Why do PCB boards require test points

For people who learn electronics, it is natural to set up test point on the circuit board, but for those who are mechanical, what is the test point?

Why do PCB boards require test points?

Maybe a little more foggy.I remember the first time I worked in a PCBA factory as a process engineer, and I asked a lot of people about it for this test point.Basically set up the test point is for the purpose of the test circuit boards have components conform to the specifications and welding, for example to check the resistance of the circuit board if you have any questions, the simplest way is to take the multimeter measure the two head can see.

But In mass production Of the factory is 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 have 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 such testing generally all parts Of the board only need 1 ~ 2 minutes time to complete, depending on the Circuit components on the amount, the longer the more parts.

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, on the surface of the solder 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, regularly take a hard blow air gun, or with alcohol to wipe the need to test.

In fact, the test point of wave soldering can also have 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.

But with the evolution of science and technology, the size of the circuit board is becoming more and more small, small circuit board under the light above the milking so many electronic components have some difficult, so the test points take up the question of circuit board space, often on design and manufacture the tug of war between, but later have the opportunity to come again to talk about this issue, etc.The appearance of the test points is usually round, because the probe is also round and relatively good, and it is easy to make the adjacent probe close to the point so that the needle density of the needle bed can be increased.

There are some institutional limitations to the use of a needle bed for circuit testing. For example, the minimum diameter of a probe has a certain limit, and a needle with a small diameter can break it easily.

The distance between the needle also to have certain restriction, because every needle out of a hole, and the back-end of each needle to weld a flat cable, if the adjacent hole is too small, and in addition to the needle will have contact between short circuit problem, interference of flat cable is also a big problem.

There is no needle for some high parts.If the probe distance too close to high parts will have the risk of collision high parts damage, because other parts is higher, often also in the test fixture seat hole to avoid needle bed, also indirectly caused can't plant the needle.More and more difficult to fit the test points of all parts on the circuit board.

As the boards are smaller and smaller, the test points are frequently discussed. Now there are some methods for reducing test points, such as Net test, test Jet, Boundary Scan, JTAG, etc.There are other test methods that want to replace the original needle bed test, such as AOI, X-ray, but at present every test seems to be unable to replace ICT 100%.

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.