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62 SMT Magazine • August 2015 then allow the formation of a conductive path on or through the moisture layer. A copper dendrite often creates an inter- mittent fault which can be very difficult to pinpoint. Contamination testing and surface insulation resistance (SIR) assessment are two techniques often used to monitor and control the levels of harmful contamination on fin- ished products to help avoid the possibility of corrosion. Contamination that causes this fail- ure mode is not only from the flux; it can come from the cleanliness of the printed board prior to use. It can also be caused by the design of the board, the way it is mounted in a product, and exposure to changes in temperature and humidity. X-ray and CT were produced on a DAGE system. Copper Pad Erosion/Dissolution Although examples of copper erosion have been highlighted in the industry, there is little evidence of this being an issue. In the case of single-sided boards, the apparent ero- sion may have been due to preparation of the copper for OSP treatment. In this case cop- per is mildly etched, and excessive prep may have removed more copper around the pads as other areas of the tracking would be pro- tected by the solder mask. Where mechanical cleaning is used and incorrectly controlled, the copper can be reduced around the hole leading to an apparent copper reduction. Fur- ther investigation of the problem and the ex- amples circulated in the industry will be fur- ther reviewed and where appropriate further trials conducted. It is interesting to note that the defects highlighted have not been shown to cause failures. At first examination we would con- sider them all rejectable based on our existing knowledge of tin/lead joints. That knowledge is again not necessarily based on failures but the inspection criteria for solder joints in circula- tion today. Perhaps we do need to re-examine Figure 8: The images show the same defect using three different FA techniques to x-ray (left), back microsectioning (center) and cT (right). figure 9: examples of copper erosion. FeAture CASE STuDY: DETECTING PROCESS DEFECTS continues