Generally, a circuit test essentially consists in the application of certain inbound stimuli, in the detection of the responses and in comparing what is read with what is expected as a correct response; naturally, the latter must be known, for example by calculating it beforehand. Each difference constitutes an error. A “physical fault" is defined as the cause of this error.

A fault situation may take place at any time of the life of a circuit, but generally the nature of the malfunction, and consequently the type of test to be used, will be different depending on when the same fault occurs.

The fundamental moments of the life of a circuit are basically as follows:

1) creation of the printed circuit: in this case, it concerns interrupted or shorted rails, bays connected among each other, breakages, etc.. Some of these faults may also be intermittent, for example short circuits due to mechanical stress.
Part of these malfunctions can be considered as logical faults;

2) assembly: here typically there may be errors in the assembly and welding of components, some may be defective, etc... Some defective elements may also be detected only after assembly;

3) storage: the causes of fault are linked to the environmental conditions: humidity, salinity, temperature, aging; the resulting faults are of a parametric type rather than logical;

4) use: in addition to the factors seen for the storage, other causes of fault appear, linked to poor heat dissipation, mechanical and electrical stress, deterioration of the connections due to aging.

Please note that the faults often do not occur by chance, but are a consequence of an imperfect manufacturing process; the detection and localisation of faults thus becomes a tool to improve the manufacture and construction of the circuits.

The fundamental test philosophies are: the in-circuit test, the functional test and the signature analysis.
This document only examines the in-circuit test.