From the latest video game to high-performance aircraft, nearly every product built is tested before it is put into service. Testing will vary depending on the product’s use and what kind of measurable results are required. Typically, some type of test will be run to verify the product’s ability to function as designed. PCB manufacturers also test printed circuit boards they have built before they ship out of the assembly facility.
Manufacturers test printed circuit board assemblies to validate the assembly process and verify their ability to run as designed. PCB contract manufacturers have different tests that they can run, but sometimes their ability to test the completed circuit boards is hampered by design errors or other problems. Here we’ll look at some common printed circuit board problems with testing and how you can avoid them with good PCB design and layout tactics.
The Importance of PCB Testing
Although automated PCB assembly processes routinely manufacture circuit boards without any concerns, there is always a chance for a solder defect or other problem to occur. To guard against this, contract PCB manufacturers will use a combination of inspection and test processes to validate assembled circuit boards.
Circuit board inspection
Solder defects in circuit board assembly can include shorts between nets, poor solder joints, or insufficient solder. Many times these problems can be detected through various inspection techniques starting with manual inspection by technicians. However, for a closer examination, PCB manufacturers will use automated optical inspection equipment (AOI). These systems scan the board with high-powered optics and compare the results to a known good board. AOI systems are used for solder paste inspection before assembly, solder joint inspection, and verifying that the correct parts have been installed on the board. The manufacturer will use an x-ray system for a more detailed examination of those components with solder joints concealed underneath them like large BGA parts.
Assembly verification
The manufacturer will use automated testing systems to test and validate an assembled circuit board, including an in-circuit test (ICT), flying probe, and cable scan. Both ICT and flying probe testing rely on probe tips contacting small testpoints on the circuit board to test each net in the board. The ICT system uses a test fixture with a test probe for each testpoint on the board to quickly test each net simultaneously. The flying probe system maneuvers a handful of probes around the board to conduct individual testing of each testpoint. Cable scan plugs into all of the connectors of a circuit board to test the nets through their connections.
Functional test
In addition to the assembly verification testing, the PCB manufacturer can also perform functional testing of the board. The functional test is designed to mimic the operating behavior of the board while under power, allowing the test operator to determine if it has been manufactured correctly according to its specifications. However, the testing process can run into some big problems without the right design criteria for testability, which we will look at next.
Six Common Printed Circuit Board Problems During Testing
These six problems can derail the testing of a circuit board and can be avoided by following good design for test (DFT) processes:
- Lack of a test strategy: For the manufacturer to adequately test the board, there needs to be a well-prepared test strategy in place. Not only does this mean providing the functional test specifications of the board, but also a full documentation package. The documents should include the schematic, artwork files, the bill of materials, and netlists files for the design.
- Insufficient testpoint coverage: The circuit board must have full testpoint coverage for ICT or flying probe testing, meaning that every active net must have a testpoint attached. Testpoints can be specialized design objects like surface mount pads, or the testpoint can be assigned to existing thru-hole pins or vias.
- Inadequate testpoint spacing: To access each testpoint automated test probes must be placed with enough spacing around them. 50 mils are often used for clearance between testpoints and other design objects such as component outlines or pads, while 100 mils are used for clearance between the testpoint and the edge of the board.
- Unavailable probe points for manual testing: Manual testing of nets for either debug or functional test requires probe points that a technician can access. These probe points are usually larger pads or holes than what a testpoint requires, and in some cases, will have a stake pin inserted for the technician to clip a probe. Probe points should be clearly marked on the silkscreen with the net they represent and be provided with enough clearance for ease of access.
- Lack of a test fixture or an incorrect or outdated version: ICT requires a test fixture which can be expensive to build or modify. Although the ability to quickly test thousands of production-built circuit boards warrants the cost, new fixtures for limited runs will slow down testing time while driving up costs. Alterations to existing test fixtures for board changes can also be expensive, and these changes need to be carefully weighed in light of the need to modify the fixture. For small production volumes or prototype builds, a different test procedure is often the better choice.
- Outdated components: Using older components can create problems for test engineers conducting functional tests on the circuit board. The components may not yield the expected results making it more difficult to verify the board to its specifications. In cases like these, the better solution is to work with your PCB contract manufacturer to find newer components or circuitry changes. Your PCB CM can help with many design decisions you need to make to support a good PCB test strategy.
Getting Additional Testing Help with Your PCB Design
It’s always a good idea to work with your manufacturer to create a good test strategy and avoid the problems listed above. They will be able to advise you on the optimum PCB spacing and placement rules for testpoints and screen your bill of materials for older and outdated parts. High-yield production of circuit boards is just as important to your manufacturer as it is to you, and they are highly motivated to help your design succeed.
At VSE, we’ve been helping circuit board designers like you for over 30 years, and we fully understand the various requirements of a PCB test. By working together, we can create a test strategy customized for your design that will contribute to the overall success of the assembly operation.
