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82 SMT007 MAGAZINE I MARCH 2022 One of the most common physical repairs 1 (restoring functional capability of a defec- tive PCB while not complying to meet origi- nal specifications) on a PCB is the repair of sol- der mask. Solder mask's purpose is to prevent solder from flowing from one point to another during the original assembly process. Dam- age to solder mask (Figure 1) can be aesthetic or functional in nature, such as the case when the mask preventing solder from flowing down the "dog bone" of a BGA causes the BGA ball solder joint to be "starved," thereby causing a defect. Solder Mask Repair Techniques for PCB Repair ere are several ways in which solder mask can become damaged. Missing mask can occur due to a breakdown in the mask adhesion to the PCB. Mask can also become damaged when an uncontrolled heat source and solder braid is used to remove solder from a pad loca- tion, thereby scratching off the mask. In cases where the mask is exposed to a high number of heat cycles, the mask can become brittle and break down (Figure 2). Poorly adhered solder mask may cause solder shorts, other solder- ing anomalies, or result in solder mask flowing down the dog bone from a BGA pad. Knocking Down the Bone Pile by Bob Wettermann, BEST INC. Figure 1: Solder mask defects.

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