Power electronics is a "hot" issue in the true sense of the word. Amplifiers and circuit-breakers can suffer considerable power loss due to excessive heat generated within the circuitry. This heat must be dissipated in order to prevent the destruction of the sensitive electronics. The heat is often dissipated using square or circular molybdenum blanks that act as base plates for semiconductor modules. For optimum heat dissipation, these blanks must be in excellent surface contact with the electronics.
This requirement led to an interesting order for our partner IMSTec. A manufacturer needed to inspect large numbers of these blanks for dimensional accuracy, surface quality and planarity before they were processed further. Only if the blanks met all the stringent quality requirements would the power electronic components that used them, work without error.
The demands placed on the proposed system were many and varied, and were not limited to the accuracy requirements in the individual tests and inspections. For instance, the blanks needed to arrive in bulk so that the system could be easily loaded. Due to the number of blanks requiring inspection, cycle times had to be less than 0.7 of a second. The blanks had to be sorted by quality, based on the inspection results and any items that did not meet the quality characteristics had to be separated out.
In addition, the system had to run un-attended for at least one whole shift with only one loading. The manufacturer also required product statistics covering quality and defect distribution in the form of reports and histograms. IMSTec soon realised that imaging technology was going to be essential to meet many of these requirements.
Highly accurate 2D and 3D inspections
The blanks must comply with tight strength, size, surface quality and planarity specifications. Because the blanks are available in an uncoated version as well as with various metal coatings, the specifications had to include the inspection of these coatings for quality. The system has now been implemented and is used to inspect strength, geometric dimensions, surface quality, and planarity along two orthogonal lines.
The resolution achieved for geometric measurements is approximately 5 m for the edge length and a range from 3,500 to 16,000 m for the diameter. The quality threshold was also high for the planarity tests, where out-of-tolerance deformations of just a few m can be reliably detected.
Moreover, both surfaces of each blank are inspected for colour, coating defects, discolouration and superficial scratches. Using stereoscopic surface and topography techniques, the blanks are also tested for blisters, deep scratches and bent corners or edges before being classified.