Reworkable underfill investigation on flip chip packages

This paper investigates the feasibility of reworkable underfill in flip chip packages. Conventional underfills are not reworkable after post cure. As a result, faulty packages are often disposed of if failure occurs. Reworkable underfill on the other hand, enables packages to be reworked. In this paper, information covering the equipment and setup, die removal and substrates cleaning methods, process issues, as well as reliability results are discussed. Two reworkable underfill materials were evaluated. The materials were compatible with eutectic-type solder systems whereby the rework temperature was specified around 220°C. A rework system was used to remove both underfill materials. Two site-cleaning techniques, namely hot gas cum vacuum method, and abrasive rotary brush method were used. Die shear after rework, temperature cycling at -55°C/125°C for 2000 cycles, and high temperature storage at 150°C for 1000 hours, were conducted to evaluate the performance of the reworkable underfill.