I think that the engineers who have instalLED the machine or followed the PCB production line should have a deep understanding of why PCB printing needs positioning and anti dazzle. I won't say much about it here. Or you can say that the manufacturing process will solve the problem. The designers need not worry about it. Some cases must be solved immediately through the manufacturing process, but this is not necessarily the desired result. It involves cost, effICiency and other aspects. Generally, We should consider from the beginning of design, which is actually a part of DFA.
PCB positioning is usually based on the following two considerations.
First, it is easy to assemble, focusing on limit, also known as rough positioning, and has low requirements on the assembly and positioning accuracy of PCB;
The second is precise positioning, focusing on the assembly positioning accuracy of PCB.
No matter which positioning method is used, the uniqueness of PCB assembly position must be ensured when the assembly is completed, that is to say, it is necessary to prevent fooling and avoid reverse installation. (Except for special cases, such as PCB board arrangement and component arrangement, there is little symmetry in the connection function
Good positioning structure has the characteristics of anti dazzle, which can be quickly installed in place at one time to avoid the situation that only positioning is not anti dazzle, or only positioning is not anti dazzle.
The internal function PCBA usually uses the rough positioning of PCB board, which is mainly for the convenience of assembly. The requirement for positioning accuracy is not high, and the rebar position is usually used for positioning.
Although the fixed structure of the PCB board above is positioned, there is no foolproof structure. The shape 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 when rotated 180 degrees. Move the two fixing holes on one side inward for a certain distance, so that the fixing holes on the left and right sides are asymmetric. When the PCB is rotated 180 degrees, it can be installed in place, but the holes and studs can't be aligned, and the screws can't be installed at last. Although this treatment avoids reverse installation, it has trial and error costs, and it is not easy to find the foolproof characteristics, so the assembly efficiency is not high.
The PCB board edge shall be provided with notch anti dazzle, and the shape of the notch can be square or sEMIcircular, so that the notch anti dazzle feature can be easily found, and only the correct assembly direction can be installed in place at one time.
It should be specially pointed out that in the case that the positioning feature and anti fool feature are designed separately, anti fool feature should be guaranteed before positioning, that is, in the PCB assembly process, the anti fool function time should be earlier than positioning, which is conducive to the improvement of correct assembly efficiency, for example, making the following anti fool feature higher.
At the same time, we should also pay attention to avoid only being foolproof and not positioning or not positioning enough,
The precision positioning of PCB is often used in the situation where the space is compact or the positioning accuracy of some components on the PCB is required to be high, and positioning posts are usually used for positioning.
Since the DC external interface and USB external interface need to locate the corresponding openings on the box, there is a requirement 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 positioning posts and the board holes is 0.05, and the number of positioning posts is generally 2, which should not be too SMAll. The larger the size tolerance of the PCB board and the positioning posts is, the higher the requirements for the dimensional tolerance of the PCB board and the positioning posts are, resulting in over positioning, and the more difficult the assembly is, To avoid over positioning, one of the plate holes can also be designed as a round hole
It should be pointed out that for PCB boards with symmetrical shape, positioning posts and positioning holes are generally arranged diagonally, and at least one positioning hole's X value or Y value is larger or smaller than another positioning hole's X value or Y value, that is, the positioning holes of PCB boards are not symmetrical along the central point of PCB boards, so as to avoid reverse mounting of PCB boards. The correct design of the positioning column should play the role of positioning and anti stupidity at the same time.
Why is the positioning with positioning column more accurate than that with reinforcement?
First, start with the manufacturing dimensional tolerance of PCB.
The following is a calculation example of the position accuracy of the intermediate components of the PCB in the following two positions (the following only takes the position accuracy calculation in the length direction as an example, but not in the width direction).
The length of PCB board is 50 ± 0.2, the spacing between locating bars is 50.4 ± 0.2, and the spacing between PCB board and locating bars is 0.2.
Through calculation, the positioning accuracy of intermediate components of PCB board is ± 0.4, and the larger the size of PCB board is, the lower the positioning accuracy is.
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.
Through calculation, the positioning accuracy of PCB board is about ± 0.2, and 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 controlled.
In a word, the positioning of reinforcement is large in size, and the accuracy is difficult to control, so the positioning error is large; The positioning column adopts small size positioning, and the accuracy is easy to control, so the positioning error is small.
Requirements for positioning hole of circuit board:
At least two locating holes shall be set at the opposite corners of the PCB, and the over position problem shall be noticed.
Second, the positioning hole is a smooth hole, that is, a nonmetallic through-hole, which can connect it to the plate with high accuracy, and its diameter error is generally within 0.1mm.
The edge distance between the positioning hole and the plate shall be at least 2mm, which is not easy to crack under the condition of ensuring the PCBA strength
In fact, the above is not only the positioning of PCB, but also the positioning and anti dazzle of other parts.