Design for test guidelines and basics, DFT

- an overview or tutorial of the basics of design for test DFT with guideleines for design for test to enable circuits to be tested in the most effective manner.

Design for test, DFT must be a central element of any design process these days. With circuit complexity increasing and component size decreasing, testing electronics circuits is becoming ever more demanding, and 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.

Design for test will impact all areas of the design of a product. Design for test, DFT will impact many areas from the basic concept, through to the choice of components used, board layout, mechanical design, as well as areas such as the choice of test equipment. 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 of test, or test system that is to be used. Different types of tester 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. These might include manual inspection, automatic inspect, ICT, FATE, etc. Each type has its own advantages and disadvantages and choices should be made using a knowledge of these.

Links to information about the types of test and types of tester can be found in the related articles list just below the main left hand menu.

Design for test circuit considerations

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. This could be the topic of an extensive article in its own right. Often special test connectors may need to be added to give additional access. For ICT it may be necessary to add resistors in the pull up or pull down path for inputs to various chips to enable them to the controlled by the tester. For boundary scan additional paths may need to be included to enable the scan to take place properly.

Mechanical considerations

Although the mechanical considerations may not be as many as any electrical ones, they are equally important in any design for test, DFT, test strategy. Tests that require accurate fixtures usually need location points on the board. These are generally holes that are placed at either side of the board and are accurately referenced to the artwork itself. It is not sufficient to reference the fixture to the board edges as these are never accurately toleranced.

It is also necessary to ensure that mechanical access is provided for any test points. This is particularly important for ICT where large components may shield test points making them inaccessible. Accordingly sufficient clearance should be allowed around all large / high components to enable the test pins to access the test points.

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 guideleines 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.