Issue link: https://iconnect007.uberflip.com/i/535707
70 SMT Magazine • July 2015 paste down on to the land each and every time. Typically auger-driven systems ensure as repeat- able of a dispensing volume as possible without indiscriminant "tails" or "gaps" in the solder deposition due to the sometimes inconsistent incoming pneumatic pressure. Finally, custom solder performs are another way to ensure con- sistent solder paste volume deposition along the lip of the shield. This is an expensive approach but one in which the various reflow methods previously described can be used to reflow the solder to prevent any gaps. Open the Can Approach Another approach in which the potential damaging side effects of the reflow process can be avoided is to selectively cut out a portion of the shield, rework the location directly under the area in which the cut-out was made and patch the shield afterwards. By using the proper laser wavelength and a very fast cutting profile, a select area can be cut out. After removal of the cut-out shield area, the hand soldering tool or rework nozzle can be used to remove and replace the device in question. This greatly reduces the likelihood of neighboring or underside compo - nents to go in to reflow or from being damaged. After replacement a special shielding tape can be affixed over the opening in order to close up the hole and maintain the efficacy of the shield. Conclusion With the proliferation of handheld wireless devices, the PCB assembly processes, including rework has become more challenging. A variety of methods can be employed in order to make the rework processes of reflow, site preparation and solder deposition easier given the complex- ity of the PCB and the volume of rework re- quired. As devices get even more densely packed these challenges will only increase, requiring further developments in rework processes. SMT Figure 6: Shield scribed with laser. figure 7: Hole opened in shield top. figure 8: after removal, replacement tape "patch" eliminates need for reflow of replacement shield. bob Wettermann is the principal of bEST inc., a contract rework and repair facility in chicago. His column, Knocking Down the Bone Pile, will appear bi-monthly in SMT Magazine. KnoCKinG doWn the Bone Pile A REWORK DIlEMMA: PCB SHIElDS continues