Why do PCB circuit boards need test points for electric meter pcb boards

2021-01-05 12:29:09 hongling

Why do PCB circuit boards need test points for electric meter pcb boards




For people studying electronics, it is natural to set test points on the circuit board, but for those who are mechanical, what are the test points when making electric meter pcb boards?


Why do PCB boards need test points?


Maybe a little foggy. I remember the first time I was a process engineer in a PCBA factory, I asked many people about this test point. Basically set the test points to test the circuit board components for compliance with specifications and soldering, such as checking the resistance of the circuit board. If you have any problems, the easiest way is to measure both ends with a multimeter to see.


But in mass production factories, there is no way to let you use meters to measure each board slowly. Each measurement of resistance, capacitance, inductance, and even the IC circuit is correct, so the so-called ICT (in-circuit test)-automatic test machine, It uses multiple probes (usually called "needle bed (bed-nail)" fixture) and all parts of the contact board need to be measured, and then measure the characteristics of electronic parts through SPC and sequence-based, parallel auxiliary methods, usually this This kind of test generally only takes 1 to 2 minutes to complete all parts of the board. Depending on the number of circuit components, the longer the time, the more part.


But if you allow the probe to directly touch the board on the electronic components or higher than its legs, it may crush some electronic components, on the contrary, so smart engineers invented "test points", the extra part of the end leads to a pair of round points, no welding (mask ), the test probe can enter these small points without direct contact with electronic parts.


Early circuit boards were in the era of traditional plug-in (DIP). It is really possible to use the soldering part of the foot as a test point, because the foot of the traditional soldering part is strong enough, not afraid of the needle, but there are often misjudgments with poor contact probes, because generally After wave soldering of electronic components, wave soldering or SMT soldering, the solder on the surface usually forms a layer of solder paste residual film flux. The impedance of this film is very high, which often leads to the investigation of poor contact, so at that time, it is a kind of For the production line of common test operators, the air gun must be blown carefully regularly or wiped with alcohol to be tested.


In fact, the test points of wave soldering can also have probe contact problems. After the prevalence of SMT, the misjudgment situation of testing has been greatly improved, and the application of test points has also been greatly rewarded, because SMT parts are usually weak and cannot bear the direct contact pressure test probe, and the probe cannot be directly contacted with the test point. The part, the welding foot, not only protects the parts from damage, but also indirectly promotes reliability testing because there are fewer misjudgments.


However, with the development of science and technology, the size of the circuit board has become smaller and smaller. It is difficult to milk many electronic components under the light above the small circuit board. Therefore, the circuit board space of the test point problem is often in the design and manufacturing A tug-of-war match between, but then I have a chance to talk about this issue again, etc. The appearance of the test point is usually round, because the survey is also round, which is relatively good, and it is easy to make the adjacent detector close to the point, so that the needle density can increase the needle bed.


There are some institutional limitations with the needle bed for circuit testing. For example, the minimum diameter of a detector has a certain limit, and a small diameter needle can easily break it.


The distance between the pins is also limited, because each pin hole and the back end of each pin are welded to the flat cable. If the adjacent holes are too small, and in addition to the pin contact short-circuit problem, the interference of the flat cable is also a big problem. .


Some tall parts have no needles. If the distance between the probes is too close, there is a risk of collision. Because other parts are high, the needle bed is usually avoided in the valve seat hole of the test fixture, which indirectly leads to the inability to implant the needle. It is increasingly difficult to adapt to the test points of all components on the circuit board.


As the boards are getting smaller and smaller, test points are often discussed. There are some methods to reduce test points, such as net test, test jet, boundary scan, JTAG, etc. There are some other test methods that want to replace the original needle bed test, such as AOI, X-ray, but it seems that every test cannot replace ICT 100%.


The ability of flocking needles should be asked by the ICT fixture manufacturer, that is, the minimum distance between the smallest diameter test point and the adjacent test point. There is usually a minimum hope and the ability can reach the lowest, but the scale supplier needs the smallest test point And the minimum score, the distance between the test points cannot exceed and the fixture is easily damaged.