This is a somewhat classic case of solder joint fatigue failure, but with the added factor of gold embrittlement. The solder alloy is SN63, the package is a J-lead PMIC, and it is soldered to an alumina ceramic substrate.

The elemental spectrum of the bulk solder joint suggested it contained ~ 3 wt% of gold, which is considered a threshold for gold embrittlement.

The thermal fatigue fracture showed classic characteristics such as grain boundary separation and propagation through the bulk solder joint.

The AuSn4 intermetallic compound is clearly visible in the solder joint microstructure at about 12% by area in the section. This harder phase in the Sn-Pb matrix likely accelerated the thermal fatigue failure.

The analysis results suggest that the thermal fatigue damage was likely due to the combination of (1) CTE mismatch between the J-lead package and the alumina substrate and (2) Au-Sn IMC in the solder joint microstructure at the threshold for gold embrittlement.

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Residues on PCBAs are unfortunately common place. SEM/EDS analysis helps to determine the nature of the residue.

The image above shows residue on the surface of the PCBA, but it is not clear what is the nature of the residue.

EDS analysis shows that the residue is likely tin bromide, suggesting corrosive bromide flux activator was left behind after assembly.

This is a BSE SEM image of some of the residue, which shows dendritic growth. The dendrites suggest that electrochemical migration (ECM) was a factor meaning ionic contamination, electric field, and moisture were likely at play. If the dendrites migrate between normally isolated signals (e.g. PWR & GND) then the circuit fails.

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A client provided two sensor assemblies for failure analysis of capacitor C4 on the internal PCBA.

This is an optical micrograph of the capacitor as mounted on the PCBA.

This is an optical image of an intermediate grinding stage during microsection preparation. A bending fracture was noted under one of the terminations.

This is a BSE SEM image of the capacitor microsection.

The fracture appeared to be due to flexure of the PCBA.

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Ionic contamination, moisture, and electrical bias can combine to create electro-chemical migration (ECM) shorts under an IC device as shown in this example.

This is an optical image showing residue on the bottom surface of the FPGA package.

This is a BSE SEM image of the same corner location. The residue bridges several signals at this corner.

An EDS spectrum of the residue suggest that it contains chlorine and bromine. Part of the bromine signal likely originates from the brominated epoxy molding compound. The chlorine & perhaps some of the bromine is most likely residual solder flux activator.

The analysis results suggest that the most likely cause of failure was external corrosion and ECM due to halide contamination. The corrosion and ECM was likely exacerbated by elevated temperature and humidity conditions.

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A client provided a flex cable sample to SEM Lab, Inc. for SEM/EDS analysis of the crimp contact plating.

EDS of the contact plating revealed that the plating was pure tin, which explains in part the many tin whiskers associated with the crimped region of the flex cable.

The longest whisker in this image is very close to shorting two adjacent signals.

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An LED lamp assembly reportedly “flickered”, meaning there was likely an intermittent high resistance in the LED circuit.

Here are summaries of the destructive physical analysis of three suspect LEDs.

The analysis results for all three samples were similar. Thermal-mechanical damage was found including (1) thermal decomposition of the lens material mid-span on the anode bond wire on Sample A & Sample C, (2) adhesive failure (separation) between the phosphor-filled-die-encapsulant and the cup on all samples, and (3) fracture of the phosphor-filled-die-encapsulant that intersected the cathode ball bond of Sample B. All of these observations are consistent with the thermal-mechanical effects of electrical overstress.

The client reported that the LEDs were being operated at 150% of the maximum forward current, which likely was the primary factor causing premature failure.

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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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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.

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.

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

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

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.

Below is a higher magnification image of the breakdown site…

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

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

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