材料期刊网

对球栅阵列封装(ball grid array, BGA)组件进行热循环实验, 通过EBSD对焊点取向进行表征, 观察不同取向焊点内晶粒取向演化情况. 同时采用Surface Evolver软件对 BGA焊点进行三维形态模拟, 并基于实际焊点内的晶粒构成, 对热加载条件下BGA组件热应力应变分布进行计算. 实验和模拟结果表明, 晶粒取向对焊点可靠性和失效模式产生非常显著的影响. 对于单个晶粒构成的焊点, 应力应变主要集中在靠近界面的钎料内部, 该处发生明显的再结晶并伴随着裂纹的萌生和扩展; 而对于多个晶粒构成的焊点, 应力应变分布则依赖于晶粒取向, 再结晶和裂纹倾向于偏离界面沿原有晶界向钎料内部扩展. 部分特殊取向焊点, 当原有晶界与焊盘界面垂直时, 不利于形变, 表现出较高的可靠性. 当晶界与焊盘夹角为45^o时, 在剪切应力和焊点各向异性的双重作用下, 原有晶界处产生较大的应力应变集中, 加速了裂纹的萌生和扩展, 导致焊点最终沿着原有晶界发生再结晶和断裂, 焊点发生早期失效的可能性增加.

During service, coefficient of thermal expansion (CTE) mismatch between different materials in electronic devices can lead to stress and strain concentration, and the creep and fatigue damage will accumulate, leading to final failure of solder joints. The main constituent of Pb-free solder joint is β-Sn, which is body-centered tetragonal metal. There is big difference in CTE and elastic modulus along different directions of β-Sn, showing strong anisotropy. Therefore, solder joints with different orientations show quite different thermo-mechanical responses. In this study, ball grid array (BGA) assemblies were subjected to thermal cycling, and the orientation of the solder joints was characterized by EBSD to track the orientation evolution in different solder joints. Surface Evolver was adopted to simulate the three-dimensional shape of the solder joint. Based on the shape and grain structure of real lead-free solder joints, the thermal stress and strain distribution in BGA assemblies under thermal loading were computed. Sub-model based on grain numbers and orientation distribution is solved to get the strain distribution of the three typical solder joints. The experimental and simulated results show that grain orientation significantly influences the solder joint reliability and failure mode. For single-grained solder joints, stress and strain concentration is located in the solder bulk near the interface, where recrystallization accompanied with initiation and propagation of cracks.  However, for multi-grained solder joints, the distribution of stress and strain depends on grain orientation. Recrystallization and cracking tend to divert from the interfacial region into the solder bulk along the pre-existing grain boundary. Some special solder joints with grain boundary perpendicular to the interface are not favorable for deformation, exhibiting higher reliability. When the grain boundary inclined at 45^o to the pad, the original grain boundaries produce large stress and strain concentration under combined action of shear stress and anisotropy of Sn grains, accelerating the crack initiation and propagation, and fracture occurred along the original grain boundaries, increasing the probability of early failure.