Why PCB board need to have a test point ?

- Nov 30, 2016-

      Set of test points is designed to test the circuit board components have not meet specifications and welding, for example to check a resistance on the circuit board, there is no problem, the easiest way is Nawan meter measuring the two can know.

      But in a large number of factories have no way to let you use the meter slowly to measure each piece on the board every resistance, inductance, even IC circuit is correct, so there is the so-called ICT (In-Circuit-Test) automated test platform, it makes a probe (generally known the "needle") (Bed-Of-Nails) fixture contact parts all lines on the board need to be measured at the same time, and then through the program in sequence, parallel characteristics as means to measure these sequential electronic components, usually this test all parts of board need only 1~2 minutes to complete, depending on the circuit board the number of parts and components, the more the longer.

      But if the probe in direct contact with the electronic parts above board or the welding foot, is likely to be destroyed some electronic components, but just the opposite, so smart engineers invented the "test", in parts of both ends of additional leads to a pair of circular dots, there is no solder (mask). The probe can make testing with the exposure to these points, without direct access to those measured electronic parts.

      SMT prevailed misjudged test situation has been greatly improved. The application of the test point has also been greatly given the task, because the SMT parts are usually very weak, unable to withstand the test pressure in direct contact with the probe, the use of test points can not let the probe into direct contact parts and welding feet, not only protect the components from hurt, also greatly enhance the reliability of the indirect test, the situation is less because of false positives. The appearance of the test point is usually round, because the probe is also round, good production, is also relatively easy for adjacent probe on a near point, so that it can increase the density of the needle bar.

      Use the needle to do circuit test there will be some mechanism of the restrictions, for example: the minimum diameter of the probes have a certain limit, the small diameter needle break easily damaged. The needle distance is limited, because each sewing has to come out from a hole, and the rear end of each needle was still welding a flat cable, if the adjacent hole is too small, in addition to a contact short circuit between the needle and the needle, flat cable interference is one of the major problems.

      Some high parts can not be planted next to the needle. If the probe distance high parts too close will have high risk of damage caused by collision of parts, other parts are usually higher because, in the test fixture seat hole to avoid the needle, but also indirectly cause cannot pin. The test points of all parts that are more and more difficult to hold on the circuit board.

      Because the board is more and more small, the number of test points to save the waste has been discussed, there has been some methods to reduce the test points, such as Net test, Test Jet, Boundary Scan and JTAG; there are other test methods to replace the original testing, such as AOI, X-Ray, but at present each the test seems to still can not replace the ICT 100%.

      The needle on the ICT plant capacity should ask with fixture manufacturers, is testing the minimum distance test point and minimum diameter of adjacent points, usually there will be a hope of minimum minimum and capability can be achieved, but the scale of the manufacturers will require the minimum test point and minimum distance between test points can be more than the number of points or fixture is easy to damage.

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