Effect of heat affected zone on the mechanical properties of copper bonding wire

Abstract

Wire bonding as one of the dominant interconnection technologies is gradually predominated by copper wire as a result of higher performance with lower cost than gold wire. Heat affected zone (HAZ) was formed due to the thermal conductivity during wire melting and free air ball (FAB) solidifying for ball bonding. The influence of the grain growth and recrystallization accompanied with HAZ formation would make the HAZ properties significantly different from that of thermally stable zone. However, it is still unclear how the material properties change with the HAZ. In the present study, mechanical properties of HAZ affected by different electronic flame-off (EFO) parameters were investigated. The length of HAZ was identified based on the results of micro-Vicker hardness as well as the grain size change from SEM micrographs. A patent-pending design of silicon clamp, which was fabricated with lithography and deep reactiveion etching (DRIE), was used to conduct the tensile test of HAZ. Deflection test was employed to accurately measure the Young's modulus of the HAZ using the nano-indenter tip. The beam length was controlled by the silicon clamps in the range of the length of HAZ ± 10 μm to make sure the Young's modulus was taken from the HAZ. The copper samples without EFO were used as control in the mechanical tests for comparison. The average of Young's modulus for the control sample was 149 GPa while the Young's modulus of the HAZ is 110 GPa. No notable difference in Young's modulus was observed among the HAZ samples made under different EFO settings.