Development of laser bonding as a manufacturing process for inner lead bonding

摘要

The conventional thermo-compression bonding processes for bonding tape-automated-bonding (TAB) leadframes to silicon die has inherent reliability drawbacks due to the high pressures and temperatures necessary to produce a good metallurgical joint. Whether tin- or gold-plated tape is used, the bonding process can cause damage to the underlying structure of the device which results in device failures. The use of a laser source for inner lead bonding (ILB) has the advantage of providing a very localized temperature input to the bonding site with minimal contact force. The resulting mechanical stresses on the device are consequently low and the overall temperature extreme to which the device is subjected is similarly low. Advanced Micro Devices, Inc., has undertaken a program to qualify a gold/tin laser bonded ILB process as a viable manufacturing alternative to thermo- compression bonding. The initial evaluation has defined thresholds for laser input energy necessary to produce a good fillet around the TAB beam and a void-free interface. This is the first necessary step to provide the degree of gold/tin alloying necessary to prevent Kirkendall voiding during subsequent high temperature storage. Among the parameters critical to the bonding process is the wafer bump surface topography. The quality of the bonding process has been monitored using bond strength data and visual examination before and after high temperature storage and temperature cycling tests. The test samples used were 154 and 160 lead production TAB tape and device designs with $APEQ 200 $mu lead pitch and a 410 lead experimental tape with 102 $mu lead pitch.