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

Abstract

• Relationship between β-Sn grain morphology and process parameters was studied systematically. • Two nucleus models were proposed to explain the formation of β-Sn morphologies. • Relationship between spatial position of nucleus models and β-Sn orientation was established. • Large θ angle β-Sn grains were theoretically and experimentally proved to have a formation probability higher than 77 %. Due to the anisotropy characteristic of β-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 β-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 β-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 μm) and single grain was mainly presented in large sized solder balls/joints (≥ 700 μm). 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 θ angle β-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 Cu 6 Sn 5 grains, cooling rate and solder ball size.