Process dependence and nucleus models of β-Sn grains in SAC305 freestanding solder balls and BGA solder joints

Due to the anisotropy characteristic of beta-Sn grain, the microstructure and orientation of Sn-based solder joints are closely related to the reliability of the solder joints. In this paper, the effects of cooling rate and solder ball size on beta-Sn morphology and orientation in Sn-3.0Ag-0.5Cu (SAC305) freestanding solder balls and SAC305/Cu ball grid array (BGA) solder joints were firstly investigated to reveal the process dependence of beta-Sn grain features. Three dominant solidified morphologies, i.e., interlaced & beach ball-like grains with three orientations, multiple twin grains with three orientations and single grain with one orientation, were found in the solder balls/joints. Solder ball size showed more significant impact than cooling rate on the morphologies that interlaced & beach ball-like grains and multiple grains were mainly presented in small sized solder balls/joints (<= 400 mu m) and single grain was mainly presented in large sized solder balls/joints (>= 700 mu m). These three solidified morphologies were nucleated and developed from two nucleus model. That is, the interlaced & beach ball-like grains and multiple twin grains morphologies were attributed to {101} nucleus model, while the single grain morphology was attributed to single grain nucleus model. Regardless of whether the solder balls/joints nucleated and solidified based on the {101} or single grain nucleus model, large theta angle beta-Sn grains were proved to have a formation probability higher than 77 % both theoretically and experimentally, and this formation probability had little relationship with interfacial Cu6Sn5 grains, cooling rate and solder ball size.

Automated summary

This paper investigates the effects of cooling rate and solder ball size on the morphology and orientation of beta-Sn in Sn-based solder joints. Three different solidified morphologies were found in the solder balls/joints, and their formation was dependent on the solder ball size. Regardless of the nucleus model, the formation probability of large theta angle beta-Sn grains was proved to be higher than 77% and had little relationship with other factors.