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 Cu₆Sn₅ grains, cooling rate and solder ball size.

由于β-Sn晶粒的各向异性特性，Sn基焊点的微观结构和取向与焊点的可靠性密切相关。本文首先研究了冷却速率和焊球尺寸对 Sn-3.0Ag-0.5Cu (SAC305) 独立焊球和 SAC305/Cu 球栅阵列 (BGA) 焊点中 β-Sn 形态和取向的影响揭示了 β-Sn 晶粒特征的过程依赖性。在焊球/接头中发现了三种主要的凝固形态，即具有三个取向的交错和沙滩球状晶粒、具有三个取向的多个孪晶晶粒和具有一个取向的单个晶粒。焊球尺寸对交错和 沙滩球状颗粒和多颗粒主要出现在小尺寸焊球/焊点（≤ 400 μm）中，单颗粒主要出现在大尺寸焊球/焊点（≥ 700 μm）中。这三种凝固形态是由两个核模型成核和发展而来的。即交错和沙滩球状晶粒和多个孪晶形貌归因于{101}核模型，而单晶粒形态归因于单晶核模型。无论焊球/焊点是否基于{101}或单晶核模型成核和凝固，大 交错和沙滩球状晶粒和多个孪晶形态归因于{101}核模型，而单晶粒形态归因于单晶核模型。无论焊球/焊点是否基于{101}或单晶核模型成核和凝固，大 交错和沙滩球状晶粒和多个孪晶形态归因于{101}核模型，而单晶粒形态归因于单晶核模型。无论焊球/焊点是否基于{101}或单晶核模型成核和凝固，大θ角β-Sn晶粒的形成概率在理论上和实验上均高于77%，且该形成概率与界面Cu ₆ Sn ₅晶粒、冷却速度和焊球尺寸关系不大。