Category Archives: Uncategorized

Solder Joint Fatigue

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.

solder fatigue 1

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

solder fatigue 2

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

solder fatigue 3

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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Failure of IR LED

LEDs tend to develop high levels of thermal expansion mismatch stress internally due to gross CTE mismatch between the lens polymer, the die, the lead-frame/cup, and the bond wires. In the case shown below, a silicone polymer was used to fill the cup perhaps to provide a stress buffer, but it did not appear to be effective.

Failure of IR LED 1

This view (photo above) is looking down through the lens at the die area. The damage shown is at the interface between the body material and the silicone polymer that fills the cup and encapsulates the device die.

Failure of IR LED 2

There was no evidence of damage at the wedge bond.

Failure of IR LED 3

This view (above) is a look through the bottom (lead) side of the device. There appeared to be some crazing at the rim of the cup.

Failure of IR LED 4

This image (above) was obtained at an intermediate stage in the microsectioning process. There is a fairly significant kink in the bond wire above the ball bond.

Failure of IR LED 5

The silicone encapsulant adhesively failed at the cup/silicone interface and the silicone/lens polymer interface.

Failure of IR LED 6

The image above shows a segment of silver plating that has separated from the underplating on the cup.

Failure of IR LED 7

Above is an optical image showing the location of the open at the kink, which is the high stress region of the gold bond wire.

Conclusion – The analysis results suggest that the LED failed due to an open bond wire just above the ball bond on the device die. The damage was likely caused by a thermal overstress event where the thermal expansion rate of the silicone die encapsulant and the peak temperature are the primary drivers. The kink in the bond wire and the adhesion strength between the silicone die encapsulant and the cup were also a likely contributing factors.

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Residue on PCBA

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

residue on PCBA 1

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

residue on PCBA 2

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

residue on PCBA 3

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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MLCC Bending Fracture

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

MLCC Bending Fracture _1

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

MLCC Bending Fracture _2

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

MLCC Bending Fracture _3

This is a BSE SEM image of the capacitor microsection.

MLCC Bending Fracture _4

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

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FPGA ECM short

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

ECM short 1

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

ECM short 2

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

ECM short 3

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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Tin Whiskers

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

tin whiskers 1

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.

tin whiskers 2

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

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LED Lamp Flickers

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

LED lamp 1

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

LED lamp 2

LED lamp 3

LED lamp 4

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

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.

gold-embrittlement-1

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

gold-embrittlement-2

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.

gold-embrittlement-3

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.

mlcc-flawed-1

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.

mlcc-flawed-2

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

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