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88 SMT007 MAGAZINE I JANUARY 2020 One of the toughest rework challenges is removing and replacing components on PCBs with underfilled components. Many times, underfill is used to provide a shock barrier to component solder joints of handheld electron- ics, such as notebooks, tablets, and phones. This underfill is added post-test in the assem- bly process and is dispensed underneath com- ponents, such as BGAs, QFNs, and LGAs. There are several aspects of reworking PCBs that have underfilled components that make them a real challenge when it comes to rework. The underfills' softening or liquidus tempera- ture is less than that of the reflow tempera- ture of the solder on the PCB. This means that under the BGA, as well as any other under- filled devices, the material softens and expands (Figure 1) before the solder reaching a liquidus state. The pressure this creates then forces out the solder in neighboring solder joints, thereby pushing out the solder when it reaches reflow temperature (Figure 2). The result is a mess of open solder joints, solder balls, and excessive solder in and around the rework area. The second large challenge in reworking a PCB containing underfilled components lies in the removal of the underfill itself. Due to mechanical forces, either through prying with a removal tool or via a specialty vacuum noz- zle removing the underfill, the board may end up with extensive pad and/or laminate dam- age. In addition, the prying of the component from the board during the removal process may damage pads and laminate underneath the underfilled component. The final challenge is in the tack adhesive strength of the under- fill, as this may be of such a high value that the pads are pulled off of the board. This phenom- enon may be most pronounced for pads that are no-connects underneath the device. Knocking Down the Bone Pile by Bob Wettermann, BEST INC. Underfill Rework and Solder 'Squirt Out' Figure 1: Disturbed solder balls in underfilled BGA. Figure 2: Disturbed solder balls in underfilled BGA.

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