What Are PCB Test Points and How Are They Used?

A test point can be defined as a small amount of copper to which a probe can be connected. A test point could be either an oscilloscope probe in development or a contact pin in production. These points are typically located on the bottom of the printed circuit board. However, test points may be on either side of dense or more complex boards. Setting test points is necessary to verify that the circuit board's components comply with specifications. If you are trying to determine if there is resistance on a circuit board, then the easiest way to test it is to measure the ends using a multimeter. It is essential in PCB design to incorporate test points.

A series of probes connected to test points on a PCB.

In mass-produced factories, however, measuring each resistance, capacitance, and inductance on every board is impossible using an ammeter. The so-called ICT (in-circuit test) automatic test machine was created. Multiple probes, commonly known as the "bed of nails," are used to contact all electronic components on the board. The program control then determines the characteristics of the electronic components using the line as the primary and parallel, respectively, as the auxiliary. This method usually takes 1-2 minutes to test every component of a board. The time required to test all circuit board components will vary depending on how many there are. These probes could contact electronic components and solder pins on the boards. The probes can crush some electronic components.

Brilliant engineers created the "test point," which is a pair of dots at the ends of each part. It does not have a solver mask so that the probe can touch these dots rather than directly touching the electronic components. It was established that solder pins were used to test parts in the early days for traditional plug-ins (DIP). This was because standard components' solder pins were strong enough to withstand pinpricking. However, it was not uncommon for probes to be misread. This is because the electronic components of general electronics after wave soldering or SMMT eat tin. The solder surface of these components will often form a layer or solder paste flux residual film. This film impedance can cause poor contact with the probe.

A flying probe PCB test points

After the popularity of SMT, test misjudgment was significantly reduced, and the application test points became a difficult task. We often see production line testers blowing with an air gun or wiping out areas that need to be tested with alcohol. Poor contact probes can also be problematic with an after-wave soldering test point. The components of PCB are often very fragile and cannot bear the pressure of the probe. Therefore, test points cannot make the probe contact the components or their solder pins. This protects them from damage and improves the test's reliability by allowing for fewer misjudgments.

However, circuit boards are shrinking due to science and technology. It is challenging to squeeze so many electronic parts onto a small circuit board. The problem of test points taking up space on a circuit board is often a tug-of-war between the end and the end. The test point's appearance is often round due to the probe being round. This is better for production and makes it easier to place the adjacent probes close to the needle bed to increase its density. A probe's minimum diameter must be within a specific limit. Needles with too small a diameter can easily break or cause damage. A pin must be placed in a hole. The back end of each pin must also be welded using a flat cable. The interference of flat cables can also be a problem if the holes adjacent to them are too small.

High-resolution devices cannot be placed next to the needle. The probe should not be placed too close to the high-resolution component. This will increase the risk of the probe colliding with the high component. Making a hole in the needle bed seat is often necessary to avoid needle implantation. It becomes more challenging to hold all test points on the circuit board. The number of test points on the board is decreasing in size. This has been the subject of repeated discussions.

There are several ways to reduce the number of test points. These include net tests, boundary scans, and JTAG. You can also use other methods to replace the original needle-bed test, such as AOI or X-ray. However, they cannot replace ICT 100%. We should inquire about the ICT's needle planting capability. This is the minimum dimension of the test point and the distance between adjacent test points. The ability to reach the desired minimum and maximum value will generally be set. Large-scale PCB producers will insist that the distance between the minimum and minimum test points cannot exceed how many points. Otherwise, the fixture could be easily damaged.

The usage of test points can speed up development and provide you with quality control reassurance.