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68 SMT Magazine • September 2017 of the PCB device. This means that under the BGA, as well as any non-protected device, the underfill softens and expands prior to the sol- der reaching liquidus state. This along with the resulting tackiness presents challenges in the re- moval of the device. During removal, the un- derfill will squirt out as local pressure pushes out the solder when it reaches reflow tempera- ture. The result is a mess in the underfilled de- vice being reworked. Due to mechanical force, either through prying with a lever arm or a spe- cialty nozzle in removing the underfilled device from the PCB, the board may end up with ex- tensive damage. Due to the mechanical force, either through prying with a lever arm or a spe- cialty rework nozzle in removing the under- filled device from the PCB, the board may end up with extensive damage (Figure 6). In addition to this prying action caus- ing damage to the PCB there may be damage around and underneath the area of the device in removing the underfill from the PCB. This damage may be caused by the solder wick or nozzle, which may scratch or damage the pads of the solder mask when removing remnant un- derfill. In addition, the tack adhesive strength of the underfill may be of such a high value that the pads are pulled off the board. This phenom- enon may be most pronounced for pads that are no-connects underneath the BGA. One of the ways to overcome the extensive damage caused by preparing the BGA site loca- tion after device removal is using a high-speed milling system. In this approach, no direct heat is used to soften the underfill, but rather a high- speed milling operation will grind off the cured underfill material as well as the remnant solder balls. This requires precision in this mechanical operation (Figure 6). By not grinding far enough, it will leave too much of the underfill at the BGA device lo- cation making the pads non-solderable. Care must be taken during this rework method to en- sure the mechanical vibration, and stress of re- work, does not cause a decrease in the reliabil- ity of the PCB. Extensive Solder Mask Damage Solder mask damage underneath a BGA location occurs due to several reasons. This damage is in the form of missing mask or a breakdown in the mask adhesion to the PCB. This can be caused by using solder braid dur- ing the site prep process, an uncontrolled heat source to remove the BGA, an abnormally high number of heat cycles applied to the PCB or by poor initial solder mask adhesion. The result- ing problems upon component replacement is solder flowing down the dog bone pattern and 'starving' the solder joint/flow underneath. The poorly adhered solder mask will cause solder shorts or other soldering anomalies. Solder mask can be repaired via a variety of techniques, each with its own advantages and disadvantages. A simple technique for spot sol- der mask repair is using a repair pen (IPC 7721 2.4.1), which is typically cured by air drying or a bake-out cycle. Another method, as outlined in IPC 77212.4.1, is via the use of liquid sol- der mask. This can be spread onto the areas that need repair. It is then either heat or UV-cured. Another technique, which repairs the mask un- derneath the BGA, is a stay-in-place stencil. This serves as a reliable way to place the BGA while simultaneously repairing the mask. Lastly, there is the mask repair stenciling technique. This method saves repair time as an ultra-thin sten- cil defines the area where the mask is repaired. Replacement mask material is then squeegeed into the apertures and then cured. Post curing, TOP 5 BGA REWORK CHALLENGES TO OVERCOME Figure 6: Mechanical grinding of underfilled BGA results.