What Are The Test Points On A PC

PCB Design: Why test points are needed on a PCB?

But In mass production Of the factory have no way to let you use the meter to slowly on the measurement Of each board to each measuring resistor, capacitor, inductor, and even the IC Circuit is correct, so there is the so-called ICT (In Circuit Test) - automated Test machine, it USES the dogan probe (commonly called "needle Bed (Bed - Of - Nails)" fixture) and contact the board all parts need to be measured on line, and then through the SPC is given priority to with sequence, tied for the auxiliary method In measuring the characteristics Of the electronic parts,Usually this test of the general board all parts only need 1~2 minutes or so time can be completed, depending on the number of parts on the circuit board, the more parts the longer time.

But if you allow the probe contact directly to the board on electronic components or above its leg, is likely to be crushing some electronic components, the opposite, so clever engineers invented the "test point", in the ends of the additional parts to elicit a pair of round dots, there is no welding (mask), can make a test probe access to these small points, without direct contact to the measurement of electronic parts.

Early on the circuit board are traditional plug-in (DIP) s, can really take the welding parts feet as test points, because traditional parts of welding feet enough strong, not afraid of needle, but often have poor contact probes of misjudgment occurs, because the general electronic components after wave soldering, wave soldering or SMT solder, the solder surface usually form a layer of solder paste flux residual film, the film of the impedance is very high, often cause poor contact probe, so at that time, are a common test operators of production line,Often blowing hard with an air gun or rubbing alcohol on areas that need to be tested.

In fact, after the wave soldering test point will also have bad probe contact problem.Later after SMT prevailed, test misjudgment situation had very big improvement, the application of the test point has also been greatly rewarded, because of the SMT parts are usually very weak, cannot bear the direct contact with the pressure of test probe, using the test point can not let the probe in direct contact to the parts and its welding feet, not only to protect parts from harm, also indirectly promote reliability testing, because the misjudgment of less.

With the evolution of technology, the size of circuit boards has become smaller and smaller. It is already difficult to squeeze so many electronic components onto a small circuit board, so the issue of test points taking up space on the circuit board is often a tug of war between the design and manufacturing sides, but this topic will be discussed later.The appearance of the test point is usually round, because the probe is also round, which is easier to produce, and it is easier for the adjacent probe to get closer, so as to increase the needle density of the needle bed.

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 has to come out of a hole, and the back end of each needle has to be welded with another flat cable. If the adjacent hole is too small, in addition to the problem of contact short-circuit between the needles, the interference of the flat cable is also a big problem.

Needles cannot be inserted 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 of the high part, holes are usually cut in the needle bed seat of the test fixture, which also indirectly results in the failure of needle implantation.The test points of all the parts on the circuit board are getting harder and harder to fit.

As the boards are getting smaller and smaller, the storage and waste of test points are discussed time and again. Now there are some methods to reduce test points, such as Net test, Test Jet, Boundary Scan, JTAG, etc.There are other test methods to replace the original needle bed test, such as AOI and X-ray, but none of them seem to be able to replace ICT 100% yet.

Flocking needle ability should ask about ICT fixture manufacturers, namely the minimum diameter of test point and the minimum distance between adjacent test points, usually will have a hope of minimum value and ability you can achieve the minimum, but have scale vendors will require minimum test points and minimum how many points, the distance between the test point can not exceed or fixture is easy to damage.