Why Do PCB Boards Need Test Points? (2)

However, with the evolution of technology, the size of the printed circuit board has become smaller and smaller. It is already a bit difficult to squeeze so many electronic parts on the small circuit board. Therefore, 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 bring the adjacent probes closer, 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 diameter is easy to break and damage.

The distance between the needles is also limited, because each needle must come out of a hole, and the back end of each needle must be soldered with a flat cable. If the adjacent holes are too small, except for the gap between the needles, there is the problem of contact short circuit, and the interference of the flat cable is also a big problem.

Needles cannot be implanted next to some tall parts. If the probe is too close to the high part, there is a risk of collision with the high part and cause damage. In addition, because of the high part, it is usually necessary to make a hole in the needle bed of the test fixture to avoid it, which indirectly makes it impossible to implant the needle. Test points for all parts that are increasingly difficult to accommodate on the circuit board.

As the boards are getting smaller and smaller, the number of test points has been repeatedly 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 which wants to replace the original needle bed test, such as AOI, X-Ray, but it seems that each test cannot replace ICT 100%.

Regarding the ability of ICT needle implantation, you should ask the matching fixture manufacturer, that is, the minimum diameter of the test point and the minimum distance between adjacent test points. Usually there will be a desired minimum value and a minimum value that the ability can achieve, but there are large-scale manufacturers will require that the distance between the minimum test point and the minimum test point cannot exceed a few points, otherwise the jig will be easily damaged.