In PCB design, pad is a very important concept, PCB engineers must be familiar with it. However, although familiar, many engineers know little about pads. Today, ban Er Mei takes you to understand the types of pads and the design standards of pads in PCB design.
A partially conductive pattern used for electrical connection, device fixation, or both.
Types of PCB pads
1、 Common pads
1. Square pad: it is often used when the components on the printed board are large and few, and the printed wire is simple. When the PCB is made by hand, it is easy to use this pad.
2. Circular pad – widely used in single – and double-sided PCB with regular components. If the density of the plate allows, the pad can be larger, so that it will not fall off during welding.
3. Island pad the connection between the pad and the pad is integrated. It is often used in vertical irregular arrangement installation. For example, this kind of pad is often used in radio recorder.
4. Polygonal pad – used to distinguish pads with similar outer diameters but different apertures for easy processing and assembly.
5. Oval pad – this pad has sufficient area to enhance peel resistance and is commonly used in dual in-line devices.
6. Open pad: it is commonly used to ensure that the pad hole of manual repair welding will not be sealed by solder after wave soldering.
2、 Special pad
1. Plum blossom pad
The plum blossom pad is usually used in the grounding position of large via hole. There are several reasons for this design:
1) The fixed hole needs to be metallized and connected with GND. If the fixed hole is fully metallized, it is easy to block the hole during reflow soldering.
2) If the inner metal screw hole is used, the grounding may be in a bad state due to installation or multiple disassembly and other reasons. No matter how the stress changes, good grounding can be ensured by adopting plum blossom hole pad.
2. Cruciform pad
Cruciform pad is also called hot pad and hot air pad. Its function is to reduce the pad’s outward heat dissipation during welding, so as to prevent false soldering or PCB peeling caused by excessive heat dissipation.
1) When your pad is grounded. Cruciform can reduce the area of connecting ground wire, slow down the heat dissipation speed and facilitate welding.
2) When your PCB needs a machine patch and is a reflow welder, cruciform pads prevent PCB peeling (because more heat is needed to melt the solder paste).
3. Teardrop pad
When the wiring of pad connection is thin, it is often used to prevent pad peeling and disconnection from pad. This type of pad is often used in high frequency circuits.
Design standard of pad in PCB design
1、 PCB pad shape and size design standard:
1. Call PCB standard package library.
2. The minimum diameter of one side of the pad shall not be less than 0.25 mm, and the maximum diameter of the whole pad shall not be greater than 3 times of the element aperture.
3. Try to ensure that the distance between the two pad edges is greater than 0.4mm.
4. Pads with an aperture of more than 1.2 mm or pad diameter of more than 3.0 mm shall be designed as diamond or plum shaped pads.
5. In the case of dense wiring, oval and oblong connection plates are recommended. The diameter or minimum width of the single panel pad is 1.6 mm; for the weak current circuit pad of the double-sided board, only the hole diameter plus 0.5 mm is needed. If the pad is too large, it is easy to cause unnecessary continuous welding.
2、 PCB pad via size standard:
The inner hole of the pad is generally not less than 0.6 mm, because the hole less than 0.6 mm is not easy to process when punching. Generally, the diameter of the metal pin plus 0.2 mm is taken as the diameter of the inner hole of the pad. For example, when the diameter of the metal pin of the resistance is 0.5 mm, the diameter of the inner hole of the pad is 0.7 mm, and the diameter of the pad depends on the diameter of the inner hole.
3、 The key points of reliability design of PCB pad are as follows
1. In order to ensure the balance of surface tension of molten solder, the pads at both ends must be symmetrical.
2. If the pad spacing is too large or too small, it will cause welding defects. Therefore, it is necessary to ensure that the distance between component end or pin and pad is appropriate.
3. The residual size of the pad, the residual size after the component end or pin is overlapped with the pad, must ensure that the solder joint can form a meniscus.
4. The pad width should be basically the same as the width of component end or pin.