For many people who have studied electronics, setting up a test point on a circuit board is very easy. But for those who study mechanics or institutions, they may not know what a PCB test point is. The following passage will provide a detail description of PCB test point.
The function of PCB test point
The basic purpose of setting up PCB test point is to test whether the components on the circuit board are in line with the specifications and whether the parts are well welded. For example, if you want to check whether there is a problem with a resistance on the circuit board, the simplest way is to measure the solder joints at both ends of the universal electric meter. Other parts such as capacitance and inductance are similar.
Why the PCB test point has been born
In mass-produced factories, there is no way to use an electricity meter to slowly measure the correct circuit of every resistor, capacitor, inductor, or even IC on every board. Hence the emergence of automated test machines called ICT(in circuit test) or MDA(manufacturing defect analyzer).
It uses multiple probes (commonly referred to as bed of nails fixtures) to simultaneously touch all parts of the board that need to be measured, and then measures the electronic properties of those parts in a sequence-oriented, side-by-side, program-controlled manner. Usually, testing all the parts on a board only needs about 30-120 seconds to complete, depending on the number of parts on the circuit board, the more parts, the longer the time needed, of course, the fewer parts, the shorter the test will be.
However, if you let these probes directly contact the electronic parts on the board or the welding feet, it is likely to destroy some electronic parts. Instead, engineers invented the test spot, which leads to a pair of solder mask at both ends of the part. The test probe can be placed in direct contact with these dots, rather than directly touching the electronic parts that need to be measured.
The benefits of PCB test point
In the early days of traditional DIP on the circuit board, the welding feet of the parts were indeed used as PCB test point, because the welding feet of the traditional parts were strong enough to be not afraid of needles. However, misjudgment of poor contact of the probe often occurred.
After wave soldering or PCB soldering process, a residual film of solder paste flux is usually formed on the surface of the solder of general electronic parts. The impedance of this film is very high, which often causes poor contact of the probe. Therefore, test operators of the production line are often seen blowing hard with the air spray gun. Or use alcohol to wipe the areas that 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 SMT process became popular, the situation of test misjudgment was greatly improved, and the application of test points was also greatly entrusted with the task.
Because SMT parts are usually fragile and cannot withstand the direct contact pressure of test probe, the use of test points can avoid the direct contact of 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 misjudgments.
However, with the evolution of science and technology, the size of the circuit board is getting smaller and smaller. It is difficult to squeeze so many electronic parts on a small circuit board. Therefore, the problem of the space occupied by the test site of the circuit board is often discussed between the design side and the manufacturing side
What does PCB test point look like
The appearance of the PCB test point is usually round, because the probe is also round, it is easier to produce, and it is easier to make the adjacent probe closer, so as to increase the needle planting density of the needle bed.
Limitations of using a needle bed for PCB test point
The minimum diameter of the probe has a certain limit, too small diameter of the needle is easy to break and damage.
The distance between needles is also limited, because each probe has to come out of a hole, and the back end of each needle has to be welded with another row of wires. If the adjacent holes are too small, there will be short circuit contact between needles, and interference of the row of wires is also a major problem.
If it is a large number of fixtures, consider making additional circuit boards instead of leads.
Needles cannot be inserted next to some of the taller parts. If the probe is too close to the high part, there will be the risk of damage caused by collision with the high part. In addition, because of the high part, the test fixture needle bed seat usually needs to be opened for avoidance, which also indirectly causes the failure of needle planting.
It’s getting harder and harder to fit all the parts on the circuit board. As the board is getting smaller and smaller, the problem of the number of PCB test point and the existence of waste is often discussed. Now there are some methods to reduce the Test points, such as net test, test jet, boundary scan, JTAG… and so on. There are other testing methods that want to replace the original needle bed testing, such as AOI test and X-Ray, but so far none of them seem to be able to replace ICT 100%.
Based on the above passage, it’s easy to get to know the significance of PCB test point in PCB Assembly factory. The emergence of PCB test point has significantly improve the productivity of workers and save costs.