This project looked at the scenario where lead free boards and Koki Lead Free Solder Paste S3X58-M406 were used along with tin lead finished QFPs.
The pictures show 2 leads from a Sn15Pb 0.65 pitch QFP alongside two pads with only lead free solder paste.
It can clearly be seen that lead from the QFP finish is leaching into the lead free solder paste and eventually induces the paste to start melting at around 210°C which is below the melting point of the paste itself. The final alloy of the finished solder joint would then be a tin lead silver copper alloy or mixture of alloys of unknown thermal and mechanical characteristics.
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Leaching of SnPb from leads started around
195°C. |
Solder paste with SnPb plated leads started to melt at around 210°C. |
Solder paste without leads started melt at around
218°C. |
The pictures show 2 leads from a Sn15Pb 0.65 pitch QFP alongside two pads with only lead free solder paste.
Depending upon the amount of lead which leaches from the component leads, combined with the rate of cooling after reflow, may lead to the above problem. This is caused by an alloy of Sn 36Pb 2Ag forming within the joint as it cools and as it will have the lowest solidification point, will be last to solidify. This alloy may concentrate at the interface with the pad and impact on the joint bonding strength.
By Gordon Clark, Director Global Support
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