In PCB design, PAD is a very important concept, PCB engineers must be familiar with it. However, although familiar, many engineers are only half – baked in the knowledge of pad. The types and design standards of pads in PCB design will be introduced in detail below.
Type of pad
Generally speaking, pads can be divided into 7 categories, which are distinguished according to their shapes as follows:
Square pad — the components on the printed board are large and few, and the printed wire is simple to use. When making PCB by hand, it is easy to use this type of pad.
Circular pad – widely used in single and double-sided printed boards with a regular arrangement of components. If the density of the board allows, the pad can be larger, welding will not fall off.
Island pad – the connection between the pad and the pad is integrated. Often used in vertical irregular alignment installation. For example, cassette recorders often use this type of pad.
Teardrop type pad – when the pad is connected to a thin wire, it is often used to prevent the pad from peeling and disconnect the wire from the pad. This pad is commonly used in high-frequency circuits.
Polygonal pad – used to distinguish between pad with close outer diameters and different apertures, easy to process and assemble.
Oval pad – these pads have enough area to enhance strip-resistance and are often used in double-line inserts.
Open-end pad – to ensure that after wave soldering, the hole of the manually repaired soldering pad is not closed by soldering tin.
Design standards for the shape and size of pads in PCB design
- The minimum side diameter of all pads shall not be less than 0.25mm, and the maximum diameter of the whole pads shall not be more than 3 times the aperture of the element.
- The spacing between the edges of the two pads shall be greater than 0.4mm as far as possible.
- In the case of dense wiring, an elliptic and oblong connecting pad are recommended. The diameter or minimum width of single-sided board pad shall be 1.6mm; Double-sided board weak-current circuit pads only need to add 0.5mm in hole diameter. Oversized pads may cause unnecessary welding. Pads with an aperture over 1.2mm or diameter over 3.0mm should be diamond or plium-shaped ones.
- For plug-in components, in order to avoid copper foil breakage during welding, and the single-sided connecting pad should be completely covered with copper foil, And double-sided minimum requirement should fill the teardrop.
- All organic insert parts shall be designed as Teardrop type pads along the bend direction to ensure full welding spots at the bend.
- Daisy-shaped pads should be used on large areas of copper skin to avoid virtual welding. If there is a large area of ground and power cord on the PCB (more than 500 square mm), it should be partially solder-mask opening or designed to fill the grid.
Requirements of PCB manufacturing process for pads
- Test points shall be added to the end of SMT components that are not connected with inserted components. The diameter of the test point shall be equal to or greater than 1.8mm, so as to facilitate the online tester test.
- If the IC footpad with dense foot spacing is not connected to the plug-in pad, a test pad shall be added. For example, if it is an SMT IC, the test point shall not be placed into the SMT IC screen printing. The test point diameter is equal to or greater than 1.8mm for the online tester test.
- If the spacing between pads is less than 0.4mm, white oil shall be applied to reduce the continuous welding during the wave peak.
- Tin primer shall be designed at both ends and ends of SMT components. The width of tin primer shall be recommended to use 0.5mm wide, and the length shall generally be 2 or 3mm.
- If there is a hand welding element on a single panel, the tin slot should be removed in the opposite direction from the tin passing direction. The size of the width of the apparent hole should be from 0.3mm to 1.0mm.
- The spacing and size of conductive rubber keys shall be in accordance with the size of the actual conductive rubber keys. The PCB connected with the conductive rubber keys shall be designed as Goldfinger and the corresponding gold-plating thickness shall be specified.
- The size and spacing of the pads shall be basically the same as that of the SMT components.