A functional test checks the behaviour of the assembled board against its functional specification, with power applied and with simulated or special test signals connected to the input/output lines. It is often combined with calibration and set-up adjustments. For low-volume products you will normally write a test procedure around individual test instruments, such as voltmeters, oscilloscopes and signal generators. You may go so far as to build a special test jig to simulate some signals, interface others, provide monitored power and make connections to the board under test.
The disadvantage of this approach is that it is costly in terms of test time. This puts up the overhead cost of each board and affects the final cost price of the overall unit. It is cheap as far as instrumentation goes, since you only need a simple test jig, and you will normally expect the test department to have the appropriate lab equipment to hand. Hence it is best suited to low production volumes where you cannot amortise the cost of automatic test equipment.
A further, hidden, disadvantage may be that you don't have to define the testing absolutely rigorously but can rely on the experience of the test technician to make good any deficiencies in the test procedure or measurement limits. It is common for test personnel to develop a better "feel" for the quirks of a particular design's behaviour under test than its designer ever could. Procedural errors and invalid test limits may be glossed over by a human tester, and if such information is not fed back to the designer then the opportunity to optimise that or subsequent designs is lost.
Testing may more easily be carried out by automatic test equipment. In this case, the function of the human is reduced to that of loading and unloading the unit, pressing the "go" button and observing the pass/fail indicator. This is comprehensively de-skilled; the total unit time is reduced to a few minutes or less. This minimises the test cost. The costs occur instead at the beginning of the production phase, in programming a test fixture. The latter is similar to (in some cases may be identical to) the bed-of-nails fixture which would be used for in-circuit testing. Or, if all required nodes are brought out to test connectors, the fixture may consist of a jig which automatically connects a suite of test instrumentation to the board under the command of a computer-based test program. The skill required of a test technician now resides in the test program, which may have been written by you as designer or by a test engineer. In any case it needs careful validation before it is let loose on the product, since it does not have the skill or expertise to determine when it is making an invalid test. The cost involved in designing and building the test fixture, programming it and validating the program, and the capital cost of the unit itself needs to be carefully judged against the savings that will be made in test time per unit. It is normally only justified if high production volumes are expected.
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