Why PCB Boards Need Test Points
Have you wondered why your PCB boards need test points? The purpose of adding test points is to test whether the components on the circuit board meet the specifications and solderability.
Checking the resistance on a printed circuit board is not a problem. The easiest way is to measure the two ends with a multimeter. But in a mass-production PCB factory, there is no way for you to measure the resistance, capacitance, inductance, or even the IC circuit of each board by using an ammeter. Thus, the PCB In-Circuit-Test (ICT) automated tester was created. It uses multiple probes (generally called "Bed-Of-Nails" fixtures) to simultaneously check all the needs on the board. Usually, testing all components on a common PCB board takes only about 1 to 2 minutes to complete.
However, if these probes directly contact the electronic components or their soldering pins on the PCB board, it is very likely destroy some of the electronic components. So smart engineers invented "test points." An extra pair of small round dots on both ends of the PCB components, with no mask on them, allows test probes touching these dots without directly contacting the electronic components which need to be measured.
The appearance of the test point is usually round, because the probe is also round, which is relatively easy to produce. However, with the evolution of technology, the PCB board size is getting smaller and smaller, thus setting test points has its drawbacks now.
1.The use of bed-of-nails for circuit testing has some mechanical limitations. For example, the minimum diameter of the probe has a certain limit, and the nail with a small diameter is easily broken.
2.The distance between nails is also limited because each nail comes out of one hole and a flat cable is soldered to the back of each nail. If the adjacent holes are too small, in addition to the problem of contact shorting between pins, the interference of the flat cable is also a problem.
3.No nails can be placed next to some high components. If the probe is too close to a high component, there is a risk of collision that can damage high components.
4.The PCB board size is getting smaller and smaller. It is already a bit difficult to arrange so many electronic components on a small printed circuit board. Therefore, it can not be ignored that test points occupy the circuit board space. There are already some ways to reduce test points, such as Net test, Test Jet, Boundary Scan, JTAG, etc.; other test methods, that are intended to replace the bed-of-nails tests, are AOI, X-Ray, etc.
EPCB regards that PCB boards need test points to ensure the final PCB board quality, but the number of them shall be as small as possible.