PCB Technical - What is the use of test points on the PCB board

Some people may ask: "PCB circuit board design: Why do I need to have test points on the PCB?" Maybe they are still a bit confused. I remember that when I first worked as a process engineer in a PCBA processing plant, I asked many people about this test site to understand it. Basically, the purpose of setting test points is to test whether the components on the circuit board meet the specifications and solderability. For example, if you want to check whether there is any problem with the resistance on a circuit board, the easiest way is to measure with a multimeter. You can know it by measuring both ends. details as follows:

Measuring Voltage

Select the appropriate gear: Adjust the voltage measurement gear of the multimeter to the desired range.

Connect the test pens: Connect the red test pen to the positive terminal on the board and the black test pen to the negative terminal.

Read the voltage value: Read the voltage value on the multimeter.

Measure current

Disconnect the current line:Adjust the current measurement setting of the multimeter to the desired range, but be careful to disconnect the current line on the circuit board first.

Connect the test pen:connect the red test pen to the positive terminal on the circuit board and the black test pen to the negative terminal.

Turn on the circuit:Turn the circuit lines on.

Read the current value:Read the current value on the multimeter.

Measure the resistance

Select the appropriate gear:Adjust the resistance measurement gear of the multimeter to the desired range.

Connect the test pens: Connect the red test pen to one end point on the board and the black test pen to the other end point.

Read the resistance value: Read the resistance value on the multimeter.

Precautions

Disconnect the power supply:When measuring the circuit board,you should first make sure that the power supply on the board has been turned off,so as not to cause damage to the circuit board or pose a threat to your personal safety.

Choose a suitable multimeter: Use a multimeter with high internal resistance and many stops (e.g. digital meter),avoid using a multimeter with low internal resistance and few stops (e.g. pointer meter).

Anti-slip measures: the meter pen or probe should take anti-slip measures,available bicycle valve core set on the pen, and should be longer than the tip of the pen about 5mm.

However, in mass production factories,there is no way for you to use an electricity meter to slowly measure whether each resistance, capacitance, inductance, and even IC circuits on each board are correct.So there is the so-called ICT (In -Circuit-Test) .The emergence of automated test machines,which use multiple probes (generally called "Bed-Of-Nails" fixtures) to simultaneously contact all the parts on the board that need to be measured.Then the characteristics of these electronic parts are measured sequentially through the program control with sequence as the main and side by side method. Usually, it only takes about 1 to 2 minutes to test all the parts of the general board, depending on the number of parts on the circuit board. It is determined that the more parts, the longer the time.

But if these probes directly touch the electronic parts on the board or its solder feet,it is likely to crush some electronic parts, which will be counterproductive. So smart engineers invented "test points", which are located at both ends of the parts.A pair of small circular dots are additionally drawn out without a solder mask (mask), so that the test probe can touch these small dots instead of directly touching the electronic parts to be measured.

In the early days of PCB design,there were traditional plug-ins (DIP).We did use the solder feet of parts as test points. Because the solder feet of traditional parts were strong enough, they were not afraid of needle sticks,but there were often probes.The misjudgment of poor contact occurs, because after general electronic parts undergo wave soldering or SMT tin, a residual film of solder paste flux is usually formed on the surface of the solder,and the resistance of this film is It is very high, which often causes poor contact of the probe. Therefore, test operators on the production line were often seen at that time, often holding an air spray gun to blow desperately, or using alcohol to wipe these places that needed to be tested.

In fact, the test points after wave soldering will also have the problem of poor probe contact. Later, after the popularity of SMT, the misjudgment of the test was greatly improved, and the application of test points was also given a great deal of responsibility, because the parts of SMT are usually very fragile and cannot withstand the direct contact pressure of the test probe. Use test points. This eliminates the need for the probe to directly contact the parts and their solder feet, which not only protects the parts from damage, but also indirectly greatly improves the reliability of the test, because there are fewer misjudgments.

However, with the evolution of technology, PCB size has become smaller and smaller. It is already a bit difficult to squeeze so many electronic parts on a small circuit board. Therefore, the problem of test points occupying circuit board space is often on the design side. There is a tug-of-war with the manufacturing side, but this topic will be discussed later when there is a chance. The appearance of the test point is usually round, because the probe is also round, which is easier to produce, and it is easier to bring the adjacent probes closer, so that the needle density of the needle bed can be increased.

The use of a needle bed for circuit testing has some inherent limitations on the mechanism. 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 must come out of a hole, and the back end of each needle must be soldered with a flat cable. If the adjacent holes are too small, except for the gap between the needles There is the problem of contact short circuit, and the interference of the flat cable is also a big problem.

Needles cannot be implanted next to some tall parts. If the probe is too close to the high part, there is a risk of collision with the high part and cause damage. In addition, because of the high part, it is usually necessary to make holes in the needle bed of the test fixture to avoid it, which indirectly makes it impossible to implant the needle. The test points of all the components that are increasingly difficult to accommodate on the PCB circuit board.

Test points are set at specific locations on the PCB that allow engineers to perform electrical tests during the manufacturing process to check and verify the integrity of the circuit, including:

Checking the quality of soldering: Test points can help to check whether the soldering points are up to standard and ensure that all components are connected correctly.

Functionality testing: Test points help to check the functionality of the board and ensure that the components are working as intended.

Troubleshooting: When a board has a problem, test points provide a diagnostic path to help identify the source of the problem.

Benefits of Testing

The benefits of using test points include:

Increase test efficiency:it is designed to increase test efficiency, reduce test time, and allow multiple components to be tested at the same time.

Protecting component integrity: it protect the integrity of sensitive components by avoiding direct contact with them during testing.

Ensure product quality: Through comprehensive testing and inspection, test points help ensure the quality and reliability of the final product.