Gold Embrittlement

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.

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We also diagnose gold embrittlement by using BSE SEM images to estimate the area fraction of AuSn4 intermetallic compound in the solder joint.

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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.

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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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MLCC manufacturing flaw

This is a microsection of an MLCC as mounted on a PCBA. There is a fracture in the top of the MLCC, which is unusual.

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The fracture was clearly associated with some type of damage introduced while the ceramic dielectric was in the “green” state, i.e. prior to firing during fabrication of the multilayered ceramic structure. This type of damage cannot have occurred after firing when the ceramic is no longer deformable as it was as green tape.

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It is interesting that this defect escaped any visual and electrical inspections that might have been performed at the factory.

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Diode Short

The image below is a “stud diode” that had failed shorted.

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The diode was chemically decapsulated and the device die was examined in the SEM. A breakdown site was found near one corner of the die.

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Below is a higher magnification image of the breakdown site…

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… which shows a striking resemblance to the “Face on Mars”.

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Conclusion – the short was caused by an alien intelligence far greater than ours.

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