PCB Tech - Improvement measures for PCB micro short circuits

Improvement measures for PCB micro short circuits

Some finished boards with micro-shorts and short-circuit phenomena cannot be tested with ordinary low-voltage computer testers to ensure that they will not flow into the hands of customers to complain to customers. Usually, circuit board manufacturers push this problem to the computer testing machine supplier, thus promoting the development of high-voltage computer testing machine. However, 100% pass rate cannot be guaranteed with a high-pressure testing machine.

Sometimes the circuit board test is all OK when the low voltage test circuit is used for the first time, and the 300V high voltage test is used for the second time to test for a short circuit. 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. Therefore, it should be regarded as a complete short circuit instead of 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 (it should be the reason that the finished product has solder mask ink).

From the test process and its resistance value, it can be determined that the edge is etched by the side etching, and then the edge is broken by the grinding process to form a bridge between the wires, and then printed with green oil to make the bridge not a complete 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 (because the copper wire is very small, it can be welded to form a short circuit without high power. If the testing machine manufacturer can increase its short circuit test current The bridge contact can be blown with sufficient power, and it is difficult for us to see this phenomenon). So the third low-voltage computer tester can test the short-circuit point, and its resistance value is only an average of 6.7 ohms.

As mentioned earlier, the 300V high-voltage board tester also complained about short-circuit phenomenon. Checking the returned bad short-circuit board showed that the bad short-circuit board measured for the third time was the same reason. Because the solder mask (green oil) screen printing will increase the insulation of the copper wire, the high-voltage computer testing machine cannot be used to measure it, and the bridge will form a short circuit 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 was no high-voltage computerized board tester, because I was not at ease, the ok board after the test was retested, and the short-circuited board appeared. Most of the problems lie in the side etching of the etching, the pattern production and the solder mask ink pretreatment line. To solve this problem, the first two processes are difficult to solve, mainly to ensure the line width and line spacing during the pattern production., To prevent some lines from being too close, and to ensure the quality of side etching during etching. Reduce the phenomenon of micro short circuit during high voltage test.

The simplest and most economically feasible method is to purchase a new plate grinding machine or to improve an existing plate grinding machine, adopt swing brushing and use volcanic ash buffing or nylon brush with low mesh, and then use high-pressure washing, especially in the water washing section. It is better to have a filter device to prevent foreign matter that can cause a short circuit from contaminating the board surface again. The plate grinding machine can remove the copper wire and the protruding edge formed by the side erosion (the dust sticking machine also has a certain removal effect). At the same time, a high-voltage machine should be used for the newly purchased computer testing machine, which can reduce the occurrence of micro short circuits and short circuits on the circuit board. This short-circuit phenomenon usually occurs in single-sided boards and high-density circuit boards.