Design for test, DFT
an overview of the basics of design for test DFT which enables 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 requirements are incorporated into the design at
the earliest stages. By adopting a test strategy and applying the design for
test requirements, 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.
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