March, 2019
www.us-
tech.com
What is Inspection Coverage in Comparison with Test Coverage?
By Christophe Lotz, President, ASTER Technologies T
est coverage is a key set of met- rics designed to ensure that electronic products are deliv-
ered defect-free. Inspection has his- torically been considered a part of process control, but with increasing performance it is becoming part of the defect detection process. From a test coverage perspec-
tive, ICT was the first type of machine where the definition was clear. Around 1970, it was defined as the number of tested devices, divided by the total number of devices. Around 1990, with growing complex- ity, it became the number of tested faults, divided by the total number of possible faults. This formula takes into account the device, but also the pins, with a list of faults that has been expanded over time.
Test Limitations Electrical tests do have some
limitations. ICT cannot test bypass capacitors, but a 3D AOI is capable of verifying the presence, polarity and the formation of solder joints. Inspection has now become a part of the manufacturing line to enhance test coverage. SPI helps to reduce defects,
because verification of solder paste, at the relevant stage, ensures that less solder faults occur. 2D AOI is evolving to 3D AOI with significant improvements in defect detection and reduced false calls.
Inspection has now become a part of the manufacturing line to enhance test coverage.
AXI is key to inspecting solder
joints when they are invisible, as are the joints required by BGA or J-lead devices. X-ray inspection also con- tributes to checking for voids in ther- mal pads to ensure proper power dis- sipation. Inspection techniques (SPI, 2D
or 3D AOI, and 2D or 3D AXI) pro- vide important coverage metrics on historical defect classes, such as presence, orientation, open, and shorts, but also introduce new ones, including misalignment, tombstones, height, and planarity issues. It is time to update coverage
metrics to consider all defect classes, which would then contribute to deliv- ering more defect-free products.
Possible Improvements Unfortunately,
inspection
machines are not always up to date with state-of-the-art CAD import and coverage reporting. Here there are opportunities for improvements. To begin with, the future of CAD-to- CAM conversion should be based on intelligent CAD files, and not GER- BER or centroid files, ensuring that the proper pin number (numerical or alphanumerical) is stored in the machine database. Next, the areas where shorts
occur are usually examined as bridges on ICs, where it should be expanded to all locations, including shorting dis- tance between any exposed copper. Lastly, the future of coverage report
The best defect detection results from a manufacturing test line with inspection strategies contributing to coverage, along with electrical test techniques.
formats should not be a massive spreadsheet with a hundred columns and 10 kilobytes per line. The addition of new columns
and local language in the header makes the import process fail. XML- based reporting ensures easy exten- sion and allows XSL file conversion to an advanced, color-coded report or simple text. The coverage reports must include part reference, pin
Continued on page 63
Page 59
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