Why Do PCB Boards Need Test Points? (1)

For people studying electronics, it is natural to set up test points on the circuit board, but for people studying machinery, what are the test points?

Maybe many people are still a little confused. I remember that when I first worked as a process engineer in a PCBA processing plant, I asked many people about this test site 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 measure with a multimeter. You can know it by measuring both ends.

However, in mass-produced factories, there is no way for you to use an electric meter to slowly measure whether each resistance, capacitance, inductance, and even IC circuits on each board are correct, so there is the so-called ICT (In -Circuit-Test) emergence of automated test machines, which use multiple probes (generally called “Bed-Of-Nails” fixtures) to simultaneously contact all the parts on the board that need to be measured. Then the characteristics of these electronic parts are measured sequentially through the program control with sequence as the main and side by side method. Usually, it only takes about 1 to 2 minutes to test all the parts of the general board, depending on the number of parts on the circuit board. It is determined that the more parts, the longer the time it will take.

But if these probes directly touch the electronic parts on the board or its solder feet, it is likely to crush some electronic parts. It will be counterproductive. So smart engineers invented “test points”, which are located at both ends of the parts. A pair of small circular dots are additionally drawn out without a solder mask (mask), so that the test probe can touch these small dots instead of directly touching the electronic parts to be measured.

In the early days when there were traditional plug-ins (DIP) on the circuit board, the solder feet of the parts were indeed used as test point., Because the solder feet of the traditional parts were strong enough that they were not afraid of needle sticks. But there were misjudgment of poor contact of probe occurs. Because after general electronic parts undergo wave soldering or SMT tin, a residual film of solder paste flux is usually formed on the surface of the solder, and the resistance of this film is very high, which often causes poor contact of the probes. Therefore, test operators on the production line were often seen at the time, often holding air spray guns to blow desperately, or rubbing alcohol on these places that needed to be tested.

In fact, the test points after wave soldering will also have the problem of poor probe contact. Later, after the popularity of SMT, the misjudgment of the test was greatly improved, and the application of test points was also given a great deal of responsibility, because the parts of SMT are usually very fragile and cannot withstand the direct contact pressure of the test probe. The use of test points eliminates the need for the probe to directly touch the parts and their welding feet, which not only protects the parts from damage, but also indirectly greatly improves the reliability of the test, because there are fewer misjudgments.