Why Are There Test Points on the Printed Circuit Board?
The boards we design go through some tests, which determine their build quality. In order to perform this test, the board must be prepared with specific elements accessible by the test probes, these elements are called test points and they are very important in PCB design. In this article, we mainly introduce the specific functions of the test points for your reference.
The purpose of setting test points on the printed circuit board 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 on a circuit board, the easiest way is to use a universal power supply. You can find out by measuring the two ends of the meter.
However, in mass-produced factories, there is no way for you to use an electric meter to slowly measure whether the circuit of each resistance, capacitance, inductance or even IC on each board is correct, so there is a so-called ICT (In-circuit Test) the emergence of automated test machines.
We can use multiple probes (commonly referred to as bed-of-mails) to simultaneously contact all the component circuits on the board that need to be measured, and then sequentially measure these electronic components in a sequence-based, parallel-assisted manner through program control characteristics.
Usually, it only takes about 1 to 2 minutes to test all the parts of the general board in this way, depending on the number of parts on the circuit board, the more parts, the longer the time.
However, if these probes directly contact the electronic parts on the board or their solder pins, it is likely to crush some electronic parts.
So engineers invented "test points", which lead out a pair of small circular points at both ends of the part. There is no solder mask on it, so the test probes can touch these small points without direct contact. to those electronic parts being measured.
In the early days when there were still traditional plug-ins (DIP) on the circuit board, the solder feet of the parts were indeed used as test points, because the solder feet of traditional parts were strong enough to not be afraid of needle sticks, but there were often probes. Misjudgment of poor contact occurs.
Because general electronic parts are subjected to wave soldering or SMT tin, a residual film of solder paste flux is usually formed on the surface of the solder. The resistance of this film is very high, which often causes poor contact of the probe.
In fact, after wave soldering test points, there will also be problems of poor probe contact. Later, after the prevalence of SMT, 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 very fragile and cannot withstand the direct contact pressure of the test probe, the use of test points eliminates the need for the probe to directly contact the parts and their solder legs, which not only protects the parts from damage but also indirectly greatly improves the test performance. reliability, because there are fewer false positives.
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 of the test point occupying the circuit board space is often a tug of war between the design side and the manufacturing side.
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 a diameter is easy to be broken and damaged.
The distance between probes is also limited because each probe has to come out of a hole, and each probe has to be soldered to the back end of another flat cable.
If the adjacent holes are too small, in addition to the problem of contact shorting between the probes, the interference of the flat cable is also a big problem.
Probes cannot be planted next to parts with high positions. If the probes are too close to the parts with high positions, there will be a risk of collision with the parts and damage. In addition, because the positions of the parts are high, it is usually necessary to test the needle bed seat of the jig. Avoiding the upper opening also indirectly makes it impossible to plant needles, making it more and more difficult to accommodate the test points of all parts on the circuit board.
There are also other test methods that want to replace the original bed of needles tests, such as AOI and X-Ray, but each test seems to be unable to replace ICT 100% at present.
Regarding the needle implanting ability of ICT, you should ask the manufacturer of the matching fixture, that is, the minimum diameter of the test point and the minimum distance between the adjacent test points. Usually, there will be the desired minimum value and the minimum value that the ability can achieve, but there are manufacturers of the scale who will require a minimum distance between test points and a maximum value.
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