Complicated and various components can be discovered on PCB. Sometimes, we usually wonder some questions like whether this component can be welded and conform to the specification. A qualified PCB finished product must accord with the requirement, or it might bring malfunction. Then, it is particularly critical to test before manufacturing. Hence, the PCB test points play a critical role in PCB manufacturing. It is a measure to test whether up to the standard and solderability. If you do not know the test point before or you plan to use the test point, this passage is going to show you what makes PCB test points become necessary and give you some suggestions as well.
What is PCB test point?
There is a testing probe placed on the board to allow test signal injecting and circuit checking on the board. This kind of testing probe on the circuit board is PCB test point. You can easily discover it is usually placed on the bottom and both sides of the board.
What are the types of PCB test points?
- Bare board test:
Like its name, this test is used after the board is finished fabrication for ensuing the electrical connectivity of the board.
- In-circuit test:
When the board is assembled, ICT can be adopted. The circuit board will be placed on the ICT fixture which is full of probes.
- Flying probe:
This type is also used after the board is assembled. It adopts 2 to 6 probes to move around each test point while ICT adopts fixed probes in the fixture.
Why PCB test point is needed?
If we want to check whether the components on the circuit board are up to the specifications and whether the components are well welded, the PCB test points can make full use of its strength to solve these problems. Generally, a multimeter can be used to test the solder joints at both ends whether a resistor of a board fails to work. This way can also be used on other components like capacitors and so on.
Actually, for mass production circumstances, it is hard to spend much time and energy to particularly detect each resistance, capacitance, inductance and IC circuit. Here is a question. How can we measure each component on the circuit board to keep the quality and save more time and energy? Some automatic testing machines like ICT(In-circuit test) or MDA(Manufacturing defect analyzer) can achieve win-win results
How does the machine work? It through several probes ( commonly called Bed of Nail fixture) to contact those components on the circuit board that need to be measured. Under program controlling, which mainly relies on sequence, and takes paralleled as auxiliary, to sequentially check and test their electronic performance and capabilities. Usually, the time spend on testing all the components on the circuit board through the automatic machine depends on the number of components. That is, more component absolutely takes up much time.
If these probes are allowed to directly touch the electronic components on the board or solder feet, it is likely to crush some electronic components, which is counterproductive. So, PCB test points are adopted. A pair of circular small points are additionally drawn out at the parts on both ends. The test probe allows you to directly touch these small points without directly contacting the electronic parts that need to be measured.
However, it can be founded that some electronic components are probably to be crushed when the probes have directly contacted the components and solder parts. It will bring some damage to the circuit board. PCB test is exactly appeared and adopted to better cover up this dilemma. A small wire loop is placed at both ends of the components without a solder mask on it. Then next step is to use the probe to measure components. It is equivalent to having an invisible septum to prevent direct contact with the components measured.
How are PCB points developed?
When there is a traditional plug-in (DIP) period, with the advantages of firmness and better probe-bearing capability, the solder feet of the components were usually taken to act as test point. Although this can take some effects, there are still appear misjudgments due to poor probe contact. After wave soldering or SMT soldering of some electronic components, the surface of the solder will form a residual film of solder paste and flux with high resistance. This is why it might cause poor probe contact circumstances. So, if you saw a test operator were blowing through a tool called an air gun or wiping by alcohol on tested parts is common.
Actually, wave soldering is also hard to avoid poor contact of the probe. This situation is greatly improved under the widely used of SMT process. Besides, the PCB test points are gradually adopted in the manufacturing process. With characterized of fragile components of the SMT, they fail to sustain the direct contact air pressure that came from the test probe. With the help of a test point, the probe fails to contact the components and solder feet directly. The components can be better protected. Moreover, it also improves testing accuracy and lowers misjudgments.
Have you ever considered why the appearance of the test point and the probe are designed as round. Because it is easy to manufacture and adjacent probes become closer for increasing the needle density.
Here is an example. There must be a certain limitation of the minimum diameter of the probe. If the diameter is designed too small, it might lead to break easily.
How about the distance between the probe? It also has the requirement that each needle is welded with another cable and comes out from a hole. If the adjacent holes are designed too narrow, we need to consider some additional situations. For example, the short circuit problem between those probes and cables.
For some taller components, it is not easy to plant the needles. There is a risk of collision of those taller components if the closer distance between the probe and these kinds of components. Furthermore, there taller components need to make holes to detour while testing the fixture, which is also difficult to plant the needles.
Nowadays, PCB is designed as more compact. Thus, there is less space to place PCB test points for components. Some methods for reducing the test points are put forward such as Net test, Test Jet, Boundary Scan and JTAG. There also have other new test methods like AOI or X-Ray which are used to replace the original needle tests. Currently, no matter which new test methods, they still can not replace ICT test.
The needle implanting capability of the ICT, including the minimum diameters of the test points and the minimum distance of the adjacent test points, needs to ask the fixture suppliers. Sometimes, an experienced supplier will require the distance between the minimum test point and minimum test distance to keep on a certain standard for avoiding damaging the fixture.
What can be considered when placing test points?
- For the circuit board
PCB test points should be placed on the same side of the board. It is efficient to save more time and money for the testing procedure.
- For PCB test points distance:
It is recommended to hold a minimum distance in 0.1 inches between PCB test points which is beneficial to improve the test efficiency.
- For the layout:
They should be distributed evenly on the whole PCB board, which is good for undergoing the test through several probes.
- For tall components:
You can designate a specific area for those tall components, in order to keep them from hampering test points to carry out.
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