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56 SMT007 MAGAZINE I APRIL 2023 • Inspecting the multilayer-stacked packages means that more advanced X-ray systems and more experienced operators are required for inspecting the packages post-rework. • The thin packages mean the packages have high MSL (moisture sensitivity levels), which requires a high degree of care. • Many of these packages are underneath or near RF shields, which in and of themselves are challenging to remove and replace without damaging underside or neighboring components. Many times the proximity of underfill or components to the shield wall can cause challenges of neighboring components being negatively impacted during the rework process. The BGA rework equipment needs to be properly adjusted in order to maintain the proper profile when reworking these stacked packages. If the top package to be singled out is to be removed and replaced, then the rework system must have the ability to precisely control the hot air time-temperature profile in order to reduce the possibility of package warpage. These PoP-capable rework systems need to have a zero force or near zero force removal capability in order to not disturb the lower package if the upper package is the one requir ing ref low. Since many of these component packages are small, the o p t i c a l s p l i t v i s i o n system—along with the BGA placement system—must be able to precisely place the component. In addition, the BGA re-work system should be equipped with a non-contact scavenging system f o r m o r e c o n s i s t e n t results in terms of site preparation. In the case of underfilled components, the low or no heat "cold" removal machines—which could be lasers or milling machines—will be key to a successful rework program. Furthermore, the X-ray inspection used to check the rework should have the ability for isometric viewing. This allows the rework technician to inspect the PoP and "see" each of the balls on the individual layers. These are some of the critical rework tools for reworking on PoP packages. Not only does the equipment need to be capable of reworking stacked packages, but the right experienced engineering and technician support needs to be available. Experience in the nuances of time-temperature profile development, along with the experience of using the right paste fluxes or dipping solder pastes, is part of getting to a robust rework process for the PoPs. The technicians need to be experienced in mitigating the effects of component warpage caused by the heating during placement and removal. Furthermore, the rework process technicians should be very disciplined in properly handling the components given higher than normal MSLs. One of the other decisions that needs to be made in the complete rework process is PoP testing. Experience will help guide the functional testing and whether the PoP should be tested prior to being placed or be tested in-circuit to confirm functionality. In addition to the right equipment set and the right operator experience, the rework supplier must have a deep understanding of a variety of different materials for PoP rework. In many cases, "dippable" solder paste is used for the individual package rework in order to ensure more consistent solder paste deposit volume, which widens the process window. There are a variety of fluxes which can be used to make Figure 2: Physical damage to PoP will require rework to be performed.

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