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58 SMT007 MAGAZINE I FEBRUARY 2023 component removal process. During reflow, the tack of the underfill holds the BGA tightly to the PCB. Removing the BGA requires the rework technician to either "cut through" the underfill using a knife or to use specially designed nozzles to pull the BGA off the board. Either of these operations can result in damage to the PCB pads or solder mask. Once the BGA has been removed, this underfill material, as well as the remnant solder, needs to be manu- ally cleaned from the board. is labor-inten- sive process is necessary to ensure that the site is adequately cleaned prior to replacement of the BGA. Newer methods such as cold removal using a highly controlled milling process or laser abla- tion of the component can overcome some of these rework challenges. ese advanced meth- ods, while faster and repeatable, require both capital and higher-end programming and pro- cessing skills on the part of the rework supplier. Challenge #4: Mirrored BGAs W hen BGA s are placed " back-to -back " directly opposite one another on both sides of a double-sided PCB, the configuration is known as "mirroring" (Figure 2). is is a con- tinuing challenge in BGA rework as board den- sities increase. In BGA rework, it is necessary to preheat the PCB from the opposite side of where the BGA is to be removed and replaced. is necessitates heating the BGA underneath the rework location causing the underside BGA to be exposed to a minimum of 125°C. In addition, this causes the underside BGA to be exposed to a longer duration heat cycle, which increases the potential for damaging or warp- ing the BGA. It may also cause the underside component to go into reflow resulting in sol- dering anomalies and solder joint embrit- tlement. Another danger is the potential for larger more massive BGA components to "fall off " the oppositely located BGAs as the sur- face tension may no longer be able to over- come gravity, which is pulling down on the component. Summary e challenges for BGA rework will con- tinue to evolve. Large BGA packages, BGAs in close proximity to neighboring devices, underfilled BGAs, and mirrored BGAs cur- rently present the greatest challenges in BGA rework today. SMT007 References 1. "Shielding Effectiveness of Polyimide Tape Dur- ing Rework," by Adam Gaynor and Bob Wettermann, Circuits Assembly Magazine, Oct. 1, 2014. 2. "Electronic Board Level Underfill Material Mar- ket," by Future Market Insights Global Consulting, 2022. Bob Wettermann is the prin- cipal of BEST Inc., a contract rework and repair facility in Chicago. For more information, contact To read past columns, click here. Figure 2: Example of mirrored BGAs.

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