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124 SMT007 MAGAZINE I JULY 2019 corners of the BGA down. Moisture in the BGA may also contribute to warp- age as the component wants to spread in the middle. In these cases, the cor- ners may curl upwards. Through a series of design of exper- iments, you may want to confirm which part of the equation—the BGA or the PCB—is warping. Isolating which surface is doing the pulling and pushing will be useful in determining how to fix the problem. How to Mitigate For BGA warping, the corners will see the largest displacement, which can cause numerous opens and bridges to occur. Similarly, the circuit board can warp up or down and push into the solder paste, causing bridges or opens. These are caught either in visual or X-ray inspection. One of the methods to minimize warpage is to slow down your heating and cooling pro- cesses. You ramp up during your preheat sec- tion and cool down during your cooling sec- tion. Now, of course, in cooldown, you don't want to go too slow because you don't want to create a coarse-grained structure. As with many things in electronics manufacturing, it's a trade-off. Controlling MSD devices— including the boards and com- ponents—is another way to help mitigate the impact of warpage. The J-STD-0033 and JEDEC guidelines for moisture handling will be the best place to refer to for proper MSD han- dling. Prebaking of boards and components followed by keep- ing them in a dry environment will mitigate the problems if they are associated with mois- ture absorption. Limiting the exposure time and understand- ing the MSD level of the board and components will also go a long way to mitigating the warpage associated with mois- ture absorption. By using a solder paste that is formulated to reduce head-in-pillow and allows for the proper coalescing of the solder paste, solder ball warpage impacts will be limited as well (Figure 2). Through properly engineering the volume of solder paste deposited at each pad loca- tion, some of the problems related to PCB and device warpage can be limited. In some cases, overprinting some of the pads—while in other Figure 2: Head-in-pillow defect. Figure 3: Shadow moiré.

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