In the PCB industry, it is natural to set up a test point on the circuit board, but what is the test point for the new person who just contacts the PCB? It is inevitable that it is a question, so today the PCB manufacturer Xiaobian will take you to understand why the test point is set on the PCB board.
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In simple terms, the purpose of setting the test point is mainly 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 the mass production of Circuit board factories, there is no way for you to use an electric meter to slowly measure whether each resistor, capacitor, inductance, 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.
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However, if these probes directly contact the electronic parts on the line 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", at both ends of the part extra lead out a pair of circular dots, there is no anti-welding (mask), you can let the test probe 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, PCB manufacturers do use the welding foot of the part as a test point, because the welding foot of the traditional part is strong enough and is not afraid of needles, but there are often misjudgments of poor contact of the probe. Because the general electronic parts after wave soldering (wave soldering) or SMT eat tin, the surface of the solder usually forms a residual film of solder paste flux, the impedance of this film is very high, often cause poor contact of the probe, so the test operator of the circuit board factory production line is often seen. Often take the air spray gun to blow hard, or take alcohol to wipe these need to be tested.
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.
The use of a needle bed for circuit testing has some inherent limitations on the mechanism, for example: the minimum diameter of the probe has a certain limit, and the needle with too small a diameter is easy to break and destroy.
The distance between the needles is also limited, because each needle has to come out of a hole, and the back end of each needle has to be welded with 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.
Needles cannot be placed 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.
As the circuit board is getting smaller and smaller, the storage 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 tester, X-Ray, but at present, each test seems to be unable to 100% replace ICT.
The minimum diameter of the test point and the minimum distance of the adjacent test point, usually there will be a desired minimum value and the minimum value that can be achieved, but the scale of the circuit board manufacturers will require the minimum test point and the minimum test point distance can not exceed a number of points, so PCB manufacturers will leave more test points in the production of the board
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