Low-Silver BGA Assembly Phase II - Reliability Assessment Seventh Report: Mixed Metallurgy Solder Joint Thermal Cycling Results

摘要

Some ball grid array suppliers are migrating their sphere alloys from SAC305 (3% Ag) or SAC405 (4% Ag) to alloys with lower silver contents and often with "micro alloying" additions. There are numerous perceived reliability benefits to this change, but process compatibility and thermal fatigue reliability have yet to be fully demonstrated. The current study has been undertaken to characterize the influence of alloy type and reflow parameters on low-silver SAC spheres assembled with backward and forward compatible pastes and reflow profiles. This study combines low-silver sphere materials with eutectic tin-lead and lead-free SAC305 solder pastes. This is the seventh report in a series being published as data become available, and presents the results of the thermal cycling of mixed metallurgy solder joints. Thermal cycling conditions include both 0 to 100"C and -40 to 125°C, with 10 minute dwell times. The accelerated thermal fatigue reliability of mixed Sn-Pb/Pb-free solder joints with varying Ag and "micro alloying" element concentrations are compared to those of 100% Sn-Pb and 100% Pb-free joints for four different package types. Further, the impact of thermal cycle conditions on the rank order of the reliability for the different solder joint compositions is presented. The implications of the data regarding the efficacy of using BGAs balled with low Ag alloys and soldered with Sn-Pb paste, and areas for future work are discussed.