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54 SMT007 MAGAZINE I APRIL 2023 Editor's Note: Though this may be Bob Wettermann's last installment of Knocking Down the Bone Pile, Bob's column will carry on under the expert guidance of the subject matter experts at BEST. Package-on-package (PoP) is an electronic component-stacked package type consisting of vertically stacked ball grid arrays most commonly in a two-high stack. The package closest to the board is the logic/CPU component and is more commonly known as the "bottom" package. The "top" package sits on top of this module and is the memory Package on Package Rework—Skill Required module. These packages are generally found in consumer electronics (mobile devices) such as smartphones, tablets, and netbooks. These packages tend to be high IO count, fine pitch, very thin packages. The rework and subsequent inspection of such components requires precise control of temperature as well as skilled repair and rework technicians and a broad understanding of a variety of materials that will be of assistance in reworking PoPs. There are numerous challenges to reworking these components, including but not limited to the following: • Due to the thin sizes of the packages (they have to fit into a slender consumer package in most cases), the thermal stresses of the reflow cycle can cause warping of the packages. This can lead to opens and shorts post-rework. • Many of these consumer devices are underfilled in order to prevent solder joints from cracking, as the handheld devices are prone to the mechanical shock of being dropped. The removal of such packages using a heat source causes the solder to be pushed around, creating soldering anomalies such as shorts. Knocking Down the Bone Pile by Bob Wettermann, BEST INC. Figure 1: Package on package (PoP).

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