Underfills protect the active surface of the die of flip chips, BGA ad CSP package types while improving their reliability by distributing stress away from the solder interconnects. This increases the performance of products in meeting drop, shock and bend criteria. Newer underfills are specifically designed to minimize the need to scrap entire boards with high cost devices bonded on them because testing has determined that a device is defective. However, the ability of these devices to be reworked once they have been underfilled is both challenging and time-consuming.
The goal of this type of rework project is to remove the underfilled device replacing it with a good die. The removal of this material can be accomplished either with mechanical grinding or through high temperature vacuum extraction or hand tools depending on the modulus of elasticity of the underfill. In some cases softening agents that are formulated by the underfill developer help to quicken the time it takes to remove the underfill and prep the site location. There are other cases where the rework of the device requires complete ablation of the component by laser or mechanical means.
The rework process begins with the even heating of the substrate to a temperature above the softening point of the underfill. The package is mechanically gripped or pried with enough torque to break the fillet’s adhesions to the board. The chip undergoing rework is then heated above the solder reflow temperature to melt the solder connections and break down the underfill. The device is then removed from the PCB. Residual solder and underfill are cleaned off the substrate. Cleanup after chip removal removes any underfill residue and excess solder on the substrate. This part of the process must be done with extreme care in order to not damage the pads and adjacent components on the substrate. The site is then cleaned prior to inspection. Once cleanup of the substrate is complete, a new chip can be aligned, reflowed, and underfilled.
BEST has been involved in other techniques which can allow customers to rework underfilled components. In some cases precision milling of devices may be required as the adhesive strength of the underfill is such that when an attempt is made to pull the component off the PCB pads, traces and mask come with the remnant underfill. In other cases lasers can micromachine in or around components to selectively get at the underfilled areas. When there are underfilled devices neighboring a to be reworked component the reflow process may disturb solder joints of the underfilled component. BEST has a variety of shielding and temperature reduction techniques in order to mitigate the problems caused by the "squirting out" of this underfill.