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For PCBAs that have correct pad design, even if a little skew takes place in the process of surface mounting, it can be corrected under the effect of surface tension of melting soldering tin, which is called auto-positioning or self-correcting effect. However, if pcba design manufacturing pad is incorrectly designed, even if mounting position is quite accurate, soldering defects still come across such as component position shift and tombstoning. Therefore, the following aspects have to be delicately considered in terms of SMT pad design.

Symmetry of pad. In order to avoid problems of position shift and tombstoning after reflow soldering, for chip components 0805 or below, pads at both ends should maintain symmetrical in terms of pad size and heat absorbing and dissipation capacity in order to keep the balance of surface tension of melting soldering tin. If one end is on the large copper foil, it's suggested that single-line connection is applied to connect pad on large copper foil.

Space between pads. In order to ensure suitable lap joint size between component ends or pins and pad, soldering defects tend to be caused when space between pads is too large or too small.

Residual size of pad has to ensure the meniscus shape of soldering points after lap joints between component ends or pins and pad.

Width of pad should be basically compatible with that of component ends or pins.

Through-hole vias mustn't be placed on pad. Otherwise in the process of reflow soldering, melting tin will possibly flow away along through-hole vias, producing pseudo soldering and tin insufficiency. It may flow to the other side of board to cause short circuit.

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