Reliability assessment of thermally compression bonded copper pillar on organic and ceramic substrates

The ultimate goal of this study is to address and overcome the critical barriers for implementing a low cost high yield manufacturing process using thermal compression bonding of copper pillars to a high density organic substrate with the aid of a nonconductive paste. First, a conventional reflow bonding of the copper pillars to the organic and ceramic substrate was conducted and reliability after air-to-air thermal cycling was compared for baseline evaluation. The selected test vehicle for this step was a pressure sensor module where ASIC and MEMS chips were mounted on both sides of a 6-layer organic and a 6-layer ceramic substrate using a solder paste and an underfill material. Thermal cycling of the constructed functional module showed superior reliability for the ceramic substrate compared to the organic. In the second step, a design of experiments was conducted to optimize process conditions for thermal compression bonding of the copper pillars in ASIC chips to both ceramic and organic substrates using a nonconductive paste in lieu of an underfill. The assembled substrates were thermally cycled under similar conditions to evaluate thermal reliability performance and mechanical integrity of the bonding. Results show better performance for the ceramic substrate compared to the organic. The conditions for dispensing a nonconductive paste and thermal compression bonding using an automated machine have been optimized for further evaluation and scaling up to manufacturing where dispensing of the nonconductive paste, placement of chips, and thermal compression bonding can be achieved in-situ with a single assembly tool with reduced cycle time. The substrate for high throughput manufacturing is a 3-layer (dielectric) high density organic allowing direct landing of fine pitch copper pillar to the conductor traces without any landing pads.