Silver alloy wire bonding

Abstract

In semiconductor packaging, wire bonding is the main technology for electrical connections between chip and leadframe or substrate. Gold wire bonding has the advantages of a fast bonding process, excellent electrical property and stable chemical property. It has been widely used in various electronic packages. Gold prices have been raised significantly over the last few years. Many manufactures have been investigating ways to replace the conventional gold wire with various new materials. Copper wire bonding is an alternative interconnection technology. Cu wire has superior electrical and thermal conductivities as well as higher tensile strength, elongation and better “ball neck” strength. On the other hand, the higher hardness of Cu wire requires higher ultrasonic power and bonding force, which lead to high risk of cratering for ball bonding and tearing for wedge bonding. These will cause some package limitation and wire bonder machine downtime or low units per hour (UPH). Ag alloy wire has low Young's modulus and hardness property. It is a low cost wire bonding solution other than gold wire. In this study, Ag alloy wire is proposed as an alternative to Au bonding wire. Emphasis is placed on the wire bonding workability and reliability of using Ag-Au-Pd alloy wire for TSOP package. Also, wire bonding parameter such as electronic flame off (EFO), bond force, ultrasonic power, heat block temperature and time for ball and wedge bonding are optimized. Furthermore, the response for parameter optimization is determined by the Dage bond tester. Package reliability is determined through environmental tests that include pressure cooker test (PCT), temperature cycle test (TCT) and high temperature storage life test (HTSL). The tested samples were studied by focused ion beam (FIB), scanning electron microscopy (SEM) and energy dispersive spectrometer analyses (EDS). Intermetallic compound growth behavior during reliability test is characterized and compared to Al-Au and Al-Cu systems. Ag-Al didn't have excessive volume variation and void occurrence to get better bonding performance during various reliability tests.