Post Mechanical Shock Test Failure Analysis on Mixed SnAgCu-BiSn BGA Solder Joints

Drop Shock Reliability of mixed alloy SnAgCu-BiSn Flip Chip BGA solder joints assembled on a Shock Test Board with two different surface finishes (Cu Organic Solderability Protectant and Electroless Nickel Immersion Gold) using three different categories of BiSn solder pastes was investigated. A metallographic analysis of the failed solder joints was undertaken to ascertain location of the cracks, their propagation paths, and to characterize microstructural features close to the cracks. Results indicated all failures occurred close to the solder-to-board land interface. The diminished reliability of solder joints on the nickel-gold based surface finish was determined to be due to the crack propagation pathway being between the intermetallic compounds and the Ni(P) layer. For the copper surface finish, the crack pathway was within the solder just above the intermetallic compound layer, and this resulted in a higher shock resistant solder joint. Bi wt% analysis in the crack region revealed that the enhancement of the solder ductility by minor quantities of elemental additions more than compensated for the higher localized Bi wt% concentration near the board lands, improving the solder joint shock reliability.