No Flow Underfill Process Development for Fine Pitch Flip Chip Silicon to Silicon Wafer Level Integration

The industry has witnessed the adoption of flip chip for its low cost, small form factor, high performance and great I/O flexibility. As the Three Dimensional (3D) packaging technology moves to the forefront, the flip chip to wafer integration, which is also a silicon to silicon assembly, is gaining more and more popularity. Most flip chip packages require underfill to overcome the CTE mismatch between the die and substrate. Although the flip chip to wafer assembly is a silicon to silicon integration, the underfill is necessary to overcome the Z-axis thermal expansion as well as the mechanical impact stresses that occur during shipping and handling. No flow underfill is of special interest for the wafer level flip chip assembly as it can dramatically reduce the process time as well as bring down the average package cost since there is a reduction in the number of process steps and the dispenser and cure oven that would be necessary for the standard capillary underfill process. Chip floating and underfill outgassing are the most problematic issues that are associated with no flow underfill applications. The chip floating is normally associated with the size/thickness of the die and volume of the underfill dispensed. The outgassing of the no flow underfill is often induced by the reflow profile used to form the solder joint. In this paper, both issues will be addressed. A very thin, fine pitch flip chip and 2x2 Wafer Level CSP tiles are used to mimic the assembly process at the wafer level. A chip floating model will be developed in this application to understand the chip floating mechanism and define the optimal no flow underfill volume needed for the process. Different reflow profiles will be studied to reduce the underfill voiding as well as improve the processing yield. The no flow assembly process developed in this paper will help the industry understand better the chip floating and voiding issues regarding the no flow underfill applications. A stable, high yield, fine pitch flip chip no flow underfill assembly process that will be developed will be a very promising wafer level assembly technique in terms of reducing the assembly cost and improving the throughput.