High Yield, Near Void-Free Assembly Process of a Flip Chip in Package Using No-Flow Underfill

The advanced assembly process for a flip chip in package (FCIP) using no-flow underfill material presents challenges with high I/O density (over 3000 I/O) and fine-pitch (down to 150 μm) interconnect applications because it has narrowed the feasible assembly process window for achieving robust interconnect yield. In spite of such challenges, a high yield, nearly void-free assembly process has been achieved in the past research using commercial no-flow underfill material with a high I/O, fine pitch FCIP. The initial void area (approximately 7% ) could cause early failures such solders fatigue cracking or solder bridging in thermal reliability. Therefore, this study reviewed a classical bubble nucleation theory to predict the conditions of underfill void nucleation in the no flow assembly process. Based on the models prediction, systematic experiments were designed to eliminate underfill voiding using parametric studies. First, a void formation study investigated the effect of reflow parameter on underfill voiding and found process conditions of void-free assembly with robust interconnections. Second, a void formation characterization validated the determined reflow conditions to achieve a high yield and void-free assembly process, and the stability of assembly process using a large scale of assemblies respectively. This paper presents systematic studies into void formation study and void formation characterization through the use of structured experimentation which was designed to achieve a high yield, void-free assembly process leveraging a void formation model based on classical bubble nucleation theory. Indeed, the theoretical models were in good agreement with experimental results.