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72 SMT007 MAGAZINE I SEPTEMBER 2018 Ball grid arrays (BGAs), chip-scale packages (CSPs), flip chips, and other component pack- ages on handheld devices are commonplace today in consumer products, including GPS, cellphones, and computers. In addition, these devices are being used in high-end handheld devices, such as military personnel communi- cators or space data collection devices. Due to handling and the brittle nature of lead-free sol- ders and packaging materials, these handheld products have used underfill to withstand the mechanical shock or impact when the devic- es are dropped or struck. The underfilling of these component packages to provide a com- pliant layer between the package and PCB is one method for increasing the reliability of these interconnections. Figure 1 shows a pack- age before being underfilled, and Figure 2 illus- trates the underfill removed. Underfill is a polymeric material used to fill the gap between the electronic component body and the PCB and encapsulate the solder joints underneath the package. This materi- al enhances the reliability of the component subject to mechanical shocks and impacts by distributing the forces. In addition, it distrib- utes the thermal stresses caused by the coeffi- cient of thermal expansion mismatch between the component and PCB. Typically, underfills have a high glass transition temperature (Tg), a high modulus (E), and a matched coefficient of thermal expansion (CTE) with respect to the solder. Underfill is the "bumper" and distrib- utes the stress more uniformly on the solder joints, thereby increasing the reliability of the solder interconnections. While there are reworkable underfills in the- ory, the practical challenges still make rework- ing an underfilled component very cum- bersome and with low resultant yields. The concept of these reworkable underfills is that the material can be thermally decomposed at Knocking Down the Bone Pile by Bob Wettermann, BEST INC. Filling the Gap: Underfill Rework Figure 1: Underfilled device prior to rework. Figure 2: Device location after component removal and site preparation.

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