Thermal Cycle Reliability of a Low Silver Ball Grid Array Assembled with Tin Bismuth Solder Paste
Authors: Richard Coyle, Raiyo Aspandiar, Michael Osterman, Charmaine Johnson, Richard Popowich, Richard Parker, Dave Hillman Company: Nokia Bell-Labs, Intel, CALCE, iNEMI, Rockwell Collins Date Published: 9/17/2017
Abstract: Pb-free solders with lower Ag content and lower processing temperatures are being considered as alternatives to neareutectic SAC305 (Sn3Ag0.5Cu), the most common alloy used for manufacturing electronic assemblies. Lowering the Ag content can improve mechanical shock performance, and lowering reflow temperatures can mitigate or eliminate certain process-induced defects. Assembling with low melting Sn42Bi58 eutectic solder paste (m.p. 138 ºC) allows for a significant reduction in reflow temperature compared to SAC305. However, high reliability end users are concerned about the thermal fatigue performance of the resultant mixed metallurgy joints created when SAC ball grid arrays are soldered with the high-Bi content alloy. This study evaluates the mixed metallurgy thermal fatigue performance of two low Ag SAC alloy ball grid arrays (BGA). In one case, the SAC components are assembled with matching SAC alloy paste and in the other case the SAC alloy components are assembled with a SnBi solder paste. Thermal fatigue is assessed using 0/100 ºC and - 40/125 ºC accelerated temperature cycling profiles. The microstructures and failure analysis are characterized using optical metallography and scanning electron microscopy.