Why PCB boards need to have test points?

For people, it is natural to set up test points on the circuit board, but for those who learn mechanics, what are the test points?

< span style="font-size: 16px;"> may be a little confusing. I remember when I first worked in a PCBA factory as a process engineer, I asked a lot of 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 there is any problem with the resistance on a circuit board, the easiest way is to use a multimeter to measure it. You can find out by measuring both ends.

But in mass-produced factories, there is no way for you to use an electric meter to slowly measure every resistance, capacitance, Whether the circuit of the inductance or even the IC is correct, so there is the emergence of the so-called ICT (In-Circuit-Test) automated testing machine, which uses multiple probes (generally called "Bed-Of- Nails)" Jig) simultaneously contacts all the parts and circuits on the board that need to be measured, and then measures the characteristics of these electronic parts sequentially through the program control in a sequence-based, parallel-assisted manner. Usually, all parts of the general board are only It takes about 1 to 2 minutes to complete, depending on the number of parts on the circuit board. The more parts, the longer the time.

But if these probes directly contact the electronic parts on the board or their solder feet, it is likely to crush some electronic parts, but instead This is counterproductive, so smart engineers invented "test points", which lead to a pair of small circular dots at both ends of the part. There is no solder mask on it, so that the test probes can touch these small dots. Instead of directly touching the electronic parts being measured.

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In the early days when the circuit boards were still traditional plug-ins (DIP), the solder feet of the parts were indeed used as test points, because the solder feet of traditional parts Strong enough to not be afraid of needle sticks, but there are often misjudgments of poor probe contact, because after wave soldering or SMT tinning of general electronic parts, a layer of solder is usually formed on the surface of the solder. The residual film of solder paste flux, the resistance of this film is very high, which often causes poor contact of the probe, so it was often seen at that time that the test operators on the production line often blew with an air gun or wiped with alcohol. These need to be tested.

In fact, the test point after wave soldering will also have the problem of poor probe contact. Later, after SMT became popular, the situation of test misjudgment has been greatly improved, and the application of test points has also been greatly assigned, because SMT parts are usually fragile and cannot withstand the direct contact pressure of test probes, using test points It is not necessary to let the probe directly touch the part and its solder feet, which not only protects the part from damage, but also indirectly greatly improves the reliability of the test, because there are fewer misjudgments.

However, with the evolution of technology, the size of the circuit board is getting smaller and smaller, and the light on the small circuit board has to squeeze so many electrons The parts are already a bit of a struggle, so the issue of test points taking up board space is often a tug-of-war between the design side and the manufacturing side, but this topic will be discussed later when there is a chance. The appearance of the test point is usually round, because the probe is also round, which is easier to produce, and it is easier to make adjacent probes closer, so that the needle density of the needle bed can be increased.

< span style="font-size: 16px;">Using a needle bed for circuit testing will have some inherent limitations on the mechanism, for example: the minimum diameter of the probe has a certain limit, and the needle with too small diameter is easy to Broken and damaged.

The distance between pins is also limited, because each pin has to come out of a hole, and the rear end of each pin has to be soldered with another flat cable, if the adjacent holes are too small, in addition to There is a problem of contact short-circuit between pins, and the interference of the flat cable is also a big problem.

Needles cannot be placed next to some tall parts. If the probe is too close to the high part, there is a risk of damage caused by collision with the high part. In addition, because the part is high, it is usually necessary to make holes in the needle bed seat of the test fixture to avoid it, which also indirectly causes the needle to be unable to be implanted. Test points for all the parts on the board that are increasingly difficult to accommodate.

As the board is getting smaller and smaller, the number of test points has been repeatedly discussed. Now there are some methods to reduce test points, such as Net test, Test Jet, Boundary Scan, JTAG.. . etc.; there are other test methods that want to replace the original bed of needles test, such as AOI, X-Ray, but each test seems to be unable to replace ICT 100% at present.