Test Strategy Example

- example of a test strategy, showing why elements of test were placed at various positions in the production flow.

Test strategy tutorial includes:

    •  Test strategy development
    •  Test strategy example
    •  Test strategy document

In order to see how a test strategy may be developed it is often more easy to look at a test strategy example. In this way, a basic format can be seen, and some of the decisions that need to be made can be understood.

The actual test strategy adopted will depend upon a variety of factors including the production process, equipment available and many other factors.

This test strategy example will show a basic test strategy and detail some of the decisions and options that can be taken.

Test strategy considerations

When developing any test strategy, it is understood that testing does not add any value to the product - it adds cost without adding any additional functionality.However it is absolutely necessary that a product leaves the production site in a working fashion - this can only be checked by testing it. Additionally the sooner a fault is found in the production process, the easier and cheaper it is to fix. As a result, any test strategy should find a fault as soon as possible in the process, while not over testing it and adding any superfluous costs.

Therefore there are a number of elements that should be included in the planning for the test strategy example:

• Minimise number of test stages:   As testing adds no value to the product, the number of stages where testing is undertaken within the overall process should be minimised
• Sufficient test stages to catch faults early:   While testing should be minimised, this should be balanced against the fact that fault finding and repair typically multiplies by a factor of ten each time the item moves to the next stage in the production process.
• Plan process around expected failure rates:   The balance between finding faults as soon as possible and minimising the number of test stages can be estimated using data from existing production lines. Using the percentages of failing boards and fault location / repair times it is possible to estimate where tests should be inserted into the process.
• Minimise test times:   During the development phase of the product testability and aggressive target test times should be set for each test stage to ensure that the most innovative test techniques are used while not compromising test coverage.
• Collect metrics and data:   As part of the testing regime, test results should not just be collected but they should also be analysed, often an almost real time to provide an up to date picture of any fault patterns, etc
• Optimise production process:   Using metrics and data obtained from testing, the process should be optimised. Often processes such as soldering can show pattern faults which should be corrected as quickly as possible. Any feedback can be used to reduce failure rates, adjust the test regimes, etc to reduce the overall production time. In some instances it may be possible to remove some test stages if almost no failures are detected there. Alternatively batch sampling may be used.

Test Strategy Example

While many different production processes will be needed to accommodate the different production requirements, the test strategy example shown below can be used to highlight some the options that can be adopted.

Test Strategy Example Flow Diagram

Test strategy example options

There are a number of stages where a test may be introduced. It is necessary to decide which options need to be adopted:

• Goods inwards testing:   This form of testing us rarely used these days. IT is time consuming and therefore costly. It would only be done where tight specifications for items such as RF filters, etc. may be involved - normally figures for failure rates can be placed on suppliers, and if high levels of failure are seen later in the process, then further investigation can be undertaken.
• PCB inspection:   The PCB inspection stage typically uses either automatic optical inspection, AOI, and / or automatic X-ray inspection, AXI. Sometimes, an AOI capability may be included in any pick-and-place system, enabling issues with the pick-and-place process to be detected almost immediately. Often, AOI / AXI is undertaken after any soldering process. In this way, both pick-and-place and soldering defects can be detected.
• In-circuit test:   An in-circuit test stage has been shown in the test strategy example. It purpose is primarily to detect manufacturing defects. It is most likely that it may be deemed not to be required if AOI / AXI is used. The drawback of the ICT is that growing board complexity reduces test access, and therefore the coverage of any in-circuit testing.
• Module functional test:   Even when a board is manufactured correctly, there is a chance that it may still not operate to its specification. This may result from component tolerances being outside their specification, or even component tolerances all being at a particular end of their allowed tolerance, and the combined effect forming a failure. Again, if failure rates are very low at this stage, deferring testing until the next stage may be a possibility. This test could also include techniques such as boundary scan.
  
  A variety of options are available for this type of test, ranging from dedicated digital functional ATE systems to those using VXI, PXI, LXI, AXIe, etc at their core. Sometimes, testers known as combinational testers utilising a variety of test techniques including ICT, functional, boundary scan, etc may be used to gain sufficient test coverage.
• Unit test:   Again, this is typically a functional test. Typically it will test the overall unit to its specification, and in some processes could be the pre-delivery test.
• Post burn-in / pre-despatch:   Some items of equipment, especially rugged items may need to undergo some form of burn-in. Often when this is undertaken background monitoring is undertaken.
  
  For the final, pre-despatch test, often a test of the major functions of the equipment may be taken to ensure it is fully operational. As some items such as final programming after the final test may be undertaken, a last test to ensure the correct operation may be needed.

Once decisions about the way the system is to be tested have been made and the systems implemented, feedback must be maintained to ensure the manufacturing process is operating correctly. Additionally it may be possible to optimise the testing to reduce times and improve its effectiveness.

Any feedback must be reviewed and monitored on a very regular basis to ensure that issues are addressed as soon as possible.

While every production process will be different, much of the rationale used in this test strategy example can be used, and employed in the way it is needed.

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