Issue link: https://iconnect007.uberflip.com/i/1424540
90 SMT007 MAGAZINE I NOVEMBER 2021 aggressive it is, the liquid can damage com- ponents and device markings as well as oth- er coatings. Post soening agent application cleaning is also a challenge as analytical testing needs to be part of the process qualification. In many cases, the reliability risks imposed by this method are not worth the process time- saving rewards. A further removal method of the compo- nent relies on laser ablation. In this method a directed laser source is programmed to scan the surface area of the laser back and forth to disintegrate or ablate the component, under- fill, and some cases, the solder balls. At low laser flux (low laser energy density, w/cm 2 ), the material is heated by the absorbed laser energy and sublimates. The laser beam is pulsed, electronically controlled, and the beam steered to not destroy neighboring components. is method is useful in applica- tions where either the vibration of the board in precision milling presents too high a reli- ability risk or when either the component or board warpage is extreme. A more recent addition to the underfilled BGA/CSP removal process list of alternatives is the use of a precision mill to cut away the device. Typically, "non-reworkable" underfill along with the component and solder is milled off the board. e board is fixtured properly to retain a flat surface, typically with the use of a vacuum fixture. Height sensors make sure the mill does not drive too far down into the board and damage it. Vacuum is also supplied near the cutting area to keep debris off the PCB. As reflow temperatures are not reached, less- er board damage results. e resulting vibra- tion of the milling process and its effect on the board need to be investigated for the end use operating environment. ere are numerous methods for rework- ing underfilled area array and bottom termi- nated components. A skilled, experienced re- work technician with several methods, includ- ing the machinery, available to them will deter- mine the highest yielding method in the fastest amount of time. SMT007 Bob Wettermann is the princi- pal of BEST Inc., a contract rework and repair facility in Chicago. For more information, contact info@solder.net. To read past columns or contact Wettermann, click here. Figure 2: Laser-ablated BGA, half height of the component. Figure 3: Pads after milling off underfilled BGA.