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86 SMT007 MAGAZINE I NOVEMBER 2021 Products such as engine control modules, drones, smartphones, and other handheld communication devices, which are designed for high reliability and require high process- ing power, oen have a BGA or CSP package as the processor. Underfill has been a solu- tion at the package level protecting these de- vices from the coefficient of thermal expansion (CTE) mismatch between the device and PCB or between the die and the component sub- strate for flip chip packages. Stress caused by CTE mismatch redistributes the stress from the bottom of the solder spheres to the entire component. Underfill also provides for a me- chanical bond between these surfaces while providing very good protection for the solder joints against both mechanical and thermal strains as the product experiences vibration, drops, or thermal shocks. Underfill is typically constructed from either an epoxy-based or other polymeric-based ma- terial and is dispensed in a controlled fashion underneath the component. rough capillary action and under increased temperature, the underfill makes its way underneath the com- ponent. It is typically cured by either thermal or thermo-sonic energy. e downside of using underfill is that it causes the rework process to be extremely dif- ficult. While some underfills are categorized as "reworkable" this does not mean that the un- derfill removal process is not without extreme challenges. ere are several problems which arise from removing an underfilled compo- nent regardless of whether it is "reworkable" or "non-reworkable." e problems are fairly nu- merous when the underfill is heated and moves around, breaking the bond between board and Methods for Underfilled Component Rework Knocking Down the Bone Pile by Bob Wettermann, BEST INC.

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