Why do PCB boards need test points?

- Dec 21, 2019-

Why do PCB boards need test points?

Summary of content: For people who learn electronics, it is natural to set up test points on the circuit board, but what are the test points for people who learn mechanics?


Why do PCB circuit boards need test points?


I may be a little more confused. I remember when I first worked in a PCBA processing plant as a process engineer, I had asked many people about this test point to understand it. Basically, the purpose of setting test points is to test whether the components on the circuit board meet the specifications and solderability. For example, if you want to check whether the resistance of a circuit board is correct, the simplest method is to take a multimeter You can know by measuring both ends.


However, in a mass-produced factory, there is no way for you to slowly measure whether each resistor, capacitor, inductor, or even IC circuit on each board is correct with an electric meter, so there is a so-called ICT (In -Circuit-Test) The emergence of an automated test machine that uses multiple probes (commonly referred to as "Bed-Of-Nails" fixtures) to simultaneously contact all the parts on the board that need to be measured. Then through program control, the characteristics of these electronic parts are measured sequentially in a sequence-by-sequence manner. Usually, it only takes about 1 to 2 minutes to test all the parts of a general board in this way, depending on the number of parts on the circuit board. The more parts, the longer the time.


However, if these probes are directly in contact with the electronic parts or soldering pins on the board, it is likely that some electronic parts will be crushed, which will be counterproductive, so smart engineers have invented "test points" at both ends of the part. An additional pair of round dots are drawn without a mask, which allows the test probe to contact these dots without directly touching the electronic parts being measured.


In the early days of traditional plug-in (DIP) on the circuit board, the solder pins of the parts were indeed used as test points, because the solder pins of the traditional parts were strong enough, not afraid of pinning, but there were often probes. The misjudgment of poor contact occurs because, after wave soldering or SMT eating of general electronic parts, a layer of solder paste flux residue film is usually formed on the surface of the solder. The resistance of this layer of film Very high, often cause poor contact of the probe, so test operators at the production line were often seen, often blowing desperately holding an air spray gun, or wiping these areas with alcohol.


In fact, the test point after wave soldering will also have poor probe contact. Later, after the popularity of SMT, the test misjudgement situation has been greatly improved, and the application of test points has been greatly entrusted. Because SMT parts are often fragile and cannot withstand the direct contact pressure of test probes, use test points. This eliminates the need for the probe to directly contact the part and its soldering feet, which not only protects the part from injury, but also greatly improves the reliability of the test, because the number of misjudgments is reduced.


However, with the evolution of technology, the size of circuit boards has become smaller and smaller. It is already a bit difficult to squeeze so many electronic parts on a small circuit board. Therefore, the problem of occupying circuit board space by test points is often found in There is a tug of war between the design side and the manufacturing side, but this issue will be discussed later. The appearance of the test points is usually round, because the probes are also round, which is easier to produce, and it is easier to make the adjacent probes closer, so that the density of the needle bed can be increased.


The use of a needle bed for circuit testing has some inherent limitations on the mechanism, for example: the minimum diameter of the probe has a certain limit, and a needle of too small diameter is easily broken and damaged.


There is also a limit on the distance between pins, because each pin must come out of a hole, and a flat cable must be welded at the back of each pin. If the adjacent hole is too small, There is a problem of contact short circuit, and interference of flat cables is also a major problem.


   Needles cannot be planted next to some tall parts. If the probe is too close to the high part, there is a risk of damage due to collision with the high part. In addition, because the part is high, it is usually necessary to open a hole in the test fixture needle bed to avoid it, which indirectly results in the inability to implant the needle. Test points for all the parts on the circuit board are getting harder and harder to accommodate.


As the board is getting smaller and smaller, the number of test points is often discussed. Now there are some methods to reduce the test points, such as Net test, Test Jet, Boundary Scan, JTAG, etc .; there are other test methods. Need to replace the original needle bed test, such as AOI, X-Ray, but currently it seems that each test can not replace 100% of ICT.


Regarding the ICT's ability to implant needles, you should ask the cooperating fixture manufacturer, that is, the minimum diameter of the test point and the minimum distance between adjacent test points. Generally, there will usually be a desired minimum value and the minimum value that can be achieved, but there are Large-scale manufacturers will require that the distance between the minimum test point and the minimum test point should not exceed many points, otherwise the fixture will be easily damaged.

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