A method to determine the slip systems in BGA lead-free solder joints during thermal fatigue

Abstract

The ball grid array (BGA) component with cross-sectioned edge row was thermally shocked to investigate the activated slip systems in the Sn–3.0Ag–0.5Cu lead-free solder joint. The microstructure and crystal orientations of Sn-based solder joints in as-reflowed and thermally shocked conditions were obtained by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD), respectively. The sample was reexamined after 200, 300 and 400 thermal shock cycles without further polishing. In this paper, one single-crystal solder joint in a BGA component was selected to analyze the activated slip systems during recrystallization under the thermally shocked cycled condition. Two steps were used to determine the activated slip systems in this study. Step one, subgrain rotation angles and axes were calculated by the Euler angles obtained by EBSD before and after thermal shock. Then several slip systems were obtained according to the calculated subgrain rotation axes. Step two, in order to further determine the accurate slip system which could cause the subgrain rotation along the axes in step one, the theoretical slip traces were obtained by calculating Euler angles. Because the results contained many kinds of theoretical slip traces, so the slip bands observed by SEM were used and assisted to select the theoretical slip traces. Based on the comprehensive analysis of the two steps, slip systems that causes the serious deformation were determined. The results showed that recrystallization occurred at different areas of the solder joint and the slip bands appeared at the corners of the solder joint. The subgrain rotation behavior was variable at the recrystallization area by analyzing the subgrain orientation after 200, 300 and 400 thermal shock cycles. By compared the as-calculated slip traces with the slip bands characterized by SEM and the rotation axes calculated by the Euler angles, (0 \(\bar {1}\) 0) [0 0 1], (1 \(\bar {1}\) 0) [0 0 1] and (1 1 0) [0 0 \(\bar {1}\)] were the slip systems that caused the subgrain rotation along a certain axis in this solder joint.