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PCB problems, even zebra strips can cause problems
2021-10-09
View:17
Author:Aure

PCB problems, even zebra strips can cause problems



 

I originally thought that [Zebra Strips] should be a simple part, but this incident was completely defeated by it. The reason is that the company has a product designed with an independent "Navigation key".


I don’t know why the design engineer chose to use the "Banma conductive tape" to connect the line between the main button and the navigation button instead of designing the navigation button and the main button on the same button board. There may be some height differences. Relationship (I guess, because the test is no longer available)!


This was originally not a big deal design, but the problem happened. The phenomenon is that after pressing some buttons on the navigation key, it will be linked to other buttons. If the linkage is also in the navigation key The up, right, down, and left buttons may be caused by poor mechanism design, which caused the malfunction of the adjacent button mechanism. But the strange thing is that the buttons on the main keyboard are connected with the panma in the middle. The conductive strip.


In order to first distinguish this phenomenon from electrical or mechanical problems, we directly removed the plastic and rubber buttons on the navigation buttons, exposed the button wires directly, and then used a conductive rod to test the buttons. , As a result, the key interlocking phenomenon still occurs in this way, so the interlocking problem caused by the key mechanism can be eliminated first.

 


PCB problems, even zebra strips can cause problems



Next, there is only the problem of the circuit board and the zebra conductive tape. First, we will cross-validate the zebra conductive tape. It is found that the button linkage phenomenon follows the zebra conductive tape. In order to make the production line resume production immediately, use it urgently. A simple jig is made for the existing product. It is hoped that this jig can screen out the bad zebra conductive tape, but the defective product cannot be detected. Remove the zebra conductive tape and take it directly for insulation resistance and micro-current testing. There is no problem, this is embarrassing! And no problems were found on the circuit board. its stuck!


 

I cut the zebra conductive tape and inspected it, and only found that the conductive tape had a cutting deviation of the conductive strip. It should be within a reasonable deviation range. How can I continue!



Later, I accidentally discovered that the gap between the gold fingers on the circuit board and the zebra conductive tape seemed a little small, and it seemed that there was no gap between at least two conductive strips as required, because the zebra conductive tape may be cut. The deviation of a conductive strip (0.18mm), mechanism assembly (upper and lower PCBA board) and production error should also be 0.2mm, and the total tolerance should be at least 0.38mm, while the actual measurement result of the minimum distance between gold fingers is only about 0.3mm. Obviously insufficient! Query the design on the PCB Gerber should have a gap of 0.5mm, because there are vias, so the gold finger is slightly off, which is really a damn error.


Review the current gold finger design, the contact pad width is 0.8mm, the gap is 0.5mm, the pitch of the zebra conductive tape is 0.18mm, and the contact pad width should only be designed to be 0.64mm [=0.18mmx3 (conductive strip) + 0.1mm (tolerance) ], you can ensure that at least three conductive strips are in contact with each contact pad, plus some allowable values, the width of the golden finger contact pad is changed to 0.7mm, and the gap is 0.6mm. This problem should be completely resolved. In addition, whether the positions of the four vias should be moved slightly, or the PCB manufacturer should pay attention to the width of the gold finger contact pads should also solve this problem, but the key point is how to prevent the search for alternative materials (2nd source) is the best policy.


By the way, you can also consider changing the picth of Banma conductive tape from the current 0.18mm to 0.10mm.