Achieving high workability in high-end package by controlling microstructure of 4N Cu wire

摘要

Nowadays, due to the increment in gold prices, copper wire is considered as a low cost material for use in various package groups. Some packages, with a low-pin count of thick size wire, have already succeeded in mass production using copper wire. In addition, copper wire has been applied in high-end packages such as BGA, QFP and QFN in small masses, and that production volume is feasible. However, the applications for copper wire require different conditions from those of gold wire, due to differences in basic characteristics, such as hardness or the tail breaking force of each metal. These characteristics are a barrier delaying the universal application of copper wire to the same degree as gold wire. The main barrier to copper wire application is 1^st bonding problem, for example metal off or pad cratering, and low 2^nd workability resulting from frequent bonding errors such as short tail or NSOL (non stick on lead). Therefore, the purpose of this study is to identify methods for hardness reduction and improvements in 2^nd workability to solve the application limitations of copper wire through wire improvement. Our results identified, firstly for FAB (free air ball) or bonded ball hardness for copper wire, the optimum composition and content to reduce hardness. In addition, regarding the low 2^nd workability, the copper wire tail bond has a lower value and bigger deviation than gold wire in bondability between the copper wire and the 2^nd lead. To improve this, a study has been conducted on controlling wire manufacturing processes and alloy composition to dramatically increase uniformity of microstructure. Fine and uniform grains of copper wire were achieved by microstructure control to minimize the quality gap in the product. Furthermore, it is helpful to reduce bonding errors during the 2^nd bond, resulting from wire grain uniformity. We have revealed that improvements in copper wire show increased work-ability and stable bonding performance, when tested on QFN, QFP and BGA.