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What Is A PCB Test Point And Why Is It Important?

A PCB test point is an important contact that allows access to board components for testing. These points are normally placed when designing the circuit board and used throughout its production process—or even after. For more about what circuit board test points are, their benefits, and how to place or use them, continue below.

What is a Test Point on PCB?

A PCB test point is a designated location on circuit board that’s meant to connect it to testing equipment. These points allow various nets to be checked for functionality using various tests such as continuity or resistance, and other tests.

PCB test points can be conducting pads that are placed along or beside traces, vias that go into the layers, thru-hole leads, or simple hooked components that are soldered to the board’s surface.

Test points are introduced in the PCB phase, using tools within the CAD software. To make them easily identifiable, they are usually clearly marked in the board design file before fabrication, and on the silkscreen after assembly.

Using a test point in PCB
Using a test point in PCB
Resource: https://www.designworldonline.com

Importance of a Test Point in PCB

Why are test points important on a PCB? During its production process, a printed circuit board undergoes numerous tests. To facilitate the testing process, conductive elements (called test points) are strategically placed on the board’s surface for the following reasons:

  • Test points allow access to PCB nets. Testing equipment can then be connected and used to check the net’s electrical statuses.
  • Being low profile, these points do not introduce complexities to the board.
  • Having a test point enhances the board’s testing efficiency. This helps cut costs through the production process.
  • During use, PCB test terminals or test points help technicians diagnose circuits and components for repair. Usually, these are deliberately placed near components that are critical to the circuit, and properly marked for easy identification.
Different types of PCB test points
Different types of PCB test points
Resource: https://electronics.stackexchange.com

Type of PCB Test Points

PCB designers and manufacturers use different types of metal contacts, or test points, to access board components. Traditionally, these are hooked components called probes. Today, most test points on a circuit board are conductive pads along traces (or off them), vias that allow internal layers to be checked, or the thru-pins for automated testing.

Probe PCB Test Points

A PCB test probe is a simple component that’s soldered to the board and supplied with a small hook. The hook forms the point where the testing probe attaches to. Using this fixture, a handheld device such as a voltage tester is connected to the board.

Normally, these test point types are designated by the name of their nets, an example being “GND” and “PWR” or power and ground plane. This can be seen in the image above, where the test points are clearly marked on the board surface or silkscreen.

Automated PCB Test Points

These types of PCB test points are meant to be used with automated testing equipment. They include pads, through-hole pins or leads, and vias. With these points, manufacturers can perform the following tests: bare board test, in-circuit test, and the flying probe test.

Adding a test point in PCB design
Adding a test point in PCB design
Resource: https://forum.kicad.info

PCB Test Point Guidelines

Various PCB test point rules apply when it comes to placing them on circuit boards. That’s because improperly allocated connection can make access difficult. In high-speed or high-frequency boards, errors could also cause interference and other issues. These rules mainly concern test point size and clearance.

PCB Test Point Size

Large points are easier to probe. However, with today’s high density boards space may not be available. Industry rules recommend 0.050 inches for PCB test point dimensions, and only slightly smaller if crucial. Square points are also preferred; they provide larger areas to make contact with testing probes.

In addition to size, the test points need to be properly well distributed on the board. You want to avoid clustered contacts, as these can put a strain on the circuit board, sometimes even cause damage to the solder joints.

It’s also advisable to keep PCB test pads short enough, especially in high-speed circuits. This is to avoid stubs that may cause electrical interference by acting like small antennas.

PCB Test Point Clearance

A distance of 0.100 inches from one test point to another is recommended. In high-density PCBs, this can go down to 0.50 inches, but not any lower. This is meant to ensure testing efficiency.

Large test point to test point distances allow sturdy probes to be used. It also makes designing the boards fixture much easier and less expensive to make. That’s in addition to enhancing testing reliability.

For the distance between a test point on PCB board and the board edges, standard practice is to leave a 0.125 inch clearance and not less than 0.100 on the lower side. This is meant to allow room for the testing fixture, among other requirements.

How to test PCB using test points and probes
How to test PCB using test points and probes
Resource: https://electronics.stackexchange.com

How to Use Test Points on PCB

PCB test points are used to check for electrical continuity or potential and solder connectivity. This is commonly done using a fixture in what’s called the “in-circuit test” (ICT), or fixtureless with the flying probe test. Each method has its benefits and drawbacks.

  • With the in circuit method, a fixture is made for the specific PCB and equipped with spring-loaded probes. Using the PCB’s design file, the probes are designed to line with the test points. This test checks all nets at the same time, thus saving time.
  • In the flying probe test, a fixture is not necessary. Instead, the testing machine hovers some test probes over and around the board, testing specific parts one at a time. This method, being fixtureless, saves on costs.

A circuit board that has been equipped with testing points can also be probed using handheld devices. These include digital multimeters and various voltage testers and other equipment.

Conclusion

A PCB test point is an important addition when designing printed circuit boards. It allows a net to be checked for functionality and other requirements, ensuring the quality and reliability PCB and their application products. To ensure their proper placement, PCB test points must be designated as per industry guidelines, which include enduring their correct sizes and clearance.

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