The pad is the basic unit of PCB surface mount assembly. On the printed circuit board, the electrical connections of all components are made through pads. So, what are the pad shape and the standards for pad size in PCB design?
1.Common Pads
A.Square pad
They are often used on the pcb with large and few components, and the printed wires are simple.
It is easy to use this kind of pad when making a PCB by hand.
B.Circular pad
Circular pads are widely used in single and double-sided printed boards with regularly arranged components. If the density of the board allows, the pad can be larger so that it will not fall off during soldering.
C.Island-shaped pad
The island-shaped pad makes the connection between the pad and the pad integrated. It is often used in vertical irregular arrangement installation, such as the PCB board in the tape recorder.
D.Polygonal pad
Polygonal pads are used to distinguish pads with close outer diameters but different apertures. It makes PCB easy to process and assemble.
E.Oval pad
This kind of pad has enough area to enhance the anti-stripping ability, and is often used in dual in-line devices.
F.Open-shaped pad
It can effectively ensure that the manually repaired pad hole is not sealed by solder after wave soldering.
2.Special Pads
A.Plum blossom pad
They are usually used in the location of the large vias on the PCB for grounding. There are several reasons for this design:
a.The fixing hole needs to be metallized and connected to GND. If the fixing hole is fully metallized, it is easy to block the hole during reflow.
b.The use of internal metal screw holes may cause the grounding to be in a bad state due to installation or multiple disassembly and assembly. However, the use of this kind of pad ensures a good grounding regardless of changes in stress.
B.Cross-pattern pad
Cross flower pads are also called thermal pads, hot air pads, etc. Its function is to reduce the heat dissipation of the pad during soldering to prevent false soldering or PCB peeling caused by excessive heat dissipation.
a.When your pad is ground. The cross-pattern pad can reduce the area of the ground wire, slow down the heat dissipation, and facilitate soldering.
b.When your PCB requires machine assemble with a reflow soldering machine, the cross-pattern pad can prevent the PCB from peeling (because more heat is needed to melt the solder paste).
C.Teardrop Pad
When the traces connected to the pads are thin, teardrop pads are often used to prevent the pads from peeling and the traces are disconnected from the pads. Such pads are commonly used in high-frequency circuits.
a.The pad can be protected to prevent the pad from falling off during multiple soldering
b.It will strengthen the reliability of the connection (can avoid uneven etching and cracks in the via offset during production)
c.It can smooth impedance and reduce the sharp jump of impedance.
3.Design standards for pads
A.Design standards for the shape and size of PCB pads
a.The single side of the pad should be larger than 0.25mm, and the diameter of the entire pad should be no more than 3 times the component aperture.
b.Try to ensure that the distance between the edges of the two pads is greater than 0.4mm.
c.In the case of dense wiring, it is recommended to use oval pad
d.For plug-in components, in order to avoid copper foil breakage during soldering, the single-sided connecting plate should be completely covered with copper foil, and the minimum requirement for double-sided panels should be filled with teardrops.
e.All machine insert parts need to be designed as drip pads along the bent leg direction to ensure full solder joints at the bent leg.
B.Reliability design points of PCB pads
a. Symmetry. To ensure the balance of the surface tension of the molten solder, the pads at both ends must be symmetrical.
b. Pad spacing. Too large or small pad spacing will cause soldering defects. Therefore, ensure that the spacing between the component ends or pins and the pad is appropriate.
c. The remaining size of the pad, the remaining size of the component end or pin and the pad after the overlap must ensure that the solder joint can form a meniscus.
d. The width of the pad should be basically the same as the width of the component tip or pin.
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