In PCB design, pad is a very important concept, which must be familiar to PCB engineers. However, although familiar, many engineers know little about pads.
Today, let’s learn about the types of lower pads and the design standards of pads in PCB design. Pad, the basic unit of surface mount assembly, is used to form the land pattern of circuit board, that is, various pad combinations designed for special component types.
Pad is a partially conductive pattern used for electrical connection, device fixation, or both.
1、 Common pads
1. Square pad
When the components on the printed board are large and few, and the printed wire is simple, it is often used. When making PCB by hand, this kind of pad is easy to realize.
2. Circular pad
It is widely used in single and double-sided printed boards with regular arrangement of 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.
4. Polygonal pad
It is used to distinguish the pads with similar outer diameter and different pore diameter, which is convenient for processing and assembly.
5. Oval pad
The pad has enough area to enhance the peel resistance, and is commonly used in dual in-line devices.
6. Open pad
It is often used to ensure that the pad hole repaired manually will not be sealed by solder after wave soldering.
2、 Special pad
1. Quincunx pad
Quincunx pads are usually used for large via grounding. There are several reasons for this design:
1) The fixing hole needs to be metallized and connected with GND. If the fixing hole is fully metallized, it is easy to block the hole during reflow soldering.
2) The use of internal metal screw holes may cause the grounding to be in a bad state due to installation or repeated disassembly and assembly. The plum blossom hole pad can ensure good grounding no matter how the stress changes.
2. Cruciform pad
Cruciform pad is also called hot pad, hot air pad, etc. Its function is to reduce the external heat dissipation of the pad during welding, so as to prevent false soldering or PCB peeling caused by excessive heat dissipation.
1) When your pad is ground. 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 a reflow welder, the cruciform pad can prevent the PCB from peeling (because it needs more heat to melt the solder paste).
3. Teardrop pad
When the routing of pad connection is thin, it is often used to prevent pad peeling and disconnection between routing and pad. This pad is often used in high frequency circuits.
Design standard of pad in PCB design
1、 Design standard for shape and size of PCB pad
1. The minimum single side of all pads shall not be less than 0.25mm, and the maximum diameter of the whole pad shall not be greater than 3 times of the element aperture.
2. The distance between two pad edges shall be greater than 0.4mm as far as possible.
3. In case of dense wiring, oval and oblong connecting plates are recommended. The diameter or minimum width of single panel pad is 1.6mm; The weak current line pad of double-sided board only needs to increase the hole diameter by 0.5mm. Too large pad is easy to cause unnecessary continuous welding. The pad with hole diameter more than 1.2mm or pad diameter more than 3.0mm shall be designed as diamond or quincunx pad.
4. For plug-in components, in order to avoid copper foil breaking during welding, and the single-sided connecting plate shall be completely covered with copper foil; The minimum requirements of double panel shall be filled with tears.
5. All machine plug-in parts shall be designed as drip pads along the bend direction to ensure full solder joints at the bend.
6. Chrysanthemum shaped pads shall be used for pads on large area copper sheets to avoid false soldering. If there is a large area of ground wire and power line area on the PCB (the area is more than 500 mm2), local windows shall be opened or designed as grid filling.
As shown in the figure:
2、 PCB pad via size standard
The inner hole of the pad is generally not less than 0.6mm, because the hole less than 0.6mm is not easy to process during die punching. Generally, the metal pin diameter plus 0.2mm is used as the inner hole diameter of the pad. For example, when the metal pin diameter of the resistance is 0.5mm, the inner hole diameter of the pad corresponds to 0.7mm, and the pad diameter depends on the inner hole diameter.
3、 Key points of reliability design of PCB pad
1. Symmetry. In order to ensure the balance of surface tension of molten solder, the pads at both ends must be symmetrical.
2. Pad spacing, too large or too small pad spacing will cause welding defects. Therefore, ensure that the spacing between component ends or pins and pads is appropriate.
3. The remaining size of the pad, the remaining 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 shall be basically consistent with the width of the component end or pin.
4、 Requirements of PCB manufacturing process on pad
1. If there are no plug-in components connected at both ends of the chip components, test points shall be added, and the diameter of the test points shall be equal to or greater than 1.8mm, so as to facilitate the test of the on-line tester.
2. If the IC pin pad with dense pin spacing is not connected to the hand plug-in pad, it needs to add a test pad. If it is a patch IC, the test point cannot be placed in the patch IC silk screen. The diameter of the test point is equal to or greater than 1.8mm to facilitate the test of the on-line tester.
3. If the pad spacing is less than 0.4mm, white oil shall be paved to reduce continuous welding when passing the wave peak.
4. Both ends and ends of the patch element shall be designed with tin lead. The width of tin lead is recommended to be 0.5mm, and the length is generally 2 and 3mm.
5. If the single panel has hand welded components, the tin bath shall be opened in the opposite direction to the tin passing direction, and the width depending on the size of the hole is 0.3mm to 1.0mm.
6. The spacing and size of conductive rubber keys shall be consistent with the actual size of conductive rubber keys. The connected PCB board shall be designed as a gold finger, and the corresponding gold plating thickness shall be specified.
7. The size and spacing of the pad shall be basically consistent with the size of the patch element.