DFT – Remember two things
To ensure the quality and correct operation of electronic products you have to be able to test them both during R&D phase (design verification) and later in production (production testing). To make sure the required testing is possible to do, and preferably even easy to do, you have to take the testing requirements into account when the product is being developed. This is called design for testability or design for testing (DFT).
In other words DFT prevents designing products that can’t be properly tested or are very difficult or expensive to test during the development or in production.
DFT is not rocket science; it essentially consists of a variety of simple solutions. Successful DFT requires two things: a named responsible person and correct timing.
Someone has to be named in charge of DFT. This can either be a DFT/design verification/production testing expert or a product development specialist who also has expertise in DFT. Something in between is the most common solution, i.e. close collaboration between the person responsible for DFT and a product development engineer.
DFT should also be included in product development right from the start, preferably starting with a project’s first meeting. DFT is unfortunately often started too late in the product development process when it is not possible to address the issues anymore.
Good DFT becomes increasingly important as the project progresses. When a product is well designed for testing, there is no need for expensive, special measurement devices, for example. In addition, the measurement results are more reliable and they can be better analyzed. At the same time, good DFT enables more comprehensive testing and makes it easier to collect information, for instance about the quality of the production process.
With good DFT, all parts of the circuit board and the final product get tested. That would not necessarily happen if no one planned testability beforehand.
DFT can also make the product development project easier. At this point, you normally want to verify the quality of the design and characterize the operation of the product. Good planning makes it possible to automate product development tests. As a result, the same tests can easily be repeated in different phases of the project for example.
Prior experience is valuable
Perhaps the biggest challenge for DFT is that you often have to add testability features to an electronic device before anyone has even had a chance to think about what needs to be tested and how. Prior experience is valuable here. A good DFT expert develops a feel for what will be necessary. This requires experience and expertise as well as knowledge of what options are available. An experienced DFT expert knows what things will probably be tested and how. Often solutions and methods can be used that have worked well before.
Over the years, we have seen that doing DFT badly or not at all can lead to very expensive solutions. It may become necessary to buy an expensive measurement device or to make a complicated mechanical construction so that at least the most important parts can be tested.
A typical mistake is not putting enough test points on the circuit board in the early stages of product development. Adding them later is a lot of work, and making changes to a product that has gone through design verification is rarely tempting. Unless DFT has been taken seriously early enough, the product may need to be sent back to the drawing board. It may again be necessary to make a new version – this time a testable one.
Picture: a project where DFT has been adressed too late.
Picture: a project where the DFT responsibility is allocated at the start of the project.
DFT for an electronic product requires technical expertise in several areas: circuit diagram, layout, mechanics, and embedded software. DFT requires hardly any changes in products, and many solutions cost almost nothing as long as they are applied early enough.
Very often DFT is about placing test points in those circuit board signals that you want to measure electrically. The measurements are impossible if you cannot access the signals with the test setup. In that case, you end up sacrificing the test coverage and some of the circuits have to be left untested.
If there is a microcontroller or microprocessor in the circuit board that needs to be tested, its software should also include testability features. It’s good to keep testing in mind and to plan connections and interfaces for embedded software that can later be used to give test commands to the device and control individual functions.
For me it is very rewarding when the DFT has been executed well. The product’s test setup can be created with easy basic solutions, and good test coverage is achieved.
About the author
Vesa Koski is Chief Engineer and DFT expert who has been doing DFT for nearly a decade.
Vesa can be reached by email at firstname.lastname@example.org