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August 2015 • SMT Magazine 63 some of the visual criteria we use in the indus- try for lead-free. Recent trials have been con- ducted on selective soldering systems with lead- free alloys. In this case, where the boards had very thin copper plating and had been solder levelled with lead-free, the copper removal was significant. After exposing the boards to a high temperature during selective soldering for an extended time, copper pads were fully dissolved from the surface of the board. This is not typi- cal and should not occur when a sound plated layer is present in a well-controlled lead-free as- sembly facility. However, it does demonstrate what can happen. A full paper on the work we have done at NPL on copper dissolution is available from the Defect Database. Broken Circuit Tracks on BGA Design Large PBGA ball was found with open circuit connections. X-ray examination found broken tracks leading to corner ball terminations. The PBGA was mounted on a standard 1.6 mm fi- bre glass FR-4 substrate. Probable cause may be expansion and contraction of the package and board during high-temperature operation; this is the likely cause for the open connections. If the pad surface had separated from the board (pad cratering), due to mechanical shock or figure 10: pbga ball with open circuit connections. figure 11: X-ray of components that have been placed in production prior to reflow is a good way of seeing any variation on placement force without disturbing any packages. FeAture CASE STuDY: DETECTING PROCESS DEFECTS continues