Design for test / testability, DFT

- key elements and guidelines in design for test, dft: improving the testability of electronics assemblies to reduce costs, improve throughput and reliability.

Design for test, DFT must be a central element of any design process these days. With circuit complexity increasing and component size decreasing, the testability of electronics circuits is more crucial as testing is becoming ever more demanding. The only way that satisfactory testing can be achieved is by employing design for test, DFT concepts from the earliest stages of the product design and ensuring the testability of the circuits is as high as possible.

Testing is a necessary element of any production or manufacturing process. While it does not add value to the product, it is essential to ensure that product leaving the factory is full functional and there are no faults. As testing can be time consuming and costly, it is essential to adopt the most effective means of test. As a result, developing the correct test strategy as well as ensuring that the product is testable are two important elements.

When products have not been designed with testability in mind, access to the circuits may be limited and it may not be easy or even possible to implement some test methodologies. This may result in testing taking longer and having a much lower level of coverage which in turn may lead to defective product being shipped. Another issue is that of fault finding. Without being designed for testability, and faults that are found can be very much more difficult to find. This can result in high levels of scrap, and increased overall test time.

As a result, design for test, or design for testability, dft, is a key element in the design of any product. While design for test, does not feature in the product specification, it should be given equal importance in terms of the internal design parameters applied.


Impact of design for testability

Design for test will impact all areas of the design of a product. As a result, design for test, DFT should be considered right from the concept stage of the product. Design for test, or design for testability, DFT will involve many areas of the product deign including:

  • Initial concept:   It is at this stage when the overall concept of the product is defined. Testability will affect key decisions about how the product is split into different assemblies. It may also affect some of the basic technologies used within the product
  • Component choice:   Design for test will affect the choice of some components. For example if boundary scan, JTAG testing was to be used, then JTAG compatible devices would be required. This will affect some of the basic choice of components that can be used, particularly within the digital / logic PCB assemblies.
  • Investment in test equipment:   Right from the beginning of the design it is necessary to look at the nature of the tests required as this may impact on the test equipment that will need to be bought for the project. In turn this may affect the overall budget. Also it is necessary to consider whether the same test equipment can be used in development and then in production. This can assist in test development, but care has to be taken not to over-specify the production test equipment.
  • Mechanical design:   The mechanical aspects of assemblies and complete units may have a significant impact on the access that is available when testing. Accordingly mechanical aspects need to be considered early in the design with respect to the effect on testability.

By employing design for test techniques, the test elements of the production costs can be reduced and this may save many times over design for test, DFT made at the beginning of the product design.


Design for test strategy

At the earliest stages of the design it is necessary to consider how the product will be tested. Decisions about the way each sub-assembly is tested (or not) must be taken at this stage to ensure that the optimum fault coverage is achieved for the lowest cost. If yields are expected to be very high then decisions can be taken not to test a sub-assembly. On the other hand the "rule of tens" should be remembered. This a very rough but very useful rule of thumb which states that at each successive stage in production it cost ten times as much to find and correct a fault.

The test strategy is an important design document in the development process. It should be prepared when the high level design of the equipment starts, although it will naturally need to evolve with the product itself. It should address subjects including the modules within the design, the type of test or tests that are most appropriate, the expected yield, test times, and so forth. In this way a sensible estimate of costing can be made and preparations for the tests can be put in place.


Types of test

One of the elements that is important in any design for test strategy is the choice of the type or types of test, to be used, and also the test systems that will be employed as a result. Different types of test system may require different decisions to be made during the design. Accordingly any design for test strategy will need to include the tester to be defined at an early stage if the product design.

In making the decision about the type of testers to be used, the design for test strategy must take on board many factors. The type of test that is optimum for any given assembly depends upon a number of factors. These might include the stage in the overall assembly of the final unit, the expected yield, the equipment that is available, the circuitry employed in the item under test, and a variety of other factors.


Note on Automatic test, ATE:

Automatic test equipment is used within virtually all electronics manufacturing environments. With assemblies and equipment becoming more complicated while costs are being reduced, automatic testing is the only way to achieve this. There are many different types of automatic testing ranging from automated optical and x-ray inspection, through in-circuit testing, to various forms of functional testing.

Click on the link for further information about Automatic test technologies


Types of test to be considered

To enable the testing on any board to be performed in the most efficient way it is necessary to take account of the test methods to be used at the design stage. Different types of test will have different requirements to enable the testing to be carried out effectively. It is necessary to consider all the different types of test that may be applied. The different design for test guidelines for each type of test can be applied as appropriate.

While there will be some overlap between design for test guidelines for different types of test system, for other types they may be totally different. The major areas of overlap occur where there are similarities between the types of test, for example where they both use fixtures or similar approaches.

It will not always be possible to implement all the design for test guidelines, the underlying reasons for the guidelines should be understood so that the cost impacts for any decisions made can be understood.


Summary

The costs of test can be one of the major costs of manufacturing a product. Accordingly it is necessary to determine the test strategy at an early stage and ensure that any design for test guidelines and requirements are incorporated into the electronics design at the earliest stages. By adopting a test strategy and applying the design for test guidelines, the costs of testing the product during manufacture can be reduced. This will enable a higher quality product to be shipped for a lower cost.

By Ian Poole


Read more automatic test, ATE, tutorials . . . . .

ATE basics PCB inspection ICT, In-Circuit Boundary scan
Flying probe Functional, FATE Test Strategy  

Return to Test and Measurement menu . . . . .

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