Gold embrittlement of solder joints still appears as a problem from time to time even though the issue has been understood for many decades. We diagnose this problem by using SEM/EDS to estimate the amount of gold in the solder joint. If the gold level exceeds about 3 wt% in the bulk solder joint then the joint is considered to be embrittled.
We also diagnose gold embrittlement by using BSE SEM images to estimate the area fraction of AuSn4 intermetallic compound in the solder joint.
There is a relationship between the wt % of gold in the bulk solder joint and the area fraction of AuSn4 intermetallic compound in the solder microstructure.
For eutectic tin-lead solder the relationship is as shown in the above figure, suggesting that 3 wt% Au corresponds with ~ 12% area fraction of AuSn4 intermetallic compound in the solder microstructure. AuSn4 is a relatively hard/brittle phase in a softer Sn & Pb matrix, so when the AuSn4 exceeds ~ 12% the integrity of the solder joint begins to suffer.
See also GOLD EMBRITTLEMENT OF SOLDER JOINTS.
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