Oxide adhesion characteristic of lead frame copper alloys

Abstract

Copper alloys have recently been finding increasing demand for lead frame due to their electrical property. There is, however, a concern about that oxide adhesion of copper alloys could be partly responsible for delamination and crack in the plastic package during reflow soldering. In this paper, the alloy-dependence on the oxide film adhesion using major lead frame copper alloy by a tape peeling test and a shear test was initially investigated, and it was found that a new high strength-high conductivity copper alloy, NK120, Cu-Cr-Zr-Zn alloy possesses the highest oxide film adhesion among the alloys. Several oxidation behaviors of the copper alloys showed that Cu-Cr-Zr-Zn alloy exhibited rougher peeled surface of substrate, higher rate of oxide film growth, higher ratio of Cu/sub 2/O/CuO and less segregation of additional elements to the interface between oxide film and substrate than other copper alloys. The relationship between oxide film adhesion and those oxidizing behaviors was discussed for the peeling mechanism. The relationship between shear strength and oxide film thickness was also investigated in detail. It indicated that the maximum adhesion has been found to exist at a certain oxide film thickness and the optimum oxide thickness varies with alloy chemistry and heating temperature. The influence of flash copper plating and anti-tarnish coating on adhesion of oxide film was then evaluated. Furthermore, a shear test and a packaging crack test were made using a practical EMC used in assembly process. Those also proved the great advantage of NK120.