Design-for-Test: Not An Afterthought

In our industry, we often hear that in many cases design-for-test or DFT is implemented right before a PCB design arrives at the assembly stage. Lots of times this happens, intentionally or inadvertently, when an OEM uses separate PCB design and assembly houses to save a few dollars or for other reasons.

Thus, DFT becomes an unwise and costly afterthought for the OEM. As a result, obvious and/or latent defects can be overlooked during assembly, creating significant and costly problems for an OEM’s product.

That’s why it is prudent for an OEM to work in tandem with its contract manufacturer (CM) or EMS provider at the outset to craft a printed circuit board test strategy ideally suited in application and cost to its particular product lines.

There is no single inspection or PCB testing system that will meet the needs of every manufacturing environment. Therefore, a number of factors must be considered in developing any given strategy. From a dollars and cents and reliability perspective, it’s important to target the proper test procedure for a particular product.

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If a product is sufficiently mature, for example, the printed circuit board should be fully functional and only minimal debug and test may be required. Testing complex PCBs, on the other hand, whether for a new or mature product, incurs considerably more detail, especially if PCBs use surface mount technology (SMT).

In this regard, DFT or tailoring the PCB test coverage to specific product modules is essential in a test strategy. This particular aspect assures sufficient test point coverage in a printed circuit board’s digital and analog modules, for example, because each requires different testing.

A test strategy’s foundation is to first get a good understanding of the types of printed circuit board testing available and where they are most appropriate from a product and cost point of view. The three main test types are in-circuit test (ICT), flying probe, and functional test.

Cost/Benefit Analysis

One way to achieve cost justification is to do a cost and benefits evaluation based on each type of test. When quantity is limited from a few boards to maybe 50 or so, a good PCB testing solution could be flying probe, whereas if the quantity is higher, generally speaking, in-circuit test (ICT) is a good choice.

Cost of manufacturing ICT fixtures ranges from a few thousand to over $50,000, and justification is demanded for making these dollar outlays. That’s when the number of product shipments factors in. If an EMS provider is shipping 50 to 100 boards a month to an OEM, for example, and it is determined that the product’s lifecycle is expected to be at least two years or more, then the cost of this expensive ICT fixture could be amortized over that time period. Keeping all other incidental costs in mind, the incremental cost of the fixture per board supplied is relatively small.

On the other hand, if monthly shipments in terms of number of PCBs are relatively large, say 250 to 300 boards per shipment, then amortization would not even take two years. Hence, the entire cost of manufacturing such an expensive ICT fixture could be amortized in less than a year. Therefore, amortization depends on ICT fixture manufacturing cost and the number of shipments scheduled for an extended time period.