Why do PCB boards need to design test points

Why do PCB boards need to design test points

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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 on the circuit board fast PCB manufacturing is wrong, the easiest way is to use a multimeter to measure both ends. Got it. However, in mass production factories, there is no way for you to use an electricity 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) The emergence of automated testing machines, which use multiple probes to simultaneously contact all the parts on the board that need to be measured. Usually, it only takes about 1 to 2 minutes to test all parts of the general board. The more parts The longer it takes.

However, if these probes directly touch the electronic parts on the board or its solder feet, it is very likely that some electronic parts will be crushed, and it will be counterproductive. Therefore, smart engineers invented “test points”, which are located on the two parts of the board. A pair of small round dots are additionally led out on the end, and there is no mask on the top, so that the test probe can touch these small dots instead of directly contacting 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 points. The solder feet of the traditional parts were strong enough not to be afraid of needle sticks, but there were often probe contacts. Poor misjudgment occurs because a thin film is formed on the surface of general electronic parts after wave soldering or SMT. The impedance of this thin film is very high, which often results in poor contact of the probe. .

In fact, the test points after wave soldering will also have the problem of poor probe contact. After the prevalence of SMT, the misjudgment of testing has been greatly improved, and the application of test points has also been given a great deal of responsibility, because SMT parts are usually very fragile and cannot withstand the direct contact pressure of the test probe. It is not necessary 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.

With the advancement of science and technology, the size of the circuit board is getting smaller and smaller. It is already a bit difficult to squeeze so many electronic parts on the small circuit board. Therefore, the problem of the test point occupying the circuit board space is often in the design. Tug of war between the end and the manufacturing end. 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 bring the adjacent probes closer, so that the needle density of the needle bed can be increased. There are some inherent restrictions on the mechanism when using a needle bed for circuit testing. For example, the minimum diameter of the probe has a certain limit, and the needle with too small diameter is easy to break and damage. The distance between the needles is also limited, because each needle must come out of a hole, and the back end of each needle must be soldered with a flat cable. If the adjacent holes are too small, except for the problem between the needle and the needle. There is the problem of contact short circuit, and the interference of the flat cable is also a big problem.

There are also other test methods that want to replace the original needle bed test, pcb online china such as AOI, X-Ray, but it seems that each test cannot replace ICT 100%.