Effects of twin grain boundaries on the subgrain rotation of the solder joint during thermal shock

Abstract

The ball grid array (BGA) component was selected as the specimen with the outmost edge row cross-sectioned to investigate subgrain rotation behavior influenced by grain boundaries under thermal shock cycles condition. To study the subgrain rotation, the crystal orientation was obtained by electron backscattered diffraction(EBSD). The results showed that a lot of subgrains were generated in the solder joints due to cyclic stress caused by the high mismatched coefficient of thermal expansion (CTE) under the thermal shock cycles condition. And the subgrains near the chip-side and the twin grain boundaries were analyzed in detail to estimate the impact of the grain boundaries on the subgrain rotation behavior. The results showed that a large number of subgrain boundaries and several newly generated subgrains appeared at the tilted twin grain boundaries and the chip-side in the solder joint after 200 thermal shock cycles. Meantime, the subgrain rotation axes and misorientation angles were both calculated, and the dislocation slip was recognized to closely relate to subgrain rotation by comprehensively analyzing the rotation axes and misorientation angles. The subgrain rotation axes of the chip-side was about Sn [101] and [001], while the subgrains rotation near the tilted grain boundaries in the same dominant grain was different from that of the subgrains near the chip-side, and subgrain rotation axes were [101], [100] and [110]. There were also a large difference in the direction of subgrain rotation between the chip-side and the tilted twin grain boundaries. The subgrain rotation axes at both sides of twin grain boundaries were similar, but the rotation directions were opposite.