Why does pcb board need to position and prevent stay

I think engineers who have installed the printed circuit board or talked to the production line should have a deep understanding of this problem. I will not say more here, or you will say that the manufacturing process will solve the problem, and designers need not worry about it. Some situations must be solved immediately through the manufacturing process, but this is not necessarily the desired result, which will involve cost, efficiency and other aspects. In general, we should start with the design, which is part of the DFA.

The positioning of printed circuit boards is usually based on two considerations.

First, the assembly is convenient, pay attention to the limit, also known as rough positioning, the PCB board assembly positioning accuracy requirements are not high;

The second is accurate positioning, focusing on the PCB board assembly positioning accuracy.

No matter what kind of positioning method, when the assembly is completed, it is necessary to ensure that the PCB board assembly position is unique, that is, to do anti-stay, avoid reverse installation. (Except in special cases, such as PCB board discharge and component arrangement, and with minimal symmetry in the switching function

Good positioning structure, itself has the characteristics of anti-stay, can be quickly installed in place to avoid the occurrence of only positioning without anti-stay, or only positioning without anti-stay situation.

Rough positioning

Internal function PCBA usually adopts the rough positioning of PCB board, its positioning is mainly for the convenience of assembly, the positioning accuracy is not high, usually adopts the rib position to locate. As shown below:

The PCB board fixed structure above is positioned, but there is no anti-dumbness structure. The appearance and fixed hole of the PCB board are symmetrical along the center point of the PCB board, and the PCB board can still be assembled by rotating 180 degrees.

As shown above, move the two fixed holes on one side inward for a certain distance, so that the fixed holes on the left and right sides are not symmetrical. Although the PCB board can be installed in place when it is rotated at 180 degrees, the holes and studs cannot be aligned, and finally the screws cannot be fastened. Although this treatment avoids the reverse installation, it has trial and error costs, anti-stay characteristics are not easy to find, and the assembly efficiency is not high.

The PCB board on the top of the edge of the notch to prevent stay, the shape of the notch can be made square or semi-circular, so that the features of the notch to prevent stay are easy to find, and only the correct assembly direction, can be installed in place.

It should be pointed out in particular that, for such positioning features and anti-stay features designed separately, anti-stay should be guaranteed before positioning. That is to say, in the process of PCB board assembly, anti-stay action time should be earlier than positioning, which is conducive to the improvement of correct assembly efficiency. For example, the following anti-stay features should be higher.

At the same time, it is also necessary to pay attention to avoid only preventing not positioning or positioning enough, as shown in the following figure.

Precise positioning

Precision positioning of the circuit board is often used in the compact space or some components on the circuit board positioning accuracy requirements, usually using the positioning column to locate. As shown below:

Since DC external interface and USB external interface need to locate the corresponding opening on the box, the positioning accuracy of PCB board is required. Two positioning columns are used here to ensure the positioning accuracy of PCB board in the XY direction. The gap between positioning column and plate hole is 0.05, and the number of positioning columns is generally 2, which should not be too small. The larger the size tolerance of PCB board and positioning column, the higher the requirements, resulting in positioning, the more difficult assembly, in order to avoid positioning, can also be designed into one of the plate holes into a round hole, as shown below, and the size of the positioning column should not be too small, too small, no strength, between the two generally between 1-2mm, should not be too large, too large, if the size is larger, can be designed into a cross bar, See the picture below.

It should be pointed out that for PCB boards with symmetrical appearance, the positioning posts and positioning holes are generally arranged diagonally, and the X or Y value of at least one positioning hole is larger or smaller than that of the other positioning hole, that is, the positioning hole of the PCB board is not symmetrical along the center point of the PCB board, so as to avoid PCB board backloading. The correct design of positioning column should play the role of positioning and preventing staying.

Why is the positioning column more accurate than the positioning bar?

Start with the PCB board manufacturing size tolerance.

The following is a calculation example of the position accuracy of the intermediate component of PCB board in the following two positions (the position accuracy calculation of the length direction is only taken as an example, and the width direction is not calculated).

The length of PCB board is 50±0.2, the spacing of positioning bars is 50.4±0.2, and the spacing between PCB board and positioning bars is 0.2.

By calculation, the positioning accuracy of PCB board intermediate component is ±0.4, and the larger the PCB board size, the lower the positioning accuracy.

The distance between the positioning column and the positioning hole is 8±0.05, the positioning column diameter is 2±0.05, the positioning hole diameter is 2.1±0.05, the spacing between the positioning column and the positioning hole is 0.05.

According to the calculation, the positioning accuracy of PCB board is about ±0.2. The positioning accuracy is less affected by the size of PCB board, as long as the relevant size of positioning column and positioning hole is well controlled.

In short, the rib positioning is the use of large size positioning, precision is difficult to control, so the positioning error is larger; The positioning column positioning is small size positioning, precision is easy to control, so the positioning error is small.