The microstructure and texture evolution in the heat affected zone of copper bonding wire

The substitution of the copper wire for gold wire has become inevitable in the electronic package, mainly due to the continuously increasing gold price. Moreover, higher electrical conductivity and thermal conductivity make the copper wire capable for high power device. After the electric flame-off (EFO), the grain growth and recrystallization in the heat affected zone (HAZ) induced by the thermal conduction along the copper wire make its material properties largely different from that in thermally stable zone. Meanwhile the thermal conduction would also give rise to recrystallization texture in the HAZ. However, the investigation on texture evolution in the HAZ has not been conducted in detail. In the present study, the microstructure variation and texture evolution in the HAZ was investigated. Starting from ball neck, a serial of cross-sections in HAZ was obtained with the interval distance of 70 μm. Electron backscatter diffraction (EBSD) technology was used for characterization. The grain orientation in the free air ball (FAB) was also investigated. Microstructure can be directly observed in the orientation-imaging-microscopic (OIM) maps. Along the inverse direction of heat conduction, the grains grew almost exponentially while <111> fiber texture component reduced in the HAZ. Subgrain was formed at the peripheries of FAB and ball neck position. The fraction of misorientation angle in the range from 15° to 35° decreased fast which may be related to the slower mobility of grain boundaries in this region. Our study emphasized that the texture evolution should not be negligible in the study of HAZ and more attention also need to be paid on variation of misorientation distribution.