We have recently demonstrated a significantly longer life in accelerated thermal cycling for Land Grid Arrays (LGAs) assembled only with SAC305 solder paste than for the corresponding SAC305 based BGA assemblies. This superior performance was shown to be a direct effect of the solder microstructure. The final Sn solidification temperature strongly affects the initial microstructure of a SnAgCu solder joint, including the Sn grain morphology, and thus the thermomechanical behavior of the joint. Right after reflow, larger BGA joints of SnAgCu alloys, which solidify at higher temperature, reveal either a single β-Sn grain or three large grains with clearly defined boundaries formed by cyclic twinning. The orientations of the highly anisotropic Sn grains are not yet controllable in manufacturing, leading to substantial statistical scatter in the performance of the solder joints. Typical LGA solder joint dimensions, however, tend to facilitate greater undercooling and the formation of an alternative interlaced twinning microstructure.
A systematic study was undertaken to identify the parameters that control the interlaced twinning microstructure. Sn grain structures were characterized by crossed polarizer microscopy and electron backscatter diffraction (EBSD). Precipitate sizes and distributions were measured using backscattered scanning electron microscopy and quantified using image analysis software. Systematic effects of solder alloy, dimensions and pad finishes were identified. Recommendations are made as to design and materials selection. The practicality of controlling the desired microstructure, as well as potential disadvantages for certain applications is discussed.
