Why do flexible circuit boards have test points?

In a mass-produced factory, 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 testing machines, which use multiple probes (commonly referred to as "Bed-Of-Nails" fixtures) to simultaneously contact all parts on the board that need to be measured, and then The characteristics of these electronic parts are measured sequentially through the program control, which is based on the sequence, and the juxtaposition is the auxiliary method. Usually, it only takes about 1 to 2 minutes to test all the parts of the general board, 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 feet, it is likely to crush some electronic parts, but it will be counterproductive, so smart engineers invented "test points" at both ends of the parts. An additional pair of small circular dots, without a solder mask, allows the test probes to touch these dots without directly touching the electronic parts being measured. In the early days when there were still traditional plug-ins (DIPs) on flexible circuit boards, 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. The misjudgment of poor needle 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 is very high, which often causes poor contact of the probes. Therefore, test operators on the production line were often seen at that time, often blowing hard with an air gun, or using alcohol to wipe the places that need to be tested. 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. It is already a little difficult to squeeze so many electronic parts on the small flexible circuit board. Therefore, the problem of the test point occupying 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 easy to be 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 flexible circuit board is 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 also other tests. The method wants to replace the original bed of needles test, such as AOI, X-Ray, but at present every test seems to be unable to replace ICT 100%.

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