Over the years, I have had far too many DIY projects fail in one way or the other simply because I was in too big of a hurry to verify my work before I finished. Now my shop is full of items like a radio-controlled helicopter that will never fly because I didn’t check my progress while there was still a chance to correct it. These mistakes have taught me the value of testing my work, a lesson that I’ve carried with me into the world of printed circuit board design.
PCBs also need to be tested, which require test points to be strategically designed into the board along with placing the components and routing the connections. Bare board continuity testing, in-circuit test, and flying probe testing all require the presence of test points on the board to facilitate their processes. Here we will look at the top seven PCB test points layout strategies that designers need to understand to build a fully testable circuit board.
Printed Circuit Board Test Points
There are two different types of “test points” used in a PCB design. The first is a small metal contact, or test point, connected to a net. This test point can be an existing thru-hole lead, via or a specifically placed metal pad, and one test point is usually assigned to each net on the board.
Once assigned, the PCB CAD system will use their XY locations and net assignments to generate data files for the automated test systems. These systems conduct either continuity testing for shorts during PCB fabrication or solder connectivity and functional testing during circuit board assembly. Two types of machines are usually used for this testing:
- In-circuit test (ICT) or “bed-of-nails”: These test machines use a fixture designed and built for each board in production. The fixture contains spring-loaded probes that simultaneously contact each test point on the board, allowing for all nets to be tested simultaneously. These machines conduct their tests very quickly, but the fixtures are expensive and time-consuming to design, build, alter, and maintain.
- Flying probe: These test machines feature three and six probes that move around the board and conduct their tests. Without designing and building a test fixture, the flying probe is an inexpensive machine to use. However, due to the individual nature of its testing, the flying probe is a much slower process and is usually reserved for limited production runs and prototypes.
The second “test point” to define is the specific points on the board accessible to technicians doing manual testing and repair. Usually, these are referred to as probe points. The following chart points out the differences between test points and probe points:
|Test point – automated testing
|Probe point – manual testing
|Smaller size contact pad
|Larger size pad for thru-hole post or loop
|Can be a via, existing thru-hole, or new pad
|Only a pad specified for the post or loop
|Is not marked on the board
|Will have a ref des, and a net name
|Can be spaced closely together
|Needs spacing for technician access
With the nature of test points now clearly defined, let’s look next at some layout strategies for placing PCB test points to help the testability of the circuit board.
Seven Layout Strategies for Successful PCB Test Points
Here are some recommendations for defining and placing test points on a circuit board. However, since manufacturers differ in their capabilities, the designer must verify these parameters before committing to them in their design.
1. Test point sizes and shapes
The larger the test point, the easier it is for the probe tips to contact them. A 0.050-inch test point is ideal, although most designers will go down to 0.035 inches. Using a square-shaped pad for a test point not only visually identifies it as a test point but offers a little more surface area for the probe to hit. Most PCB CAD systems have automated features that convert regular holes and vias to test points and change their size and shape.
2. Test point to test point spacing
The greater the distance between test points, the easier it is to build, alter, and maintain the test fixture. Test points that are spaced no less than 0.100 inches from center to center are ideal for this. It allows the use of larger probes that are more robust, making for a test fixture with greater long-term reliability. However, test points can be placed as close as 0.050 inches center to center with high-density designs, and the fixture will be configured with smaller probes.
3. Test point to component spacing
The recommended clearance from the center of a test point to the edge of a component is 0.100 inches, with 0.050 inches being the absolute minimum spacing. Another consideration to keep in mind is to refrain from placing taller components on the probe side of the circuit board. These may require additional milling and fabrication steps in constructing the test fixture, which will drive up the costs.
4. Test point to board edge spacing
Test machines often use a vacuum drawdown system to firmly secure the board to the fixture, which requires space around the board’s perimeter for a tight vacuum seal. For a good seal, it is recommended to keep test points back from the edge of the board by 0.125 inches, with the absolute minimum being 0.100 inches.
5. Test point placement
Test points should be distributed throughout the board as evenly as possible to avoid high-density probing areas. This clustering could put uneven pressure on the board, causing it to flex and possibly break solder joints. It is also important to keep test points clear of obstructions such as overhanging component features, mounting hardware, and solder mask. Test points should also be placed with high-speed design rules in mind to avoid creating antennas that will radiate interference.
6. Probe point parameters
Probe points are different from automated test points and therefore have their own parameters to consider. Their hole or surface mount pad must be large enough to accommodate the post or loop that will be soldered to them, and they must be placed in a location easily accessible by a technician.
7. Probe point identification
Thru-hole probe points can be flagged as an automated test point on the back of the board just as any other thru-hole pin but serve a different purpose on the top side of the board. These points must be identified in silkscreen by a reference designator and the net that can be probed.
Automated Testing at Your PCB Contract Manufacturer
Automated test points, and manual probe points, are essential in the manufacturability of a printed circuit board. At VSE, we have been configuring circuit boards for full testability for over 35 years, and we can help you with your questions about automated PCB testing.