Development of a rework-technology for flip-chips, using no-flow underfills

Underfill is a material used for filling the gap between a flip-chip and the PCB. Usually the material is dispensed after reflowing the solder joints. Driven by capillary force, it fills the gap and encapsulates the solder balls. No-flow underfills are a new technique to eliminate the time consuming flow and curing step of regular materials. These materials are dispensed before reflowing. The curing and soldering takes place at the same time in a reflow oven. As with regular underfills, it is still almost impossible to rework a defective chip. Our goal is not to develop a reworkable underfill material, but to rework a flip-chip using a conventional no-flow underfill. Two approaches were taken into consideration: thermal and chemical rework. A combination of both, as the most likely way to successfully rework, was tested.