Reverse Engineering PCB SMT Pad Design
SMT assembly quality and reverse engineering PCB SMT pad design has a direct, very important relationship. If the PCB pad is designed correctly, a small amount of skew can be corrected at the time of reflow soldering due to the effect of the surface tension of the molten solder (called self-positioning or self-correcting effect); conversely, if the PCB SMT pad design is not designed correctly , Even if the placement position is very accurate, but after reflow soldering component location deviation, suspension bridge and other welding defects.
According to the various components of the solder joint structure analysis, in order to meet the reliability requirements of the solder joint, reverse engineering pcb SMT pad design should grasp the following key elements:
1. Symmetry - both ends of the pad must be symmetrical, in order to ensure that the surface tension of molten solder balance.
2. Pad spacing - to ensure that the component tip or pin and pad the appropriate overlap size. Pad spacing is too large or too small will cause welding defects.
3. Residual size of the pad - the remaining size of the component tip or pin and the pad must ensure that the solder joint can form a meniscus.
4. Pad width - should be consistent with the width of the component tip or pin.
Figure 5 shows the structure of the pad.If you violate the design requirements, reflow soldering will produce welding defects, and reverse engineering PCB pad design problems in the production process is difficult or even Can not be solved. E.g:
1. When the pad spacing G is too large or too small, reflow soldering due to the soldering terminal can not overlap with the pad overlap, will produce suspension bridge, shift.
2. When the size of the pad size asymmetry. Or the two components of the end of the design in the same pad, due to surface tension asymmetry, will also produce suspension bridge, shift.
3. Through-hole design on the SMT pad, the solder will flow from the through-hole, will cause insufficient amount of solder paste