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Why do engineers set test points when designing PCB boards?

2019-10-08 19:20:02 366

For those who learn electronics, setting test points on a circuit board is natural, but what about test points for those who learn machinery?


Basically, the purpose of setting up test points is to test the components on the circuit board for non-conformity and solderability. For example, if you want to check the resistance of a circuit board, there is no problem. The easiest way is to take the meter. You can know by measuring both ends.


However, in the PCB manufacturing factory where large-scale circuit boards are produced, there is no way for you to use the electric meter to slowly measure whether each resistor, capacitor, inductor, or even the IC circuit on each board is correct. The emergence of the so-called ICT (In-Circuit-Test) automated test machine, which uses multiple probes (generally called "Bed-Of-Nails" fixtures) while simultaneously touching all the boards needed to be Measure the part line, and then measure the characteristics of these electronic parts sequentially by program-controlled and side-by-side methods. Usually, it takes only about 1~2 minutes to test all the parts of the general board. The number of parts on the board depends on the number of parts, and the longer the part, the longer it takes.


However, if these probes are directly exposed to the electronic parts on the board or their soldering feet, it is likely to crush some electronic parts, but it is counterproductive, so the clever engineers invented the "test points" at both ends of the parts. An additional pair of small dots are placed with no mask on them, allowing the test probe to touch these small points without directly touching the electronic components being measured.


In the early days when the board was also in the traditional plug-in (DIP) era, the soldering feet of the parts were used as test points. Because the soldering feet of the traditional parts are strong enough, they are not afraid of pinning, but there are often probes. The misjudgment of poor contact occurs because the general electronic parts are wave soldered or SMT after soldering, and a residual film of solder paste is usually formed on the surface of the solder. The impedance of this film Very high, often caused by poor contact of the probe, so the test workers who often see the production line at that time, often with an air spray gun desperately blowing, or alcohol to wipe these areas to be tested.


In fact, the test points after wave soldering will also have poor probe contact. Later, after SMT became popular, the test misjudgment situation was greatly improved. The application of test points was also greatly burdened, because the parts of SMT are usually very fragile and cannot withstand the direct contact pressure of the test probe. It is possible to prevent the probe from directly contacting the part and its solder fillet, not only to protect the part from damage, but also to greatly improve the reliability of the test, because the situation of misjudgment is reduced.


However, with the evolution of technology, the size of the circuit board is getting smaller and smaller. It is already a little difficult to squeeze so many electronic parts on the small circuit board. Therefore, the test point occupies the space of the board, often in the There is a tug-of-war between the design side of the board and the manufacturing end, but this topic will have a chance to talk about it later. The appearance of the test points is usually round, because the probes are also round, better to produce, and it is easier to get adjacent probes closer together, so that the needle density of the needle bed can be increased.


1. There are some innate limitations on the use of a needle bed for circuit testing. For example, the minimum diameter of the probe has a certain limit, and the needle of too small diameter is easily broken and damaged.


2. There is also a limit to the distance between the needles, because each needle has to come out of one hole, and a flat cable is welded to the back end of each needle. If the adjacent holes are too small, except for the needle and needle. There is a problem of contact short circuit, and the interference of the flat cable is also a big problem.


3. Some high parts cannot be implanted next to them. If the probe is too close to the high part, there is a risk of damage caused by high-impact parts. In addition, because the part is high, it is usually necessary to open the hole in the test fixture needle bed seat, which indirectly causes the needle to be implanted. Test points on all parts of the board that are getting harder to accommodate on the board.


4. As the board gets smaller and smaller, the number of test points is often discussed. Now there are some ways to reduce the test points, such as Net test, Test Jet, Boundary Scan, JTAG. Etc. There are other test methods that want to replace the original needle bed test, such as AOI, X-Ray, but currently each test seems to be unable to replace ICT 100%.


Regarding the needle-planting ability of ICT, it should be asked about the fixture manufacturer, that is, the minimum diameter of the test point and the minimum distance between adjacent test points. Usually there will be a minimum value of the desired minimum value and the ability to reach, but there is Manufacturers of scale will require that the distance between the minimum test point and the minimum test point should not exceed, or the fixture will be easily damaged.


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