Bad and good test effectiveness values are the percentage of PCBs that are properly distinguished as bad or good. This measure differs from test coverage, which is determined by the percentage of defects covered. Since both are measures of defect detection, factors that increase test coverage will also increase bad test effectiveness.
Good test effectiveness is a measure of properly passing1 good PCBs. Factors that affect good test effectiveness include proper fixturing and appropriate test-target size and spacing. A bad fixture may result in incorrectly failing a good PCB，due to improper fit or contact. In addition， very small target size and inadequate spacing between targets may result in false failures, due to improper contact between the tester and the PCB. A small target size of less than about 35 mil, 〇r with less than 50 mil between targets, is not considered adequate.
Future trends in testing
The increased use of higher-density PCB component technology might change some of the analysis performed in the test strategy shown in this chapter. Nodal access to some components, such as the ball grid array (BGA), is limited, since the higher number of leads has resulted in the leads being placed underneath the body of the component. These leads could be placed on the top side of the PCBs with no access to test pins, hence in-circuit testing could not be performed for the BGA connections to test whether the terminations were successfully completed. Newer testing technologies that are currently available, such as x-ray machines that can detect solder defects through the PCBs, might have to be added to the test methods and strategies. The most efficient method to reduce test costs is to increase the quality of the assemblies being tested, as shown earlier in the examples in Tables 4-5 to 4-8.