Mitigation of Corrosion and Inter-Metallic Diffusion for SAC 305 Solder Paste on Silver/Palladium Ceramic Traces by Controlling Moisture Content

Silver/Palladium metal traces are commonly deposited as pastes on ceramic substrates to form solderable connections and routing layers. However, in the presence of moisture at levels as low as 10,000 ppm and temperatures greater than 125°C, SAC 305 solder paste (96.5% Tin, 3% Silver, 0.5% Copper) can produce a corrosion assisted non-conductive and brittle inter-metallic alloy within as little as 15 days when soldered to Silver/Palladium metal traces. Studies have shown that 20 - 30% moisture content within a hermetically sealed package will produce electrical ‘opens’ within 20 - 24 hours at 150°C in those regions where the SAC 305 solder comes into contact with the Silver/Palladium traces. In some cases, internal cavity epoxies can sufficiently outgas to create 20 - 30% (200,000 - 300,000 ppm) moisture levels following hermetic sealing. However, if exposed to a proper pre-bake prior to hermetic lid sealing (typically 200°C for 1 hour) this corrosion assisted inter-metallic diffusion can be substantially mitigated by controlling the moisture to levels near 500 - 1000 ppm. This mitigation becomes a necessary step for hermetic systems with Sn/Ag/Cu solder on Ag/Pd traces, and which require higher temperature stabilization bakes (150°C) as part of a reliability screening flow.