Testability of printed circuit boards begins with the schematic. Here are some tips that will ease production testing.

For low-volume products you will probably be focusing on functional testing, where a board is tested by powering it up and ensuring that all circuits behave as expected.

Ideally, you want to make measurements that will confirm that all components are present and are the correct value. Hence, add test points that give access to the inputs and outputs of each functional circuit block. For example, you can test a filter by applying a signal of variable frequency to the input and measuring the amplitude of the output.

Consider adding a resistor between the output of one circuit and the input of the next, so that the input can be over-driven by a test signal. That can make it much easier to test individual circuit blocks and pin down a fault. This tip is good for logic circuits as well as analog ones.

Test coverage is the proportion of possible faults that can be detected by the proposed tests. The type of faults we usually concern ourselves with are opens, shorts, missing and wrong-valued components. It is not realistic to expect 100% coverage. For example, a single missing decoupling capacitor is unlikely to be detectable. However, during schematic capture, for each component added consider what test would confirm the component is correctly present. Consider adding test points to facilitate any needed tests.

Many circuits include a microcontroller. It can be very convenient, and reduce the amount of test equipment needed, to have the MCU participate in testing, either by generating test signals or performing measurements. In this case it makes sense to include provision for programming the MCU in the test fixture. Add test points, or a fixture-compatible connector, as appropriate.

Where circuits are tested using on-board loopback of signals, be aware that this does not test connectors, which can be one of the most significant sources of assembly faults. It may be better to loop the signals back off-board, ensuring that connectors are tested.

For design reviews, a good tip is to print out the schematic and then walk through the proposed tests, highlighting the components that will be tested. Only mark a component if you are sure a test would reveal that a component was missing, or the wrong value. At the end you will see the test coverage, and see where more test points might be helpful.