Why should there be test points on PCB?
In the PCB industry, it is natural to set test points on the PCB. But what are the test points for newcomers who are new to PCB? It's a question. Today, you can understand why PCB manufacturers set test points on PCB boards.
In short, 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 to use an electric meter to slowly measure the correctness of each resistance, capacitance, inductance, and even IC circuit on each board in a mass production Circuit board factory. Therefore, the so-calLED ICT (In Circuit Test) automatic testing machine appears. It uses multiple probes (commonly called "Bed Of Nails" fixtures) to simultaneously contact all parts and circuits on the board that need to be measured, Then, the characteristics of these electronIC parts are measured in sequence through program control, mainly in sequence, supplemented by juxtaposition. Generally, the test of all parts of a general board 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 solder legs on the circuit board, it is likely to crush some electronic parts, which is counterproductive. So SMArt engineers invented a "test point". An additional pair of round dots were introduced at both ends of the part. There is no mask on it, so the test probe can contact these dots, It does not need to directly contact the electronic parts being measured.
In the early days when the circuit board was still a traditional plug-in (DIP), PCB manufacturers did use the solder legs of parts as test points, because the solder legs of traditional parts were strong enough to be free from pin pricking. However, misjudgment of poor probe contact often occurred, because ordinary electronic parts were wave soldered or SMT soldered, A layer of residual film of solder paste flux is usually formed on the surface of its solder. 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 Pcb factory production line often blow hard with an air 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.
However, with the development of science and technology, the size of circuit boards is getting smaller and smaller. It is hard to squeeze so many electronic parts on Small circuit boards. Therefore, the problem of test points occupying the space of circuit boards is often a tug of war between the design side and the manufacturing side. However, this topic 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.
There are some limitations on the mechanism when using a needle bed for circuit testing. For example, the minimum diameter of the probe has a certain limit. A needle with too small diameter is easy to break.
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 big problem.
Some high parts cannot be planted with needles. 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.
As the circuit boards are getting smaller and smaller, the number of test points has been discussed repeatedly. Now there are some methods to reduce test points, such as Net test, Test Jet, Boundary Scan, JTAG, etc; There are other test methods that want to replace the original needle bed test, such as AOI tester and X-Ray, but at present, each test seems unable to replace ICT 100%.
The minimum diameter of the test point and the minimum distance between adjacent test points usually have a desired minimum value and a minimum value that can be reached. However, a circuit board manufacturer with a large scale will require that the distance between the minimum test point and the minimum test point cannot exceed many points, so PCB manufacturers will reserve more test points when producing boards