While testing is one of the last steps in the manufacturing process, it must be planned for early – as part of the design process – in order to ensure that necessary tests can be carried out and that they can be done so cost-effectively.
This DFM tip provides best practices for planning the test strategy for your electronics device.
Identify the Types of Tests
Particularly when dealing with complex products, designers must consider how the product should be tested. Each type of test will require specific considerations to be implemented into the design. So, as part of your design strategy, consider how the product will be tested. If you are outsourcing your test a contract manufacturer like OCM Manufacturing, begin by finding out the CM’s test capabilities. This may include one or more of these methods:
- Automated optical inspection (AOI)
- In-circuit test (ICT)
- Flying probe ICT
- Functional test or automated functional test
- System test
- Environmental test
There may be some overlap in test coverage between these test methods. Work with your CM to understand the applicable fault spectrum and devise a test plan that covers this spectrum effectively for your production volume. Overlap should be minimized and fault isolation weighed against production debug costs.
Access to Test Points
One of the most important design considerations with respect to the testability of your product is to ensure that the test points are easily accessible by the appropriate test probes. Specifically:
- Make the circuit nodes accessible. Don’t locate them only beneath a device where a test probe is unable to reach.
- Provide a pin, loop, or pad for probing. SMT test loops to provide grounding for scope probes are now readily available and inexpensive.
- It’s best to keep test points on one side of the board only – this simplifies testing and reduces the costs of fixturing. In the case of mezzanine boards, test pads facing the mating card are essentially useless. (Read more on this in our DFM Tip on the topic of Best Practices for Double-Sided Mixed-Technology Board Design).
- Consider implementing jumpers to allow for isolation of specific portions of the circuit. Using this method on control system feedback loops is particularly helpful.
Specific Design Considerations
The primary goal of your test strategy should be a plan that is cost-effective and screens out the most prevalent defects in the manufacturing process. This will depend in part on the manufacturer and the test methodologies, so consult first with your contract manufacturer.Depending on the test methodology, the following guidelines may be useful when planning your test strategy:
The most important aspect of AOI test design is the inclusion of fiducials for PCB registration and keep-out areas for transport rails or fixturing. It’s also a good idea to limit the use of dual-purpose footprints (e.g. for PTH or SMT, for 0402 or 0603) and to specify a preferred alternate so that component appearance varies less.
Functional test designs should be user-friendly and designed to log the right data to monitor performance of the design. Documentation should include not only the schematics but also a description of the function of the board. Block diagrams and/or circuit descriptions help when debug must be undertaken and they minimize “bone-piles” of problematic products. Functional tests should also be designed such that they isolate defects as much as possible – this allows failures to be debugged quickly. (For details about how to prepare documentation, please read our DFM Tip, “Best Practices for Preparing Documentation”.)
If the board contains a programmable device, write test-only firmware that allows the tester to put the product in a simpler mode than operating mode and verify that the basic functionality is working.
These tests must also be robust – engineering tests used in prototyping phase are rarely sufficient in a production environment. Connectors and wiring will fail quickly, producing false results and delaying shipments.
Keep test nodes on one side of the board only; provide pull-up resistors so that parts can be isolated or disabled; provide proper test pads.
System tests are the simplest tests that can be performed; they do not typically provide much information about what portion of a circuit is not performing properly. System testing is suitable as a final test but generally must be accompanied by one or more of the other tests described above.
At OCM Manufacturing, we can work with designers to ensure that their designs are testable. Contact one of our Program Managers for details about how we can help.