PCB Fabrication

How to deal with the occurrence of PCB micro short circuit

by:A-TECH      2021-03-25
In the process of PCB use, there will be occasional micro short circuits and short circuits. When these phenomena occur, we have to find a way to solve them. Next, we will introduce the solutions for micro short circuits and short circuits in PCBs.  In the process of circuit board testing, sometimes there is no problem when we first test it, but the second time we use 300V high voltage test, a short circuit occurs. For the third time, it was re-tested with ordinary low voltage, and the short-circuit board measured for the second time was also determined to be short-circuited. Use the resistance profile of a multimeter to measure that the pad point between the two wires of the short-circuit point is a short circuit, and the average resistance value is 6.7 ohms. It should be regarded as a short circuit rather than a micro short circuit. Then use a high-power magnifying glass to check the short-circuit phenomenon, and the short-circuit point cannot be accurately detected. This should be the reason why the finished product has solder mask ink.   Therefore, we can determine that the edge is etched by the side etching, and then the edge is broken by the grinding process to form a bridge on the wire, and then printed with green oil to make the bridge not a short-circuit bridge. In this way, the first test is low voltage and it is naturally impossible to detect a bridge short circuit. For the second time, the high voltage is first detected as a micro short circuit, and then the high voltage breakdown welding (the copper wire can be welded to form a short circuit with a very small power). The test machine manufacturer can increase the allowable power of the short circuit test current. It is difficult for us to see this phenomenon if the bridge contact can be blown). The third low-voltage computer tester can test the short-circuit point, and its resistance value is only an average of 6.7 ohms.   We mentioned that the 300V high-voltage board tester also complained about short circuits. Checking the returned bad short circuit board showed that the bad short circuit board measured for the third time was the same reason. The solder mask (green oil) screen printing will aggravate the insulation of the copper wire connected to it, making it impossible to measure with a high-voltage computer tester, and short-circuit the bridge during the testing process of handling, assembly, wave soldering and semi-finished products. Sometimes we use the low-voltage tester to measure the short-circuit board by bending and tapping and sometimes the short-circuit will disappear. When there is no high-voltage computer board tester, I don’t worry. If the ok board is tested again, there will be a short circuit and a bad board. Most of the problem lies in the side etching of etching and the front line of pattern production and solder mask ink. The first two processes are more difficult. The main reason is to ensure the line width and line spacing during the pattern production to prevent some lines from being too close. , Ensure the quality of side etching during etching. Reduce the phenomenon of micro short circuit during high voltage test. The easiest and most economical method is to purchase a new plate grinding machine or to improve an existing plate grinding machine, using swing brushing and low-mesh nylon brush or volcanic ash buffing, and then high-pressure washing. Of course, it is the most effective washing section. Fortunately, there is a filter device, and the plate grinding machine can remove the copper wire and the protruding edge formed by the side etching. In addition, you should recommend buying a high-voltage machine when buying a new computer test machine, so that you can reduce the chance of micro-shorts and short-circuits on the circuit board.
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