Plastic Deformation and Life Prediction of Solder Joints for Mechanical Shock and Drop/Impact Loading Conditions

Recent data [Syed, A., et al., "Alloying Effect of Ni, Co, and Sb in SAC solder for Improved Drop Performance of Chip Scale Packages with Cu OSP Pad Finish," Proceedings of the 8^th EPTC conference, 2006, pp. 404-411.] shows that significant improvement in drop performance can be achieved by utilizing solder balls of Sn1.2Ag0.5Cu0.05Ni composition on Cu OSP finish packages. Along with higher performance, the data also shows that the failure mode changes from brittle (IMC) to ductile (bulk solder) when this alloy is used as opposed to SAC305/SAC405 solder alloy. This implies that quantification of plastic deformation of solder joint due to drop/impact loading can be used in predicting the life of solder joints when this alloy is used for solder balls. This paper reports the correlation of accumulated plastic strain and plastic work (energy density) to the number of drops to failure. Board level drop tests were performed on two component types with varying component sizes, ball sizes, and ball pitch using Cu OSP finish and solder balls Sn1.2Ag0.5Cu0.05Ni composition. The drop to failure data was collected on these components at various locations on the board to generate enough spread in the data. Each of this data set was then simulated using the approach published previously [Syed, A., et al., "A Methodology for Drop Performance Prediction and Application for Design Optimization of Chip Scale Packages," Proceedings of 55^th ECTC conference, 2005, pp. 472-479.] employing high strain rate stress-strain behavior of this solder alloy. The accumulated plastic strain and plastic work in the critical solder joint was computed for each component and at various locations on the board to determine the correlation with number of drops to failure. The simulation approach and the correlations achieved can be used not only to predict solder joint response in a board level test but also for a product level drop test. Also, the same methodology can be used for li- fe prediction of solder joints using other solder alloys, as long as failure is in the bulk solder.