Test Points in the Printed Circuit Board
About the Test Points in the Printed Circuit Board
Basically, test points are designed to check whether the components are correctly mounted as required on the Printed Circuit Boards (PCBs) and the solderability. For example, checking the resistance in the PCB is not a problem, the easiest method is using the multimeter. While in the situation of mass production it would be a lack of efficiency to measure every resistor in each board, capacitance, inductance or the IC. The factory will apply a so-called ICT (In -Circuit-Test). The use of multiple probes (commonly referred to as "bed-Of-nails" fixtures) simultaneously touches all parts of the board that need to be measured. And then according to the program mainly sequence-based, paralleling to the auxiliary way to check the components, normally it would cost about 1-2 minutes testing each board, it depends on quantities on the PCBs. The bigger, the longer.
And if these probes contact the components or the soldering pads directly, they may cause some damage to the components, in case of that the clever engineer designed the "test point", stretching the traces with a dot shaped pad at the end.
When the PCB assembly was still in the era of traditional plug-in (DIP), the components’ soldering feet would act as test points. As the soldering feet were strong enough, the testing probes won’t be harmful to the components, in many cases there were false failure report because of the probes poor contact with the soldering feet. As after the wave soldering process, there will always a layer of solder past paste flux with high impedance on the surface which will lead poor contact with the probes. The test operators, often with an air spray guns in hands, have to get rid of the unwanted film or using the alcohol to clean the feet.
Later, in the prevalence of SMT, the test situation has been greatly improved, the application of test points has also been widely accepted. Most components are usually very fragile, would not withstand the direct contact with the pressure of test probes. With the test points it wouldn’t be necessary for the direct contact with the components or the soldering pads, protecting the components and enhancing the reliability of the test!
However, with the evolution of technology, the size of the PCB is getting smaller and smaller, squeezing so many components in smaller boards is harder and harder, so the test points occupied some for the limited space and that is a side-effect.
Normally test points are in round shape, similar shape with the probes, in order to increase the density of the testing probes.
There are still some restrictions for the use of test probes, such as:
1. the minimum diameter of the probe. The probes will be break if the diameter is small.
2. the distance between the probe, in the bottom of each probe is connected with a cable. If the adjacent holes are small, there will be a short-circuit contact problem, besides the interference between the cables.
3.Some components’ height problem. The probes have to avoid the area with such components.
4.there are other test methods, such as AOI, X-Ray, and still ICT is necessary.