SMD refers to the way of copper foil pad or pad bare design seen on the circuit board. This small detail of PCB pad design that no one cared about before is becoming more and more important in the trend of electronic parts becoming smaller and smaller and solder joints becoming smaller and smaller. Today's PCB pads and tracks are basically made of copper foil. However, when we design PCB, we will not expose all the copper foil, but only the pads that need to be contacted or welded, so as to avoid possible wet short circuit or other problems in future use, At this time, we usually use the so-called SMD PCBA "solder mask" to cover the copper foil that does not need to be exposed, so the position accuracy and ability of solder mask printing become very important relative to the small pad.

SMD (solder mask defined pad design) is to use the solder mask (green paint / green oil) to cover a large area of copper foil, and then expose the copper foil at the opening of the green paint (green paint is not covered) to form a pad. Because the size of the pad will depend on the size of the green hole, so it is said to be a limited pad. The actual copper foil size of SMD PCBA is larger than that of nsmd, and the solder pad will be covered with anti solder oil, so the bonding strength between the solder pad and FR4 is relatively good, and the pad is not easy to fall off due to repeated heating during maintenance or heavy industry.

Although the solder strength of SMD PCBA pad will be relatively poor. This is because the relative tin eating area is smaller, and anti solder oil is pressed around the SMD pad. When the anti solder oil flows through the reflow high temperature, it will expand and contract, which will also affect the tin eating effect at the junction of solder and green paint. However, the shape of SMD PCBA design pad is generally more complete, because it is less affected by the trace. It is suggested that SMD design should be adopted for small parts and thin pins of smallchip, such as resistance, capacitor, inductor, tvsdiode below 0402. It is strongly recommended that SMD design be adopted for 0201 and 01005.