Design standard for shape and size of the pad in PCB design
First, the type of pad
In general, the pads can be divided into seven categories.
1. Square pads -- when the components on the printed circuit board are large and small, and the printed wiring is simple, it is usually adopted. When making PCB by hand, it is easy to use this kind of pad.
2. Circular pads -- widely used in the regular arrangement of components in single and double-sided printed boards. If the density of the boards allows, the pad can be larger, so as not to fall off during soldering.
3. Island-shaped pad -- the connection between the pads is integrated. Often used in vertical irregular arrangement installation. For example, this kind of pad is often used in the recorder.
4. Teardrop pad -- it is often used when the wire connecting the pad is small, so as to prevent the peeling of the pad and the wire breaking off from the pad. This kind of pad is commonly used in high frequency circuits.
5. Polygonal Pads -- it is used to distinguish the pad with a similar external diameter and a different aperture, so as to facilitate processing and assembly.
6. Oval Pad - this type of pad has sufficient area to enhance the anti-stripping ability and is often used for double-row straight insert devices.
7. Open-ended pad -- it is often used to ensure that after wave soldering, the hole of manual resoldering pad is not sealed by soldering tin.
Second, design standard of shape and size of the pad in PCB design
1. The minimum unilateral diameter of all pad shall be no less than 0.25mm, and the maximum diameter of the entire pad shall be no more than 3 times of the element 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 long round connection plates. The diameter or minimum width of the single-panel pad is 1.6mm. Double panel lines of weak current welding plate in a hole with 0.5 mm diameter, bonding pad too easy to cause unnecessary even welding, aperture is greater than 1.2 mm or welding plate is greater than 3.0 mm diameter welding plate shall be designed for the diamond or quincunx welding pad.
4. For plug-in components, in order to avoid the occurrence of copper foil breakage during soldering, copper foil shall be completely covered on the single-side connection plate; And double boards minimum requirement should fill tear drop.
5. The organic inserted parts shall be designed as drip pad along the bend foot direction to ensure full welding points at the bend foot.
6. The pad on a large area of copper shall be chrysanthemum-shaped, not soldering. If there is a large area of ground wire and power line area (more than 500 mm square) on the PCB, it should be partially solder mask opening or designed for Nets filling. As shown in figure:
Third, the PCB manufacturing process requirements of the pad
1. The test points should be added without connecting the two ends of the patch components to the inserted components. The diameter of the test points is equal to or greater than 1.8mm, so as to facilitate the online test of the tester.
2. The IC pad with a tight pitch is required to add a test pad if it is not connected to the hand-insertion pad. For a patch IC, the test point cannot be placed inside the patch IC's silk screen. The diameter of the test point is equal to or greater than 1.8 mm to facilitate in-line tester testing.
3. If the distance between the pads is less than 0.4mm, white solder mask shall be applied to reduce the continuous welding during the over-peak soldering.
4. The tip and the end of SMD components should be designed with lead tin. The width of the lead tin is recommended to use 0.5mm wire, and the length is generally 2, 3mm.
5. If there is a hand soldering element in a single board, the tin groove should be removed. The direction is opposite to the direction of the tin passage. The width of the hole should be between 0.3mm and 1.0mm
6. The spacing and size of the conductive rubber keys should be consistent with the actual size of the conductive rubber keys. The PCB board that is 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 apds should be exactly the same as the size of the SMD components.