PCB Tech - Why do pcb boards need positioning and foolproof

The positioning of printed PCBs is usually based on the following two considerations.

First, it is easy to assemble, pay attention to the limit, also known as coarse positioning, and does not require high precision in the assembly and positioning of the PCB board;

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

Regardless of the positioning method, when the assembly is completed, the uniqueness of the assembly position of the PCB board must be guaranteed, that is, to prevent foolishness and avoid reverse installation. (Except for special cases, such as PCB board layout and component arrangement, and the connection function has very little symmetry

The good positioning structure itself has the characteristics of foolproof, which can be quickly installed in place at one time, avoiding the situation that only positioning but not foolproof, or only positioning but not foolproof.

Rough positioning

The internal function PCBA usually adopts the rough positioning of the PCB board, and its positioning is mainly for the convenience of assembly, and the positioning accuracy is not high, and the rib position is usually used for positioning. As shown in the figure:

Although the above PCB board fixing structure is positioned, there is no foolproof structure. The shape of the PCB board and the fixing holes are symmetrical along the center of the PCB board, and the PCB board can still be assembled by rotating 180 degrees.

As shown above, move the two fixing holes on one side inward a certain distance to make the fixing holes on the left and right sides asymmetrical. When the PCB board is rotated at 180 degrees, although it can be installed in place, the board holes and studs are not aligned. In the end, the screws cannot be screwed. Although this treatment avoids reverse installation, it has trial and error costs, foolproof features are not easy to find, and assembly efficiency is not high.

The side of the PCB board above is made of notches to prevent fools. The shape of the notches can be square or semicircular. In this way, the features of notches are easy to find, and only the correct assembly direction can be installed in place at one time.

It should be particularly pointed out that for this situation where the positioning feature and the fool-proof feature are designed separately, the fool-proof feature should be ensured before positioning. That is to say, in the PCB board assembly process, the fool-proof action time is earlier than the positioning., This is conducive to the improvement of correct assembly efficiency, for example, the following fool-proof features are made higher.

At the same time, please also pay attention to avoiding only foolproof without positioning or insufficient positioning, as shown in the figure below.

accurate locating

The precise positioning of the circuit board is often used in occasions where the space is tight or the positioning accuracy of certain components on the circuit board is high, and the positioning post is usually used for positioning. As shown in the figure:

Since the DC external interface and the USB external interface need to locate the corresponding openings on the box, there are requirements for the positioning accuracy of the PCB board. Here, two positioning posts are used for positioning to ensure the positioning accuracy of the PCB board in the XY direction. The gap between the post and the board hole is 0.05, and the number of positioning posts is generally two. It should not be too small. If too large, the higher the dimensional tolerance of the PCB board and the positioning posts, the more difficult it will be to assemble. To avoid over positioning, one of the plate holes can also be designed as a round hole, as shown in the figure below, and the size of the positioning column should not be too small, too small, or without strength, and the distance between the two is generally between 1-2mm, and it should not be too large., If the size is too large, it can be designed as a crossbar, as shown in the figure below.

It should be pointed out that for symmetrical PCB boards, the positioning pillars and positioning holes are generally arranged diagonally, and at least one positioning hole has an X or Y value larger or smaller than the X or Y value of the other positioning hole., That is, the positioning holes of the PCB board are not symmetrical along with the center point of the PCB board so that the PCB board can be prevented from being reversed. The correct design of the positioning column should play the role of positioning and be foolproof at the same time.

Why is it more accurate to use positioning posts than to use ribs?

Start with the manufacturing dimensional tolerances of the PCB board.

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

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

After calculation, the positioning accuracy of the middle components of the PCB board 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 diameter of the positioning column is 2±0.05, the diameter of the positioning hole is 2.1±0.05, and the distance between the positioning column and the positioning hole is 0.05.

After calculation, the positioning accuracy of the PCB board is about ±0.2, and the positioning accuracy is less affected by the size of the PCB board. As long as the relative size of the positioning column and the positioning hole is controlled.

In a word, the rib positioning adopts large-size positioning, the accuracy is difficult to control, so the positioning error is relatively large; while the positioning column positioning adopts the small-size positioning, the accuracy is easy to control, so the positioning error is small.

Circuit board positioning hole requirements:

Set at least two positioning holes at the opposite corners of the PCB board, and pay attention to the over-position problem.

The second is that the positioning hole is a light hole type, that is, a non-metalized through-hole, which can be connected to the plate with high accuracy, and its diameter error range is generally within 0.1mm.

The distance between the positioning hole and the board is at least 2mm, and it is not easy to break under the condition of ensuring the strength of the PCBA.

The above is actually not only the positioning of the PCB board but also the positioning and foolproofness of other parts.

By calling the library, the PCB model is created in Creo, without the need for manual modeling in the end.