Why should test points be set in circuit board design?
It's natural for electronics learners to set test points on circuit boards, but for mechanical learners, 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 the resistance of a circuit board, the simplest way is to take a multimeter to measure its two ends.
But in mass-produced factories, there is no way for you to slowly measure every resistance, capacitance, inductance, or even IC circuit on each board with a meter, so there is the so-called in-Circuit-Test automated test stand.
It uses multiple probes (commonly referred to as "Bed-Of-Nails") to simultaneously contact all parts on the board that need to be measured, and then sequentially measures the characteristics of these electronic parts through programmable control, with sequence as the dominant and supplementary means.
Usually, it takes only about 1 to 2 minutes to test all parts of a general board.
Depending on the number of parts on the circuit board, the more parts the longer.
But if these probes are directly touched by the electronic parts on the board or their welding feet, they are likely to crush some of the electronic parts. On the contrary, smart engineers invented "test points".
An additional pair of circular dots is introduced at both ends of the part. There is no mask on them. It allows the probe for testing to touch these dots without directly touching the measured electronic parts.
In the early days of traditional plug-ins (DIPs) on circuit boards, it was true that the welding feet of parts were used as test points.
Because the solder foot of traditional parts is strong enough not to be afraid of pinning, but there are often misjudgments of poor contact of probes, because after wave soldering or SMT tin feeding, a residual film of solder paste flux is usually formed on the solder surface of ordinary electronic parts.
The impedance of the 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 an air spray gun or wipe these places with alcohol.
In fact, the test point after wave soldering also has the problem of poor contact of probe.
After the popularity of SMT, the situation of misjudgement in test has been greatly improved.
The application of test points has also been greatly entrusted with the task, because SMT parts are usually fragile and unable to withstand the direct contact pressure of test probes.
By using the test point, the probe can not directly touch the parts and their welding feet, which not only protects the parts from damage, but also indirectly greatly improves the reliability of the test, because the misjudgement situation is less.
However, with the development of science and technology, the size of circuit boards is getting smaller and smaller. It's a little hard to squeeze so many electronic parts on small circuit boards.
Therefore, the problem of occupying PCB space by test points is often a tug-of-war between the design and manufacturing sides, but this issue will be discussed later.
The appearance of the test point is usually circular, because the probe is also circular, better production, and easier to make adjacent probes closer, so as to increase the needle density of the needle bed.
1. There are some inherent limitations in using needle bed for circuit testing. For example, the smallest diameter of the probe has a certain limit, and the needle with too small diameter is easy to break and destroy.
2. The distance between needles is also limited, because each needle needs to come out from a hole, and each needle has to weld a flat cable at the back end. If the adjacent hole is too small, besides the contact short circuit between needles and needles, the interference of flat cable is also a big problem.
3. Needles cannot be planted next to some high parts. If the probe is too close to a high part, there will be a risk of damage caused by collision with a high part. In addition, because of the high part, it is usually necessary to open a hole on the test fixture needle bed to avoid, which indirectly results in the inability to implant needles. The test points of all the parts on the circuit board are becoming more and more difficult to accommodate.
4. As the board is getting smaller and smaller, the number of test points has been discussed repeatedly. Now there are some methods to reduce test points, such as Net test, Test Jet, Boundary Scan, JTAG. There are other testing methods that want to replace the original needle bed testing, such as AOI, X-Ray, but at present each test seems unable to replace ICT 100%.
As for the ability of needle implantation in ICT, we should ask the cooperating fixture manufacturer, that is, the minimum diameter of the test point and the minimum distance between the adjacent test points. Usually there will be a hopeful minimum and a minimum that the ability can reach, but the large-scale manufacturer will require that the distance between the minimum test point and the minimum test point should not exceed much. Less, or the treatment will be easily damaged.
