Basically, the purpose of setting test points is to test whether the components on PCB meet the specifications and solderability. For example, if you want to check whether there is any problem with the resistance on PCB, the easiest way is to measure with a multimeter. You can know it by measuring both ends.
However, in a mass production factory, there is no way to use an electric meter to measure whether each resistance, capacitance, inductance, and even the circuit of each IC on each board is correct, so there is the so-called ICT( In-Circuit-Test) automated testing 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. And then measure these electronic parts in a sequence-based way through program control, and parallel as a supplementary 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. The more parts the longer the time.
But if these probes directly touch the electronic parts on the board or its solder feet, it is likely to crush some electronic parts. So some smart engineers invented test points and led out extra at both ends of the parts. A pair of small round dots without a solder mask (mask) allows the test probe to touch these small dots without directly touching the electronic parts being measured.
In the early days when there were traditional plug-ins (DIP) on PCB, the solder feet of the parts were indeed used as test points, because the solder feet of the traditional parts were strong enough that they were not afraid of needle sticks. The misjudgment of poor contact of the probes 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 holding air spray guns to blow desperately, or wipe these places that need to be tested with alcohol.
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 played an important role. Because the parts of SMT are usually very fragile and cannot withstand the direct contact pressure of the test probe. Use test points will 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
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