Issue link: https://iconnect007.uberflip.com/i/769122
64 SMT Magazine • January 2017 One of the challenges associated with BGA rework has to do with effects of device warpage, which can cause undue shorts or opens post re- work. The impact of a lead-free rework process, the continuous "thinning" of the BGA package as well as continued turnaround time pressures of rework all have led to increased propensity of this phenomenon to occur. While most of the failures in the rework process can be captured via visual or x-ray inspection, or some escapes in test can occur. For example, head-in-pillow defects can be caused by the process of the ball being "pulled" out of the oriented paste during the reflow profile. Too much warpage can also create stress on the solder ball joints and lead to reliability failures of the packages (Figure 1). Outside of opens and shorts in the reflowed and reworked BGA, there is a widely accepted analytical technique for measuring the degree of warpage. These measurements are made by coating the part with reflective paint and plac- ing a sheet of low expansion quartz glass etched with equally spaced parallel lines parallel to the sample. A beam of light is then directed onto the quartz glass and the lines create a shadow on the top of the BGA package. When the package becomes warped a Moiré pattern is produced by the geometric interference between the lines on the quartz and the shadow of the lines on the surface. These fringe patterns then can be cali- brated and are displayed as 3-D topographical part "map." A typical Shadow Moiré warpage output, from a modern measuring instrument, can be seen below (Figure 2). This method tries to emulate the behavior of the package through a thermal cycle. by Bob Wettermann BEST INC. Reducing Warpage on BGAs During Rework KNOCKING DOWN THE BONE PILE Figure 1: Typical rework profile and the potential impact of warpage at differing points.