12 February 2010
Dissolution/erosion of the plating or coating is not a new issue, but with the introduction of lead-free soldering materials, new process stages like selective soldering and higher process temperatures, it is a real issue, as the pictures show.
To date this issue has had little attention but potentially, when looking at the images above, some engineers may have concerns. Example microsections have been presented in international seminars and discussed in technical forums. In these cases perfectly formed joints have been produced, but below the surface virtually all the copper has been dissolved. To date, no specific details on the process used have been circulated. However, the IDEALS Lead-Free Project, an early European Funded Lead-Free Project, reported this potential problem.
When the issue was first discussed with other engineers, very few people had observed the phenomenon. In fact, a review of the author’s previously produced microsections showed limited evidence of the problem. During a review of many of the lead-free sections produced by National Physical Laboratory (NPL), no significant evidence of erosion could be found. Typically all the joints would have been produced under strict controls and a copper reduction of 2-4um is not untypical on joints. More evidence of the problem surfaced during the SMART Group Lead-Free Experience first run in 2003 in the UK, and this was specifically on solder levelled boards.
It is fair to say that increased copper dissolution into the solder wave or selective pot was considered inevitable due to increased soldering temperatures and higher tin content. Furthermore, we have experienced copper dissolution during rework of joints and lead-free solder joints that often have double the thickness of intermetallic layer, even on reflow, suggesting greater copper removal. However, there is no direct correlation between the intermetallic layer thickness and copper removal. Measuring the copper thickness on the barrel of a hole that has been reworked can often show a greater dissolution rate on one side where the solder and heat are applied. Solder levelled boards, when examined by microsection, tend to show copper reduction. This was especially prominent when levelling is conducted twice to overcome thermal and wetting issues.
Concerns were expressed at the impact of high copper levels in wave and selective soldering processes but this really relates to our experience with tin/lead alloys. Copper is the main contaminant found during normal production; it does impact the soldering yield by increasing solder shorts. Higher copper levels do make a difference to the visual appearance of the joint and temperatures, but it has not been shown to impact joint reliability. With lead-free copper it is already part of the alloy system; between 0.5%-0.7% is added to the mix to lower the melting point.
The NPL comprehensive project report and good practice guide on copper dissolution is now available and covers extensive trials on materials and process and can be obtained by contacting Chris Hunt at firstname.lastname@example.org.
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