METHODS OF PCB TESTS
The methods of PCB test include the auto optical inspection (AOI) and in-circuit-test (ICT)
Auto Optical Inspection (AOI)
The AOI refers to automatic optical identification. In consideration of production efficiency and inspection accuracy, replacing visual inspection with optical inspection equipment has become a basic trend. In the AOI, the image file of the control is stored in the equipment to perform the optical comparison with the DUT to automatically identify if the errors exceed the tolerance. If an error exceeds the tolerance, determine if the anomaly should be scrapped of repaired. As the circuit on a PCB becomes increasing elaborate such as to exceed the limit of visual inspection, the AOI is often used to compare the circuit layers to check for the over/under etching or impacts.
Total ICT has become the common test of almost all customers. Even if the ICT is not requested, PCB manufacturers will run the test to ensure that there are no circuit problems in their products.
The ICT can be performed in terms of:
In the flying probe ICT, a PCB is fixed on the testing machine contacting the pads and vias back and forth with 2–4 probes, in order to test if the circuit is identical with the original design or check for broken/short circuit. As the probes run back and forth very fast, it is called the flying probe ICT. This is a very basic ICT method. Its advantage is, it does not need any specific clamps, but simply fixing the PCB on the testing machine from the top and bottom or the left and right. Therefore, the flying probe ICT is very suitable for HMLV manufacturing or sample testing. The disadvantage is it takes a longer time to finish the test, and the more testing points there are, the longer the time it will take. Therefore, it is not suitable for testing mass-produced PCBs.
Traditionally, a PCB manufacturer will need to hire a tester supplier to build a tester for the product. In addition, as the probes and wires are set for each testing point and circuit in testing building, the tester cannot be disassembled after use and can only be stored in the complete set.
Universal on Grid ICT
A PCB manufacturer also needs to hire a tester supplier to build a universal on grid tester which only has holes for the testing points and without any cable connection. Before running a test, PCB manufacturers only need to insert probes on each testing hole and put the tester on the upper and lower pads of the ICT machine. During the test, the upper and lower pads only need to contact the testing points through the probes to check if the circuit on the PCB is correct. In addition, the probes can be disassembled from the universal on grid tester after use to facilitate storage.