PCBA Manufacturing Test Coverage-printed
PCBA manufacturing test coverage
The ideal PCBA manufacturing test covers (that is, successfully detects) any defect(s) present from the entire PCBA manufacturing test defect universe. However in practice it is almost impossible to test for 100% of the PCBA manufacturing defect universe due to electrical and imaging test limitations and tradeoffs. The electrical process test challenges can range from the lack of access(test pads) to components in parallel circuits (for example, hundreds of small capacitors in parallel across power and ground planes). Imaging test challenges can be component(s) and solder joints hidden by RF shielding, small outline surface mount device (SMD) (e.g. 01005) under a larger SMD (e.g. ball grid array (BGA)) and even hidden solder joints masked by underside heat sinks or another solder joints. Thus, each test technology has significant ‘blind spots’, and a goal is to combine technologies to more effectively eliminate untested defects.
PCBA manufacturing test can be compromised by:
The lack of design for test (DFT) verification prior to design freeze.
Time to market (TTM) pressures compressing the time allocated to develop, debug and verify the tests.
Lowering overall cost of test for equipment, labor and test operations.
Previously, PCBA manufacturing used a simple formula to determine the test coverage:
This formula does not account for the quality of the test for each of the components. If a test verifies only 1 pin out of 1,521 pins of a BGA, the formula will deem the component is tested ignoring potential defects to the remaining 1,520 pins of the BGA component.
The formula is not rigorous to determine:
The quality of test of each component.
The criticality of the defect in the defect universe.
Similarly, PCBA shorts coverage in the past was defined as the number of accessible board nodes divided by the total number of board nodes.
This node-oriented measure did not take into account how nodes may have many or few connection points to device pins, and how some of these could never (in practice) be shorted. Others that could be shorted were also not tested. For example, a small-value inductor, a closed switch or jumper or a small-value resistor could have a short across its terminals, but not be tested due to resolution problems with the impedance measurement.