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Working With PCB Test Points

Key Takeaways

  • Learn what PCB test points are used for.

  • Understand how the test points are created in a PCB design.

  • Consider important features in your software that will help with the generation of PCB test points.

A standard test form being filled in with a pencil

Testing is a part of everyday life.

The word “test” is not one of the more favored words in the English language. Usually, it conjures up unpleasant memories of school, a driver’s license examination, or trips to the doctor’s office. Yet, without the various trials we go through on a regular basis, we would never know our true capabilities or experience the confidence gained by passing a tough period of testing.

The circuit boards we design also have different types of testing to go through to be operationally ready for their intended use. Not only is the board tested to verify that it will function as designed but also that it has been assembled correctly and is ready to be put into service. To facilitate this testing, printed circuit boards are designed with small metal contacts called test points. Here is a brief overview of what PCB test points are and how you as a designer should work with them.

The Purpose of PCB Test Points

To verify the integrity of the component assembly process, a completed printed circuit board will go through an automated test cycle. This is designed to find component pins that are not connected to the board with a good solder joint and will use a system of probes to contact the test points designed into the board. There are two different systems that are generally used for this testing:

In-circuit test (ICT): 

This is a system that is designed to test all of the nets on the circuit board simultaneously. To do this, ICT employs the use of a test fixture that is loaded with probes to contact the test points on the board. The fixture will have one probe for each test point on the board, which enables the testing to be conducted very quickly. These test fixtures are usually configured to test the bottom of the board, but they can also test the top or both sides of the board at the same time.

Because the ICT fixture is designed to test each circuit together, it is the test method most likely to be used for production runs of a circuit board. Another advantage is that ICT can also conduct some functional testing of the board in addition to the manufacturing tests. The drawback, though, is that ICT fixtures take time to develop and are expensive to build. Alterations to existing fixtures for PCB design revisions can be costly as well.

Flying probe: 

This system differs from ICT in that it only uses between two and six probes to conduct all of the tests on the board. As the name would suggest, these probes fly around the board contacting each specific test point sequentially that they are programmed to test. Due to the more individualistic nature of the testing, the flying probe takes a lot longer to test a board than an ICT will. Flying probes also aren’t able to conduct any functional testing of the board due to the minimum amount of test points that are being probed at one time.

The advantage of the flying probe test, however, is that it is very fast and inexpensive to set up and run. Additionally, alterations to the test due to circuit board revisions are easy to incorporate. Flying probe systems can also test larger board sizes than ICT systems can.

This is the testing that the PCB test points make possible. Next, we’ll look at what it takes to create those test points during PCB design.

The menu for test point parameters in Cadence’s Allegro PCB Designer

The test point parameters menu of Allegro PCB Designer.

How Test Points Are Created in PCB Designs

To function as a PCB test point, the test point must be an exposed metal spot on an exterior layer of the board that is connected to a net. This will allow the probes of the automated test equipment to make contact with it to conduct the test. As the test probe tips are available in a variety of shapes for different testing surfaces (flat, spherical, conical, etc.), each test point on the board can be matched up with a probe that is best suited for it. This allows the designer to designate existing thru-hole pins and vias in the board as test points, as well as flat surface test points.

The next step is to flag the desired locations as test points in the CAD system. Most PCB design software tools will have special utilities built into them for doing this, and you can see in the picture above a typical menu for setting up the test point parameters. The design software typically will then go through and automatically select test point candidates if they conform to spacing and other specific requirements set up in the parameters. There are also a number of options that can be done with test points, such as changing via shapes from a round pad to square if visual identification of the test point is required.

Once each candidate has been flagged as a test point, the PCB design software is ready to export this information into a test point file. This data is used to build test fixtures and program the automated test equipment. The key to all of this, of course, is to work with PCB design software tools that allow for the easy creation of test points.

A 3D layout in Cadence Allegro PCB Designer

Complex circuit boards today need a lot of help from the design tools to enhance productivity.

PCB Design Software Tools for Creating Test Points

At one time, the creation of test points in a printed circuit board design was purely a manual operation requiring a lot of work on the part of the designer. With today’s complex PCB designs, however, that method won’t work anymore. To stay ahead of their new product introduction schedules, PCB designers need the best in automated test point generation systems. This includes the ability to automatically select and flag test point locations as well as set up a variety of parameters to satisfy the requirements of your manufacturer.

You need the most advanced PCB design system available to you for the test point functionality required on today’s designs. Allegro PCB Editor not only has the functionality you need for easy test point generation, but it also has many other features designed to increase your productivity while assuring design quality.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts