When students in the medical field graduate from their studies to become full-time medical professionals, they will have gone through a lot of testing. There are the traditional tests to cover what they have studied, as well as what they have learned in the lab. There is a constant barrage of verbal quizzes to keep them on their toes, and finally, they are evaluated on their performance as an intern. The reason for this is pretty obvious—you wouldn’t want to be the patient who, half-way through their appendectomy, had a confused surgeon throw up their hands in defeat and sulk out of the operating room. If someone is going to be a surgeon, they need to prove that they can handle the pressure through extensive testing.
In the same way, when you are building a printed circuit board, you need the assurance that it will do the job that it was designed for by having it go through rigorous testing. Yet, some contract manufacturers don’t put the same effort into testing their products as others. This can lead to intermittent problems or complete board failures. It’s bad enough to deal with these problems when you are debugging the PCB, but when these failures happen in the field, it can be a real disaster. To ensure your circuit boards don’t suffer from these kinds of problems, you need them built by a CM that will put your boards through thorough PCB testing techniques.
Key PCB Testing Techniques
There are four key PCB testing techniques you should expect your CM to provide. Each has strengths and weaknesses you should be aware of.
1. In-Circuit Test (ICT)
In-circuit test (ICT) is a printed circuit board testing system with multiple probes known as a bed of nails. The ICT probes make simultaneous contact with all of the test points on the PCB to electrically test its circuits for assembly failures, such as shorts or bad solder joints. It will also check critical component values, overall board functionality, and can be enhanced for more complex testing with other adapters and modules.
The disadvantages of ICT are mostly the expense of the test fixtures, which can cost between $8,000-$15,000 to build. Additionally, any modifications to the test head are difficult and expensive. The PCB must also be designed with good design for test (DFT) techniques to have maximum fault coverage, and an experienced technician is required to develop and use the test software and equipment to their fullest capabilities. For boards that are built in higher volume, however, ICT is usually the best test option.
2. Flying Probe
The flying probe test system uses two to six probes that move or fly around the board to conduct its tests. Flying probe has the advantage of being able to test larger boards, such as backplanes, than ICT can, and it has lower costs and development times for its setup. Like ICT, it will test for assembly problems due to solder connections, as well as the presence of components, their values, and polarities. With the simplification of the test setup, however, flying probe can easily handle PCB layout revisions.
Flying probe does have operational limitations when compared to ICT in its functionality testing, although some newer versions are improving on this by allowing some powered testing. With only half a dozen probes contacting each test point, flying probe is also inherently slower than ICT. This is not a problem, however, for low-volume builds of printed circuit boards and can even be the preferred method when comparing all of the other costs that can be involved.
For PCBs with multiple connectors, such as backplanes or other complex interface boards, the CableScan system is an excellent choice for testing. CableScan is connected to the board to be tested through connectors and will check every pin against every other pin looking for assembly problems, such as solder shorts or opens. It will also test the configuration of jumpers as well as diode functionality and measure resistance and capacitance.
The CableScan system does have some limits on how large of a connector it will test in a single pass, and there will be some minor costs for the development of a hardware interface. Mating connectors need to be obtained and wired to the system, which can cause delays depending on the lead time, complexity, or expense. Setups and test times for CableScan can be long depending on the number and complexity of the connectors involved.
4. Functional Test
Functional testing of your board is the most comprehensive test to determine its final pass/fail status. Through a combination of connected cables, test probes, and test software, the functional test fixture will simulate the board’s behavior based on your specifications. When you provide the functional test fixture, you will have the confidence that your CM will have tested the board using the precise parameters that you designed for it.
As an alternative, your CM can develop the functional test fixture (hardware and software) for you. This can be especially appealing for smaller and more simplistic designs. The CM’s test engineers will develop a bench test fixture and methodology that will target assembly defects without spending time and costs validating specific components. This type of functional test will determine if the overall circuitry is performing as expected, although a more detailed debug process is required to identify and correct faults that are found.
What You Can Expect from Your CM
At VSE, we have all of the testing equipment described above, and we are staffed with experienced engineers and technicians to wield these tools to their fullest extent. This gives you a wide range of test options and expertise that can often be difficult to find all under the same roof. Our staff continually cross-trains on these different testing methodologies to ensure all of our customers’ needs are met. We also maintain a “test engineering knowledge” (TEK) database, which captures failure symptoms, root cause, and relevant actions to assist in the training of our personnel. Here are some of the specific benefits we provide:
- ICT: Our staff has a complete understanding of ICT, including semiconductor testing, end-user applications, DFT methodologies, and ICT optimization. We have the resources and expertise to help you on the design side, as well as developing the hardware and software components of ICT that we will use to test your board.
- Flying probe: With our engineering resources and experience, we can help you with a return on investment evaluation to determine if this approach is the most effective for your design. Once decided, our skilled technicians will implement flying probe testing that is configured specifically for your board.
- CableScan: We are one of the industry leaders in implementing this test methodology for your design. As such, we have a large inventory of connector interfaces already built and available, which will help reduce the development time and cost for your board.
- Functional test: Our engineers are skilled at creating complex test schemes for your functional test using LabVIEW. We have the expertise to program in variable supply levels, frequencies, and other parameters to give your board a more thorough test and reduce the amount of hardware that would normally be required.