No-flow underfill process modeling and analysis for low cost, high throughput flip chip assembly

A new defect in which a chip "floats" over the board surface after chip placement is appearing in the low-cost, high-throughput flip chip on board (FCOB) assembly that is based on no-flow underfill. This defect has the potential to significantly lower process yield when process variables are not properly controlled. In fact, it was found that much of the yield loss observed post reflow is attributable to "chip floating." A process model has been developed that will allow an understanding of the underlying physics of the floating phenomena and identification of process variables so that this process defect can be eliminated. The critical process variables include chip placement speed, chip placement force, dwell time, deposited underfill mass and underfill material properties such as viscosity, density, surface tension, wetting speed on the board, etc. A test chip and board was specially designed so that chip floating over the board can be easily detected. To validate the model, the effects of the critical process variables on chip floating were investigated by a series of experiments, and the results were compared to the theoretical model's predictions.