Why do we need test points on the PCB circuit board?

Shenzhen xinmaolong precision circuit Co,.LtdRelease Time:2022-01-15

In the PCB circuit board industry, it is natural to set test points on the circuit board, but for newcomers who are new to PCB circuit boards, what are the test points? It is a question, so today, the editor of the PCB manufacturer will take you to understand why the test points should be set on the PCB circuit board.


Simply put, 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 of a circuit board, the easiest way is to use a universal You can find out by measuring the two ends of the meter.


However, in mass-produced circuit board factories, 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 the (In-Circuit-Test) automated testing machine, which uses multiple probes (commonly referred to as "Bed-Of-Nails" fixtures) to simultaneously contact all the parts on the board that need to be measured Then, through the program control, the characteristics of these electronic parts are measured sequentially in a sequence-based, parallel-assisted manner. Usually, it only takes about 1 to 2 minutes to test all the parts of the general board, depending on the parts on the circuit board. Depends, the more parts, the longer the time.


However, if these probes directly contact the electronic parts or their solder feet on the circuit board, it is likely to crush some electronic parts, but it will be counterproductive, so smart engineers invented "test points". A pair of small circular dots are drawn at both ends, and there is no solder mask (mask) on them, so that the test probes can contact these small dots without directly touching the electronic parts being measured.


In the early days when the circuit boards were still traditional plug-ins (DIP), PCB manufacturers did use the solder feet of the parts as test points, because the solder feet of traditional parts were strong enough to not be afraid of needle sticks, but there were often The misjudgment of poor probe 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. The impedance of the circuit board is very high, which often causes poor contact of the probe. Therefore, it was often seen that test operators on the production line of the circuit board factory often used an air spray gun to blow hard, or wiped the places that need to be tested with alcohol.


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, and it is already a little difficult to squeeze so many electronic parts on the small circuit board, so the problem that the test points occupy the circuit board space is often 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.


Using the needle bed for circuit testing will have some inherent limitations on the mechanism. For example, the minimum diameter of the probe has a certain limit, and the needle with too small diameter is easily broken and damaged.


The distance between the pins is also limited, because each pin has to come out of 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. There is a problem of contact short circuit, and the interference of the flat cable is also a big problem.


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.


As the circuit board is getting smaller and smaller, the number of test points has been repeatedly discussed, and now there are some methods to reduce test points, such as Net test, Test Jet, Boundary Scan, JTAG, etc.; there are also other test methods. I want to replace the original bed of needles test, such as AOI tester and X-Ray, but each test seems to be unable to replace ICT 100% at present.


The minimum diameter of the test point and the minimum distance between the adjacent test points usually have a desired minimum value and the minimum value that the capability can achieve. However, large-scale circuit board manufacturers will require that the distance between the minimum test point and the minimum test point is not the same. How many points can be exceeded, so PCB manufacturers will leave more test points when producing boards


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