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What is a PCB positioning hole
Circuit board positioning hole refers to determining the specific position of PCB via hole in the PCB design process, which is a very important link in the PCB design process. The function of the positioning hole is the processing reference when the printed circuit board is manufactured. There are various positioning methods for PCB positioning holes, mainly based on different positioning accuracy requirements. The positioning holes on the printed circuit board should be represented by special graphic symbols. When the requirements are not high, larger assembly holes in the printed circuit board can also be used instead.
In order to facilitate the fixing of the board during drilling and milling of the printed circuit board, as well as to facilitate online testing, many circuit board manufacturers hope that users design three non-metallized holes on the PCB. The positioning holes are usually designed as non-metallized holes with a drilling diameter of Envy mm or jealous. If the Smmo board is tight, at least two positioning holes should be placed, and they should be placed diagonally. If you make a jigsaw puzzle, you can regard the jigsaw puzzle as a PCB, as long as there are three positioning holes in the entire jigsaw puzzle. If the user does not place it, the circuit board manufacturer will automatically add it without affecting the circuit, or use the existing non-metalized holes in the board as positioning holes.
Positioning hole positioning method
Device hole interface devices and connectors are mostly plug-in components. The diameter of the through hole of the interposing device is 8-20 mil larger than the pin diameter, and the tin penetration is good during soldering. It should be noted that there is an error in the hole diameter of the circuit board factory. The approximate error is ±0.05mm. Each interval of 0.05mm is a kind of drill, and the diameter is above 3.20mm, and each interval of 0.1mm is a kind of drill. Therefore, when designing the device aperture, the unit should be converted to millimeters, and the aperture should be designed to be an integer multiple of 0.05. The board manufacturer sets the size of the drilling tool according to the drilling data provided by the user. The size of the drilling tool is usually 0.1～0.15mm larger than the formed hole required by the user. As little as possible. If it is a crimping device, the aperture should not be enlarged. It should be designed according to the data suggestion, and which crimping devices should be specified in the board manufacturing instructions, so that the circuit board manufacturer can try to control the error during the board manufacturing process and avoid some problems. Necessary trouble.
Drilling types are divided into metallized holes and non-metallized holes. There is heavy copper in the hole wall of the metallized hole, which can play a conductive role, which is represented by PTH. There is no sinking copper in the hole wall of the non-metallized hole, and it cannot play a conductive role, which is indicated by NPTH. The difference between the outer diameter and the inner diameter of the metallized hole diameter should be greater than 20mil, otherwise the welding ring of the pad is too small to be processed, and it is not conducive to welding. If conditions permit, it can be designed such that the aperture is the radius of the pad. The maximum drilling diameter of metallized holes is 6.35mm, and the maximum drilling diameter of non-metallized holes is 6.5mm. Metallized holes should not be designed on the contour line, and the distance between the edge of the hole and the contour line should generally be greater than 1mm. Heavy holes in cobalt holes are easy to damage the drill bit, so it should be avoided as much as possible. Holes that do not require soldering and have no electrical characteristics can be designed as non-metallized holes. Non-metallized holes do not need to be designed with pads. The edge of the hole is at least 1mmo away from the line or copper foil. The drilling can be divided into round holes and rectangular holes according to the shape. Most of the drilling holes are round holes. Rectangular holes are drilled several times by the drill according to the prescribed procedure. Therefore, it is best to design rectangular holes with a length of twice the width and a width of not less than 0.8mm. Rectangular holes should be designed as little as possible.
PCB positioning hole requirements:
The development of the PCB design industry has become mature, so the requirements for PCB positioning holes are also very complete. The positioning hole requirements are as follows.
1. At least two positioning holes must be provided on the diagonal of the veneer.
2. The standard aperture of the positioning hole is 3.2mm _+0.05mm.
3. The following preferred apertures can also be used for the veneers of different products of the enterprise: 2.8mm ± 0.05mm, 3.0mm ± 0.5mm, 3.5mm ± 0.5mm and 4.5mm ± 05mm. For different single boards of the same product (such as ZXJlO's DT board and PP board, etc.), if the PCB dimensions are the same, the location of the positioning holes must also be unified.
4. The positioning hole is a light hole, that is, a non-metalized through hole (except for the radio frequency board).
5. If the existing mounting holes (except the clasping mounting holes) meet the above requirements, there is no need to set additional holes.
Some common specifications and accuracy requirements for positioning holes:
1. The error range of the diameter of the positioning hole is generally within 0.01mm. How the PCB manufacturing room has a large error will cause poor contact of the probe and inaccurate alignment of the automatic mechanism of the interface connector.
2. Requirements for the diameter of the positioning hole: try to be less than 3mm, so that the positioning column will not be deformed, and it is too large to be easy to operate.
3. The distance of the positioning hole PCB network: above 1MM, so that the installation operation is not easy to short-circuit, and it will not cause damage to the product line.
4. Type of positioning hole: The positioning hole is generally a mechanical control that does not sink copper, so that it is not connected with the circuit on the board and has higher accuracy.
5. The layout of the positioning tank: it needs to be on the four corners or diagonals of the PCBA to form a multi-point surface positioning, the positioning is accurate, and the longer the distance, the better.
6. The distance between the positioning hole and the test point must be at least 2mm to prevent accidental short circuit during the test.
7. The distance between the positioning hole and the edge of the board is at least 2mm, which is not easy to break while ensuring the strength of the PCBA.
