SMT007 Magazine

SMT007-Nov2018

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78 SMT007 MAGAZINE I NOVEMBER 2018 Proper Thermal Shielding Yields Highest Rework Results There are numerous "gotchas" if the rework technician does not care for components and materials neighboring the component rework area. If this care is not taken, the rework com- ponent might have been properly reworked, but the PWB itself may not be functional due to thermal damage. While PCB rework pro- cesses using best-in-class methods, materials, and equipment may have a high yield, an even higher yield can be achieved by protecting sur- rounding components. During component rework, taking extreme care of nearby components—whether they are adjacent to the device being reworked or on the opposite side of the board—is required to avoid collateral heat damage, inadvertent reflow, and altered characteristics, such as dis- coloration or part skewing. Damage can be done to the part physically when the heat from the rework source is beyond the range of what the component is rated. This is common with connectors, relays, and batteries. In addition to component damage, de-wet- ting, pad damage, starved solder fillets, and component surface finish oxidation can result. In some cases, the damage is unseen in the form of increased intermetallic growth in the solder joint, which may impact the reliabili- ty of the assembly. In end-use environments where reliability is critical, this increased reli- ability risk must be investigated for its impact. Also, a process capability study might have to be completed to verify the rework process. Further, there can be other materials dam- aged on the assembly that are part of the prop- er operation and design for its end-use operat- Knocking Down the Bone Pile by Bob Wettermann, BEST INC. Figure 1: Skewed component due to proximity to the one being reworked and improper shielding.

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