SMT assembly quality and PCB Copy pad design has a direct, very important relationship. If the PCB pads are designed correctly, a small amount of skewing during placement can be corrected (called self-alignment or self-correcting effects) due to the effect of surface tension of the solder during reflow soldering; conversely, if the PCB copy pad design is not Correct, even if the placement position is very accurate, but after reflow soldering will appear component position offset, bridge and other welding defects.

According to the analysis of the solder joint structure of various components, in order to meet the reliability requirements of the solder joint, the PCB pad design should master the following key elements:

1. Symmetry - both ends of the pad must be symmetric, in order to ensure that the molten solder surface tension balance.

2. Pad Spacing - Ensure proper splice size of the component tip or pin and pad. If the pad pitch is too large or too small, it will cause welding defects.

3. Remaining size of the pad - the remaining size of the component end or pin after lap bonding with the pad must ensure that the solder joint is able to form the meniscus.

4. Pad design width - should be consistent with the width of the header or pin.

Figure 5 shows the schematic of the pad structure, if the design requirements, reflow soldering will produce welding defects, and PCB design problems in the production process is difficult or even impossible to solve of. E.g:

1. When the pad spacing G is too large or too small, the reflow soldering time because the component can not overlap with the welding lap overlap, will produce suspension bridge, shift.

2. When the size of the PCB Copy pad size is asymmetric. Or the two components of the end design in the same pad, due to surface tension asymmetry, also produce suspension bridge, displacement.

3. Through-hole design In the pad, the solder will flow from the through-hole, will result in insufficient amount of solder paste