate robustness after the aforementioned exposures.