Process Development for Flip Chip Bonding with Different Bump Compositions

While developing new processes in the area of microchip electronic packaging, the goal is to increase quality and throughput while lowering cost. In the traditional backend assembly process for mature technologies, the die are singulated, sorted, mounted onto packages, and then wire bonded to the next higher assembly. Flip chip technology restructures the traditional assembly flow by mounting an electronic chip face down to a substrate, eliminating the need for die mount or wire bonding. Apart from process reduction, other advantages include increased I/O density, better thermal dissipation, and better connection performance based on low impedance from a significantly shorter bond length compared to wire bonding. Northrop Grumman's Advanced Technology Lab (ATL) has explored capabilities for assembly using multiple flip chip bump materials including gold, indium, and lead-tin solders. Each of these different material categories required different approaches to yield a proper bond. For example, gold-to-gold thermocompression bonding required a bonding profile with high force and temperature in an ambient air environment, while lead-tin solder requires little to no force, but needs exposure to a flux or other oxidation-eliminating agent. This initial process assessment leads to future studies that will be done to grow the flip chip bonding proficiency. Northrop Grumman is currently exploring throughput enhancement of gold-to-gold thermocompression flip chip bonding. This study is investigating the profile parameters of force, temperature, and dwell time and how they affect the compression and bond strength of the flip chip bonded assembly. The results of this study have already improved the throughput for Northrop Grumman gold thermocompression flip chip bonding, and will also help build the method for flip chip bonding process development for future studies. In the coming year, ATL plans to expand flip chip capability by exploring current applications to enhance the process capability.