Issue link: https://iconnect007.uberflip.com/i/869081
66 SMT Magazine • September 2017 usually detected in functional testing but rath- er in the field after the assembly has been ex- posed to some physical or thermal stresses. Warping can also cause bridging and shorts, at either the outer corners or the center of the de- vice during replacement (Figure 2). This warp- ing phenomenon can cause the center of the BGA to bow upwards and the corners down, or vice versa. Sometimes, this is enough to cause the corners to move closer to the cir cuit board surface. This puts extra pressure on the corner solder joints. Later in the lifecycle of the de - vice, this may cause shorts or cracks. There are several fine tunings of the re- work process which will mitigate the impact of warped packages. The main way to lessen the impact of warping includes the adjustment of the reflow profile and solder paste chemistry. The reflow profile will help determine which areas of the device may be subject to thermal stress as well as ensuring the flux activity lev- el does not expire. The solder paste chemistry is an integral step in making sure there is plenty of flux activity if the warping occurs over a pro- longed reflow cycle. These variables are impor- tant to control during the BGA rework process to minimize warping. By aiming for a very small temperature dif- ferential across the entire BGA package—less than 10°C across the entire package is optimal— the impact of device warpage can be greatly re- duced in the rework process. One of the meth- ods for ensuring a consistent and small tempera- ture gradient across the entire package is to use a programmable multi-zone bottom-side heating source (Figure 3). Ideally, a multi-zoned heating source will heat the underside of the board to 100–120°C (lead-free processing temperatures) with the device location elevated to 140°C if the board and components can withstand this. This slight temperature differential will ensure that there is not undo thermal stress, which may de- laminate or warp the board. The heating nozzle (assuming hot gas is used as the reflow source) will bring the solder balls up to 20–30°C above liquidus for 60–120 seconds. In addition to the proper removal and re- flow profile, the PCB will need to be properly supported to mitigate any effects of deflections on the PCB during the removal or reflow pro- files. Board supports should be such that they prevent undue board deflections. An example of such a board holder can be found in Figure 4. The lack of support further exacerbates warping which may occur. Solder paste chemistry can impact the de- gree to which the device or PCB is warped dur- ing the rework process. Solder pastes, with an activation system that can provide sustainable high-temperature fluxing, can create a homoge- nous connection. This connection goes beyond the ball and the paste alloy interface, which is important for warped devices to form an inter- metallic bond. Those that are not able to sus- tain high-temperature efficacy are more prone to HiP defects. Figure 3: Multi-zone bottom heat of BGA rework system. Figure 2: Warpage and Z-height deflection on BGA. TOP 5 BGA REWORK CHALLENGES TO OVERCOME