Solder paste slump can and will lead to solder shorts during reflow. FIGURE 1 shows a 0.020"- pitch QFP with wet paste shorts present after placement. The paste may have been present between the pads prior to placement and directly after printing. In this case the printer or SPI should have picked up the faults prior to placement. It is good practice to review the paste products used to understand the degree of slump expected prior to reflow and the highest temperature just before reflow. This will provide insight as to the likelihood of short circuits.
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