What is Test Coverage?
Test coverage (also called defect coverage) is a measure of the ability of a tester to detect defects. It is the percentage of those defects that are “covered” by a test，with 100% representing test coverage of all possible defects within a particular PCB defect category. There are many factors affecting test coverage:
• “Nodal access” refers to physical access to the nodes of a circuit by a test probe. When there is less than 100% nodal access, the coverage of circuit functionality is lessened. It is dependent upon the technology 〇f the circuitry of the PCB. Coverage of analog circuitry increases approximately linearly with nodal access, whereas cover- age of digital circuitry increases in step increments, depending on whether the node controls important digital pins such as reset pins. Thus, digital circuits have a higher number of critical nodes, i.e., nodes that control or affect a large amount of functionality.
• The manufacturing technology of a PCB includes features that can affect nodal access, such as the use of surface mount technology (SMT) and the component population density of the PCB. Through- hole (TH) circuits have about 100% nodal access; SMT PCBs can have significantly less. Double-sided PCBs impede nodal access, since using the underside of the PCB for circuitry imposes a competition for PCB '*real estate*' between that circuitry and the test routes needed for accessing the top of the PCB. High-density PCBs result in less access, due to difficulty in probing the test pads.
• Strategic business decisions concerning the amount of time and financial resources budgeted for test and fixture development. A model for this effect would involve two stages: the first would assume a minimal test development time of approximately two weeks to develop 60—70% of test programs; the second stage would allow additional time of two to four weeks to complete the remaining tests.
• Design for test efforts (DFT). Test coverage can be increased by DFT efforts and built-in self-test (BIST) features. These are tests embedded inside the PCBs. The amount of memory allotted for BIST is a good indicator of good test coverage.