Check the PCB engineer's test points during PCB design

Basically, the purpose of setting test points is to test whether the components on the circuit board meet the specifications and solderability. For example, to check whether the resistance on a circuit board is correct, the simplest way is to measure the two ends of the circuit board with a multimeter.

However, there is no way for you to slowly measure each resistance, capacitance, inductance, and even IC circuit on each board with an electric meter in the production of large quantities of circuit boards. Therefore, the so-called ICT (In Circuit Test) automatic testing machine appears. It uses multiple probes (generally called "Bed Of Nails" fixture) to contact all parts and circuits on the board that need to be measured at the same time, Then, the characteristics of these PCB parts are measured in sequence by program control in the way of sequence as the main part and parallel as the auxiliary part. Generally, testing all parts of a general board in this way can be completed in about 1~2 minutes. Depending on the number of parts on the circuit board, the more parts, the longer the time.

However, if these probes directly contact the electronic parts or their welding legs on the board, it is likely to crush some electronic parts, which is counterproductive. Therefore, smart engineers invented a "test point", which leads an additional pair of round dots at both ends of the part. There is no mask on it, so that the test probe can contact these dots without directly contacting the measured electronic parts.

In the early days when the circuit board was still a traditional plug-in (DIP), it was true that the solder legs of parts were used as test points, because the solder legs of traditional parts were strong enough to not be afraid of pin pricking, but the misjudgment of poor probe contact often occurred, because after wave soldering or SMT tin eating, a layer of residual film of solder paste flux was usually formed on the solder surface of general electronic parts, The impedance of this film is very high, which often leads to poor contact of the probe. Therefore, it is often seen that the test operators of the production line often blow hard with an air spray gun, or wipe these places that need to be tested with alcohol.

In fact, the probe may have poor contact at the test point after wave soldering. Later, after the prevalence of SMT, the situation of test misjudgment has been greatly improved, and the application of test points has also been given a great responsibility. Because SMT parts are usually fragile and cannot withstand the direct contact pressure of the test probe, the use of test points allows the probe to not directly contact the parts and their weld legs, which not only protects the parts from damage, but also indirectly greatly improves the reliability of the test, because there are fewer misjudgments.

PCB design

However, with the development of science and technology, the size of the circuit board is getting smaller and smaller. It is hard to squeeze so many electronic parts on a small circuit board. Therefore, the problem of the space occupied by the test point on the circuit board is often a tug of war between the end and the manufacturing end. However, this issue will be discussed later when the opportunity arises. 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 planting density of the needle bed can be increased.

1. There are some limitations on the mechanism when using the needle bed for circuit testing, such as: the minimum diameter of the probe has a certain limit, and the needle with too small diameter is easy to break and damage.

2. The distance between needles is also limited, because each needle must come out of a hole, and a flat cable must be welded at the back end of each needle. If the adjacent holes are too small, in addition to the problem of contact short circuit between needles, the interference of flat cables is also a major problem.

3. The needle cannot be planted beside some high parts. If the probe is too close to the high part, there will be 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 on the needle bed of the test fixture to avoid it, which also indirectly causes that the needle cannot be planted. Test points of all parts on the circuit board that are increasingly difficult to accommodate.