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Streamline test plan before the design

03 Aug 2015  | George Henning

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For the most part, PCB designers hardly think about test strategy in their designs. After all, test is a manufacturing issue. In fact, all elements of PCB design eventually become manufacturing concerns if they have not been thought through in the design phase. But, by the time a product reaches manufacturing phase, designs are hardened, costs have been set, and customers have been promised delivery. It's the worst possible time to uncover problems.

If you are at the point of establishing prices and talking to customers about delivery and you don't have a test plan including test designs, you're running a high risk of cost overruns and delays. Designers who have experienced this at least once get it and don't usually make the same mistake twice.

A test plan could go into considerable detail, but in the end it needs to be matched to the product's complexity, cost and end market. A simple product aimed at a market with low performance requirements may need nothing more than a go/no-go test. A product with several PCBs and multiple functional areas needs a test strategy for each of those before assembly in the final product. This reduces the cost of isolating and reporting performance problems.

Get your test plan going early in the design and you can work collaboratively with your contract manufacturer (CM) before the design is finalised. This effort includes obtaining feedback on design-for-manufacturing (DFM) and design-for-test (DFT) features you can implement. The collaboration can yield an optimal solution that considers cost, performance and timing, saving you time and money during production.

Design for test is so important to us at OCM Manufacturing that we have created a short how-to guide for planning a test strategy. Some of the practices we recommend include:

Identify the types of test you will need: Work with your CM to identify the spectrum of faults you need to cover and the types of test that will provide that coverage with minimal overlap among tests. You will need to start by finding out your CM's test capabilities, which might include automated optical inspection (AOI), in-circuit test, automated functional test and environmental testing.

Plan access to test points: Make sure that test points are accessible to the equipment being used. A critical node located under an IC cannot be probed, for instance. Providing pins or loops specifically for probing (including ground connections) is a good idea, as is locating test points all on one side of the board where possible, to simplify testing and minimise the cost of fixtures.

Add fiducials and keep-out areas for AOI When using automated optical inspection, fiducials provide essential reference points for registering the image with the inspection template. The keep-out areas provide space for transport rails or fixtures needed to move the board through the inspection station.

If you're embarking on a new PCB design, then, a best practice is to ensure that your design process includes a test-plan gate you go through before your design can advance. In today's highly competitive market, you need every cost and time-to-market advantage you can get, and designing for test is one of those.

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