The oxidation control of copper leadframe package for prevention of popcorn cracking

Copper alloy leadframe has good thermal, electrical, mechanical and cost performance. It, however, is susceptible to package cracking during reflow soldering due to poor adhesion of the molding compound to the leadframe. In this paper we investigated the oxide film properties of copper leadframe and the effect of the film on the reliability performance of semiconductor packages. According to our experiment, oxide film growth was expressed as a function of time and temperature and the growth of the oxide film was surpressed when oxygen concentration was less than 5%. An adhesion strength test of epoxy molding compound to the oxidized copper surface showed drastic degradation of strength if the film thickness exceeded about 20 nm accompanied with delaminating surface alternation. Finally, we conducted a reflow cracking test with samples whose oxide film thickness was controlled. The results showed good correlation with the adhesion strength test. It was found that the oxide film thickness should be less than 40 nm to enhance reflow crack performance.