Test coverage is a key set of metrics to ensure that delivered electronics products are defect free. Inspection was historically considered as part of process control but with increasing performance it becomes part of the defect detection process.
From the test coverage point of view, ICT was the first type of machine where the definition was clear. Around 1970, it was defined as number of tested devices, divided by the total number of devices. Around 1990, with growing complexity, 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.
Electrical tests do have some limitations: ICT cannot test bypass capacitors, but a 3D AOI is capable of verifying the presence, polarity and well-formed solder joints. Inspection has now become a part of the manufacturing line to enhance test coverage. SPI helps to reduce defect occurrence 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. AXI is key to inspect solder joints which are invisible, such BGA or J-Lead packaged devices. X-Ray inspection also contributes to check for voids in thermal pads and to ensure good power dissipation. Inspection techniques (SPI, 2D, AOI, 3D AOI, 2D AXI or 3D AXI) bring important coverage metrics on historical defect classes, like Presence, Orientation, Open, Short, but also introducing new ones (misaligned, tombstone, height, planarity…). It is time to update coverage metrics to consider all defect classes. This will contribute to delivering defect-free products.
Unfortunately, inspections machines are not always up-to-date with the state of the art for CAD import and coverage reporting. Here are some areas of improvements:
- The future of CAD to CAM conversion should be based on intelligent CAD files (not GERBER or centroid files) ensuring proper pin number (numerical or alphanumerical) to be stored in the machine database.
- The areas where shorts occur are usually examined as bridges on ICs where it should be expanded to all locations, including short distance between any exposed copper.
- The future of coverage report formats cannot be a huge spreadsheet with 100 columns, 10Kbytes per line. The addition of new columns and local language in the header makes the import process fail. XML based reporting ensures easy extension and allows XSL file conversion to an advanced color-coded report or simple text. The coverage reports must include part reference, pin number and the inspection algorithms applied to each of the elements.
TestWay is the state of the art technology to create & qualify inspection programs. TestWay predicts inspection coverage from CAD data and converts this data to be used by inspection machines, with significant savings in AOI library matching. It will also maintain the pin number used on the machine reports, to keep them consistent with the CAD data. As one key principle of Industry 4.0, TestWay closes the loop, by reading the test coverage back from the machine, to verify that the test performance (physical world) is aligned with the early simulation results (virtual world). Feel free to contact ASTER for a demonstration on any of your inspection machines.
