Design for SMD pads

Standard Package Pad Design

     SMD Pad design follows relevant standards，such as IPC782 standard; SMD on the reflow surface, the pad of bigger components 

(such as transistors, sockets, etc.) should be appropriately increased, such as the SOT23 pads can be lengthened by 0.8-1mm. This 

will avoids the missing solder due to the  'shadow effect' of the component.

     The pads size is determined according to the size of the component. The width of the pad is equal to or greater than the width of 

the component leads, the soldering effect will be the best.

     For the through hole components, in order to ensure the best soldering effect, the gap between the pins and the apertures are 

advantageous for insertion, and a smaller capillary is required to achieve a good capillary effect, so a balance needs to be struck 

between them.

The shadow effect

       Avoid using a single large pad between two interconnected SMDs, because the solder on the large pad will pull the two 

components in the middle. The correct way is to separate the pads of the two components. Connect thinner wires between 

the two pads. If the wires are required to pass a strong current, several wires can be connected in parallel.

       There should be no through holes on or near the pads of the SMD. Otherwise, during solder reflow, the solder on the pads 

will flow away along the vias, which will result in solder joints, less tin, and may flow to the board. The other side will cause a 

short circuit.

       For IC pads with a large number of pins, it is necessary to expand the copper of the edge pads to increase the attractive force, 

so that the components will return to the middle during reflow soldering.

       In order to prevent the solder from overflowing from the through hole to the upper plate during reflow soldering, the part is

short-circuited to the ground or short-circuited between the parts. When designing the multi-layer board, attention should be paid 

to the components of the metal case, and the case and the printed board are in contact with the printed board. The other side of 

the through hole should not be opened on the solder mask layer. Be sure to cover it with solder mask or screen printing oil (for 

example, a crystal of two feet and an LED of three feet).