For people who study electronics, setting up a test point on the circuit board is a natural thing, but for people who study mechanics, what is the test point?
Maybe a little more confused. I remember when I first entered the PCBA processing plant to work as a process engineer, I had to ask a lot of people for this test point to understand it. Basically, the purpose of setting the test point is to test whether the components on the circuit board meet the specifications and weldability, for example, if you want to check whether the resistance on a circuit board has a problem, the easiest way is to take a multimeter to measure its two ends.
However, In mass production factories, there is no way for you to use an electric meter to slowly measure whether every resistor, capacitor, inductor, or even IC Circuit on each board is correct, so there is the emergence of the so-called ICT(in-circuit-test) automatic Test machine. It uses multiple probes (commonly known as "Bed-Of-Nails" fixtures) to simultaneously contact all the parts on the board that need to be measured, and then measures the characteristics of these electronic parts in a sequential and side-by-side manner through program control. Usually, the test of all the parts of the general board only takes about 1 to 2 minutes to complete, depending on the number of parts on the circuit board, the more parts the longer.
However, if these probes directly contact the electronic parts on the board or its welding feet, it is likely to destroy some electronic parts, but the opposite is true, so smart engineers invented the "test point", an extra pair of circular dots at both ends of the part, there is no anti-welding (mask), so that the test probe can contact these small points. Without having to come into direct contact with the electronic parts being measured.
In the early days of the traditional plug-in (DIP) on the circuit board, the welding foot of the part is indeed used as a test point, because the welding foot of the traditional part is strong enough and is not afraid of needles, but there is often a misjudgment of poor contact of the probe, because the general electronic parts have been wave soldering (wave soldering) or SMT after eating tin. A residual film of solder paste flux is usually formed on the surface of the solder, and the impedance of this film is very high, which often causes poor contact of the probe, so it is often seen that the test operators of the production line often blow desperately with the air spray gun, or wipe these need to be tested with alcohol.
In fact, the test point after wave soldering will also have the problem of poor contact of the probe. Later, after the prevalence of SMT, the situation of test misjudgment has been greatly improved, and the application of test points has been greatly entrusted with the task, because the parts of SMT are usually fragile and can not withstand the direct contact pressure of the test probe, and the use of test points can avoid the direct contact of the probe to the parts and their welding feet, which not only protects the parts from damage, but also protects the parts from damage. Indirectly, the reliability of the test is greatly improved, because there are fewer cases of miscalculation.
However, with the evolution of science and technology, the size of the circuit board is getting smaller and smaller, and it is already somewhat difficult to squeeze so many electronic parts from the light on the circuit board, so the problem of the test point occupying the space of the circuit board is often a tug-of-war between the design end and the manufacturing end, but this issue will have the opportunity to talk again in the future. The appearance of the test point is usually round, because the probe is also round, it is easier to produce, and it is easier to let the adjacent probe be closer together, so that the needle density of the needle bed can be increased.
1. the use of needle bed to do circuit testing will have some innate limitations on the mechanism, such as: the minimum diameter of the probe has a certain limit, too small diameter of the needle is easy to break and damage.
2. the distance between the needles also has a certain limit, because each needle has to come out of a hole, and the back end of each needle has to weld a flat cable, if the adjacent hole is too small, in addition to the problem of short circuit contact between the needle and the needle, the interference of the flat cable is also a big problem.
3. Needles cannot be planted next to some tall parts. If the probe is too close to the tall part, there is a risk of damage caused by collision with the tall part. In addition, because the part is high, it is usually necessary to cut holes in the needle bed of the test fixture to avoid it, which also indirectly causes the needle planting. It is getting harder to fit all the parts on the circuit board under the test point.
4. As the board is getting smaller and smaller, the existence and waste of test points are often 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 needle bed test, such as AOI, X-Ray, but at present, each test seems to be unable to 100% replace ICT.
On the ability of ICT needle planting should be asked to cooperate with the fixture manufacturer, that is, the minimum diameter of the test point and the minimum distance of the adjacent test point, usually there will be a minimum value and the minimum value that can be achieved, but the scale of the manufacturer will require the minimum test point and the minimum test point distance can not exceed a number of points, otherwise the fixture is easy to damage.
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