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In PCB design, pad is a very important concept, PCB engineers must be familiar with it. However, although familiar, many engineers have a little knowledge of pads. The following will introduce in detail the types and design standards of pads in PCB design.
1. Types of pads
In general, pads can be divided into 7 categories, which are distinguished by shape as follows
Square pads-more used when the printed circuit board components are large and few, and the printed wires are simple. It is easy to use this kind of pad when making a PCB by hand.
Circular pads-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.
Island-shaped pad-the connection between the pad and the pad is integrated. It is often used in vertical irregular arrangement installation. For example, such pads are often used in tape recorders.
Teardrop pads-often used when the traces connected to the pads are thin to prevent the pads from peeling and the traces are disconnected from the pads. Such pads are commonly used in high-frequency circuits.
Polygonal pads-used to distinguish pads with close outer diameters but different apertures for easy processing and assembly.
Oval pad-This pad has enough area to enhance the anti-stripping ability, and is often used in dual in-line devices.
Open-shaped pads-in order to ensure that after wave soldering, the pad holes that are manually repaired are not sealed by solder.
2. Design standards for the shape and size of pads in PCB design
1. The minimum single side of all pads is not less than 0.25mm, and the maximum diameter of the entire pad is not more than 3 times the component aperture.
2. Try to ensure that the distance between the edges of the two pads is greater than 0.4mm.
3. In the case of dense wiring, it is recommended to use oval and oblong connection plates. The diameter or minimum width of the single-panel pad is 1.6mm; the weak-current circuit pad of the double-sided board only needs to add 0.5mm to the hole diameter. If the pad is too large, it will easily cause unnecessary continuous soldering. The diameter of the hole exceeds 1.2mm or the diameter of the pad. The pads over 3.0mm should be designed as diamond or quincunx pads.
4. For plug-in components, in order to avoid copper foil breaking during soldering, and the single-sided connecting plate should be completely covered with copper foil; the minimum requirement for double-sided panels should be filled with teardrops.
5. 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.
6. The pads on the large-area copper skin should be chrysanthemum-shaped pads, not to be soldered. If there is a large area of ground and power lines on the PCB (with an area of more than 500 square millimeters), the window should be partially opened or designed as a grid filling.
Third, PCB factory manufacturing process requirements for pads
1. Test points should be added to the two ends of the chip components that are not connected to the plug-in components. The diameter of the test point is equal to or greater than 1.8mm to facilitate the on-line tester test.
2. If the IC foot pads with dense pin spacing are not connected to the hand plug-in pads, test pads need to be added. For SMD ICs, the test points cannot be placed in the SMD IC silk screen. The diameter of the test point is equal to or greater than 1.8mm to facilitate online tester testing.
3. If the distance between the pads is less than 0.4mm, white oil must be applied to reduce continuous soldering when the wave crest is exceeded.
4. The two ends and ends of the SMD component should be designed with lead-tin. The lead-tin width is recommended to use 0.5mm wire, and the length is generally 2 or 3mm.
5. If there are hand-soldering components on the single panel, remove the tin bath, the direction is opposite to the soldering direction, and the width of the viewing hole is 0.3MM to 1.0MM.
6. The spacing and size of the conductive rubber buttons should be consistent with the actual size of the conductive rubber buttons. The PCB circuit board connected to this should be designed as a gold finger, and the corresponding gold plating thickness should be specified.
7. The size and spacing of the pad should be basically the same as the size of the patch component.
