Flip chip organic substrate with metal column

Discusses the issue of stress induced from CTE mismatch between silicon and organic substrate in a flip-chip package. Although the use of underfill has greatly reduced the stress on the solder bump under die by holding the stretched substrate to silicon, the solder joints are still seeing high stress prior to the process. The flatness of the assembly becomes an issue with the stretched substrate after the underfilled epoxy gets cured. This situation will get worse with large die size. It is not uncommon to see a flip chip size greater than 20 mm. In this paper metal columns are built on the substrate instead of silicon using the existing PCB process. They are imbedded in a layer of low modulus epoxy. It forms a metal column matrix and provides a mechanical cushion for the stress to be generated after the flip chip attachment and solder reflow. The structure of the metal column was disclosed and finite element analysis was performed as the simulated mechanical analysis to quantify the improvement of stress on the die and warpage after flip chip solder bump reflow and after underfill epoxy curing. The impact of following material selections and/or properties is analyzed and conclusions drawn: 1. Matrix epoxy with different modulus 2. Thickness of matrix epoxy and height of metal column 3. Material of metal column.