Why do PCB boards need to have test points?

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The purpose of setting up 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 on a circuit board is faulty, the easiest way is to measure it with a multimeter. Then can know.

However, if the probe directly contacts the electronic parts on the board or its solder feet, it is likely to crush some electronic parts, but it is counterproductive, so there is a test point, and a pair of circular small pins are drawn at both ends of the part. The dots, without a solder mask, allow the test probes to touch these small dots without directly touching the electronic parts being measured.

In mass-production factories, it is impossible to individually test each electronic component (such as resistors, capacitors, inductors, and IC circuits) on a circuit board using an electric meter. To solve this problem, an automated test machine called ICT (In-Circuit-Test) has been developed. These machines use multiple probes, often referred to as "Bed-Of-Nails" fixtures, to make simultaneous contact with all the parts on the board that need to be measured. Then, the characteristics of these electronic parts are measured sequentially through program control. This method usually takes only 1 to 2 minutes to test all the parts of a general circuit board, depending on the number of parts present. It is important to note that the more parts there are, the longer the testing process will take.

There are some limitations to using needle beds for circuit testing. For instance, the probe's minimum diameter has a certain limit, and if it's too small, it can break easily and cause damage. The distance between pins is also restricted. Each pin should come out of a hole, and a flat cable should be welded at the back end of each pin. If the adjacent holes are too small, it can cause short circuits between pins and interference of the flat cable.

Additionally, some high parts cannot be planted next to the probes. If the probe is too close to the high part, it can damage it. The needle bed base of the test fixture should avoid the holes on high parts, and the needle cannot be planted indirectly. As the circuit board gets smaller, storing and wasting test points is repeatedly discussed.

There are methods to reduce test points such as net test, test jet, boundary scan, JTAG, etc. Other test methods can replace the original needle bed test, such as AOI and X-ray. However, at present, none of these tests can replace ICT entirely.