What are the different types of test points used on a printed circuit board?
Why are PCB test points important for the successful manufacturing of a circuit board?
How PCB design CAD tools are used to add test points to your design.
A circuit board to be tested.
Just mention the word “test” and most people will feel their heart skip a terrifying beat. There is something very intimidating about taking a test that seems to be instilled in us from our first days of elementary school. As everyone knows, though, testing is a very important part of life. You wouldn’t want to fly on a plane with a pilot who failed her flight examination, and the idea of a surgeon who didn’t pass his basic anatomy class brandishing a scalpel should scare everyone.
The circuit boards we design also have some testing to go through; and, in their case, the testing will determine how well they have been manufactured. In order for this testing to happen, the board must be prepared with specific elements that a test probe can access. These elements are called test points and their inclusion on PCB designs is very important. Let’s take a few minutes here to talk about the PCB test point and why it is so important to the success of the completed circuit board.
The Different Types of PCB Test Points
There are two main types of test points used on a printed circuit board. The first is an easily accessible point for a technician to access with hand-held testing equipment. To help with this, these points often have a post soldered into them for clipping on a test lead. These test points will be marked with a reference designator and usually the name of the net in silkscreen for easy reference, such as “GND.”
The other type of test point is used primarily for automated test equipment. The automated test points on a circuit board are thru-hole pins, vias, or small landing pads of metal that are designed to accommodate the probes of automated test systems. There are three main test systems that will use these test points on the board:
Bare board test: This is a test that is run after the circuit board has been fabricated to make sure that all of the nets in the board have electrical connectivity.
In-circuit test: ICT is run after the board has been assembled. The board is positioned on the ICT fixture which is filled with probes for each individual net. These fixtures usually test the bottom of the board, but they can also test the top or both sides together if needed.
Flying probe: Like ICT, this test is run after assembly on each net of the board by accessing the test points. Where it differs, however, is that the flying probe equipment uses two to six probes that move around to each test point instead of using stationary probes in a fixture like ICT does.
Those are the different test points that will be included in a PCB design and what they are used for. Next, we will see why this testing is so important for circuit board manufacturing.
A circuit board having its assembly verified with a flying probe tester.
Why Test Points Are Important for PCB Manufacturing
Automated testing is essential for verifying the manufacturing of printed circuit boards. While a bare board test checks that the fabricated board has good connectivity, ICT and flying probe tests will verify the board assembly and solder connections. Here is how ICT and flying probe testing accomplish that:
In-Circuit Test (ICT)
ICT is a system that uses a test fixture that is prepared exclusively for a specific circuit board design as well as accompanying software that will be run on the test system. The fixture and software are created using the test point locations and other design data generated by the CAD database, and a probe will be used for each net on the design. While the main purpose of ICT is to verify the assembly of the board by testing each net for a good connection to its associated part pin, it can also be used to test the functionality of the board.
ICT test fixtures and software are expensive and time-consuming to develop. The test fixtures are complex with thousands of probes in them and changes to a fixture for board revisions can also be very expensive. Once developed, though, the in-circuit test will test large amounts of circuit boards very quickly. For mass production, the benefits of speed and functional testing quickly offset the development time and costs of ICT.
Flying Probe Testing
This system is also designed to verify the integrity of an assembled circuit board, but it does this very differently than ICT. Instead of a test fixture, there are only two to six probes that fly around accessing each test point on the board. Without the need to build an expensive test fixture, the flying probe system can be set up very quickly and inexpensively. Also, any changes to the board that require changes to the test system can be easily reprogrammed.
The disadvantage of the flying probe is that it is very slow. Where ICT can test all of the points on the board almost instantaneously, the flying probe system has to maneuver its probes around to each individual test point. On a large board, this can take a very long time, and the ability to do functional testing is eliminated. However, for prototypes or low volume production runs, using the flying probe for testing can be very advantageous.
Now that we’ve seen what test points are and how they are used, we’ll next look at how PCB CAD tools include them in the design.
PCB design tools usually have detailed test point parameters as you see here in OrCAD PCB Designer.
Using PCB Design Tools to Assign Test Points to Your Board
Most PCB design systems will offer specific tools or features for adding test points to a design. Usually, these tools will need their parameters set up first, as shown in the picture above, before the test points are assigned. These parameters will cover various details such as how close the test points are allowed to be to each other and additional design for test rules as well. Once the parameters are set, the test points can then be added automatically or manually by the designer in an iterative process.
Test points are typically assigned first to any available thru-hole pins and then to vias. If there aren’t any vias available, the tools will give the designer the ability to add a new via onto a trace. There will also be a number of options available when assigning test points, such as adding a specific reference designator or changing the via pad shape from round to square.
One of the best PCB design systems for working with test points is OrCAD PCB Designer. With OrCAD, you will have the capabilities you need to make sure that your PCB design has full test point coverage before it goes to manufacturing.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.