For those who study electronics, it is natural to set up test points on the circuit board, but for those who study mechanics, what are the test points?
Basically, 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 there is any problem with the resistance on a circuit board, the easiest way is to use a multimeter to measure it. You can find out by measuring both ends.
However, in a mass-produced factory, there is no way for you to use an electric meter to slowly measure whether the circuit of each resistor, capacitor, inductance, or even IC on each board is correct, so there is a so-called ICT ( The emergence of In-Circuit-Test) automated testing machine, which uses multiple probes (commonly referred to as "Bed-Of-Nails" fixtures) to simultaneously contact all the parts lines on the board that need to be measured , and then measure the characteristics of these electronic parts sequentially through program control in a sequence-based, juxtaposed manner. 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. Depending on the number of parts, the time will be longer.
However, if these probes directly contact the electronic parts on the board or their solder feet, it is likely to crush some electronic parts, but it will be counterproductive, so smart engineers invented "test points" at both ends of the parts. An additional pair of small circular dots, without a solder mask, allows the test probes to touch these dots without directly touching the electronic parts being measured.
In the early days when there were still traditional plug-ins (DIP) on the circuit board, the solder feet of the parts were indeed used as test points, because the solder feet of traditional parts were strong enough to not be afraid of needle sticks, but there were often probes. The misjudgment of poor contact occurs, because after the general electronic parts are subjected to wave soldering or SMT, a residual film of solder paste flux is usually formed on the surface of the solder, and the impedance of this film is It is very high, which often causes poor contact of the probe. Therefore, test operators on the production line were often seen at that time, often blowing hard with an air spray gun, or using alcohol to wipe the places that need to be tested.
In fact, the test point after wave soldering will also have the problem of poor probe contact. Later, after SMT became popular, the situation of test misjudgment has been greatly improved, and the application of test points has also been greatly assigned, because SMT parts are usually fragile and cannot withstand the direct contact pressure of test probes, using test points It is not necessary to let the probe directly touch the part and its solder feet, which not only protects the part from damage, but also indirectly greatly improves the reliability of the test, because there are fewer misjudgments.
However, with the evolution of technology, the size of the circuit board is getting smaller and smaller. It is already a little difficult to squeeze so many electronic parts on the small circuit board. Therefore, the problem that the test points occupy the circuit board space is often caused by There is a tug-of-war between the design side and the manufacturing side, but this topic will be discussed later when there is a chance. 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 density of the needle bed can be increased.
1. There are some inherent limitations in using the 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 easily broken and damaged.
2. The distance between the pins is also limited, because each pin must come out from a hole, and the rear end of each pin needs to be soldered with a flat cable. If the adjacent holes are too small, in addition to the gap between the pins and the pins. There will be contact short-circuit problems between them, and the interference of flat cables is also a big problem.
3. Needles cannot be planted next to some tall parts. If the probe is too close to the high part, there is 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 in the needle bed seat of the test fixture to avoid it, which also indirectly causes the needle to be unable to be implanted. Test points for all the parts on the board that are increasingly difficult to accommodate.
4. As the board is getting smaller and smaller, the number of test points has been repeatedly discussed. 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 bed of needles test, such as AOI, X-Ray, but each test seems to be unable to replace ICT 100% at present.
Regarding the needle implanting ability of ICT, you should ask the manufacturer of the matching fixture, that is, the minimum diameter of the test point and the minimum distance between the adjacent test points. Usually there is a desired minimum value and the minimum value that the ability can achieve, but there are Large-scale manufacturers will require that the distance between the minimum test point and the minimum test point should not exceed a few points, otherwise the fixture will be easily damaged.