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122 SMT007 MAGAZINE I JULY 2019 Warpage of BGA packages or PCBs can occur when any heating and subsequent cooling cycle is gone through. This may leave the package to either bow in the middle. Pushing the cor- ners up or downward will show up in bridging (caught on X-ray) or cause opens that would show up on endoscopic or visual inspection. If the PCB warps, then opens or shorts on a vari- ety of other component areas may result. Reasons for Problems BGA and PCB warpage is a problem of mate- rial coefficient of thermal expansion (CTE) mismatch between the various packaging com- ponent materials, such as the substrate, silicon die, and EMC. The rate at which there are tem- perature increases can affect the temperature uniformity across the component during place- ment and removal, so this rate is indirectly cor- related with warpage differences (Figure 1). Also, the larger the package, the greater chance of this warping phenomena with all other things being equal. Certainly, the type of rework heating method (hot air rework sys- tem, IR, hot air reflow oven, vapor phase oven, etc.) also has an impact. Using low-CTE ther- mal materials, it is possible to tailor CTE, par- tially or fully eliminating this problem. Some plastic bodied ball grid arrays (PBGAs) include a heat spreader, which causes the top of the BGA package to expand at a faster rate than the bottom of the BGA; this can pull the BGA and PCB Warpage: What to Do Knocking Down the Bone Pile by Bob Wettermann, BEST INC. Figure 1: Opens due to BGA warpage.

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