by Dan Orlando & Walt Maclay
1. Recognizing the need for testing
Many people don’t realize how important testing is to the product manufacturing process. After manufacturing, each board should be tested to ensure only good products are shipped. Despite excellent quality control at the best contract manufacturers, there can be errors in PCB fabrication, PCB assembly, flaws in procedures (such as loading the code into a device), or even flaws in the design. A failure rate of 1% is considered good for an average board prior to testing and would be unacceptably high for most customers. The testing determines if the product satisfies the requirements and is ready to ship. At Voler Systems, we ensure a smooth, thoughtful transfer either to the client’s internal test group or to partners like Solution Sources Programming for test and production.
2. Leaving test as an after-thought can lead to expensive problems
We do design verification testing prior to manufacturing, which includes safety, compliance and regulatory testing as well as testing over the specified range of environmental conditions. Both manufacturing test and design verification should be planned from the beginning of the design.
3. Early testing decisions affect the design of the board
The methodology of testing is important, as it impacts design considerations on a PCB. By implementing a Design for Test (DFT) phase in product design, we can ensure a smooth transition from design to manufacturing. This DFT plan ensures each board meets functional and performance requirements.
At Voler, our design check list helps avoid common problems in transferring design into production. Some typical common problems we work to avoid are:
- No access points to test products (no thought into giving access via test points or pads for an electrical connection to probe)
- Test pads too small to probe (targets too small to accurately probe or touch to make good electrical contact)
- Test pads too close to probe (center to center spacing of test points are too close, therefore the test probes can short out to each other)
- Designing boards with no tooling holes (this prevents a board from wandering when being tested; therefore making the board under test a moving target with a small target to try to make electrical contact).
- Putting test points or pads too close to other devices or other non-testable devices such as brackets (affects the ability to put a probe on a test point, as it’s too close to such devices that it either can’t make contact due to it being blocked or it could short out to a device).
- Specifying components that should have “built in test” capability but not confirming with the chip manufacturer that it works.
- Presuming a board will be re-spun or the layout revised before production; therefore not designing for test in the first round of layout (many times the first board can be the final version if it is designed for test).
Early design and testing decisions impact the test methodology and manufacturing cost. There are different types of test, such as in-circuit test and functional test. They vary in the ability to find a defect, the time to execute the test, and the cost to develop. The selection of the right type of test depends upon the complexity of the board, the volume being manufactured, and the level of concern about a failure.
Tradeoffs between the type of testing, cost, and the need for calibration can either simplify or add to the complexity of your overall manufacturing and quality processes. For example, will code be loaded onto the board? At what point in the manufacturing process will this happen? Can it be done at the same time as testing to avoid a second operation with more handing, which is expensive? Will the device need to be calibrated? Should this be done before testing or after? Can the calibration be done at the same time as testing to avoid an additional operation with more handling?
Lastly, the assembly of the device should be carefully considered and documented. Optimizing test and assembly procedures to minimize scrap will help lower the cost to manufacture product while cutting down on test, debug, and repair time. For example, some signal wires should be routed away from noise generating parts. Some devices are glued or welded shut. After this step they cannot be repaired. It is usually desirable to test the device prior to this step, minimizing scrap.
In conclusion, working closely with partners like Solution Sources Programming, often in conjunction with our client’s internal testing group, minimizes testing cost, cuts debug and rework time, saves development time, and helps to ensure reliable and high quality products.
Founded in 1990, Solution Sources Programming, Inc has decades of experience in implementing the latest test and measurement technologies from DFT (Design for Testability), Bench-Top Boundary scan, In-Circuit Test and Functional/System Integration Testing. We design, build and deploy custom automated test solutions to validate boards and products in characterization, qualification, and manufacturing environments.