Why need to have a test point on the PCB circuit board?

- Aug 22, 2016-

     I remember the first time I entered the PCBA processing plant when the process engineer, but also had to ask a lot of people to this test point to understand it. Basically set test point is for testing circuit board components have not meet specifications and welding, for example, want to check a single circuit board resistance there is no problem, the simplest way is to Nawan with meter measuring the two will be able to 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 of the above board or the weld leg, is likely to pressure can destroy some electronic components, but just the opposite, so smart engineers invented a "test", in the parts of both ends of the additional leads to a pair of round dots above does not prevent welding (mask), can make test probe contact to the dots, without direct contact with the electronic parts that are measured.

     In the early stage of the circuit board above are traditional plug-in (DIP) in welding feet will take part as a test point to use, because the traditional welding foot parts are strong enough, not afraid of needles, but there is often a bad situation misjudgment contact probe, because the general electronic components after soldering (wave soldering) or SMT tin, in the film surface residual solder usually form a layer of paste flux, the impedance of this layer of film is very high, often caused by bad contact of the probe, so the test operator was often seen in the production line, often with air lance blowing hard, or take these places to test alcohol wipe.

     In fact, after wave soldering test points will also have poor contact probe. Later after the prevalence of SMT, misjudged test get great improvement, application of test points was also greatly given the task, because SMT parts are usually very weak, unable to withstand the test probe direct contact pressure, use the test point can don't want to probe directly contact part and a welding foot, not only protect components from harm, also indirectly greatly improve the test reliability, because fewer cases of miscarriage of justice.

     But with the development of science and technology, the size of the circuit board is also getting smaller and smaller, a little above the circuit board to squeeze so many electronic parts have some difficulty, so the test point occupied of space on the circuit board, often between the end end design and manufacture tug of war, but this issue, later will have the opportunity to talk again. The appearance of the test point is usually round, because of the probe is circular, better production, is relatively easy to make adjacent probe near a point, so that it can increase needle bed needling density.

     Using the needle bar to do the test circuit will have some mechanism to heaven limit. For example, the minimum diameter of the probe has a certain limit, too small diameter of needle break easily damaged.

     Needle distance is limited, because each root needle to come out from a hole and at the rear end of the each needle was still welding a flat cable, if the adjacent to the hole is too small, in addition to the short contact problem between the needle and the needle, the interference of the flat cable is a big ask.

Some high parts can not be planted next to the needle. If parts of high probe distance too close to be a risk of injury caused by high collision parts, also because of the higher parts, often also in the test fixture neilsbed seat hole to avoid, but also indirectly cause not needling. The test points of all parts that are more and more difficult to hold on the circuit board.

     Due to the more and more small board, test point of the amount of waste repeatedly brought up for discussion, now has the some reduces the appearance of test methods, such as net test, test jet, boundary scan, JTAG; there are also other test method to replace the original neilsbed test, such as AOI, X-ray, but the test each seem to be not 100% instead of ICT.

     Minimum distance on ICT graft needle ability should ask with fixture manufacturers, namely, a test point minimum diameter and adjacent test points, generally have a minimum value and ability can reach the minimum value of hope, but the scale of the manufacturers will require minimum test point and minimum test distance can not exceed the number of points or fixture is easily damaged.


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