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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Diode Short

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

diode-short-1

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.

diode-short-2

Below is a higher magnification image of the breakdown site…

diode-short-3

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

diode-short-4

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

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ESD Damage in Zener Diode

Sometimes we’re lucky and stumble into something like this ESD damage in a Zener diode. The diode was short circuited, but it was micro sectioned in order to determine the die thickness (not something we would normally do on a shorted diode).

zener-eds-1

A damage site was found directly under the ball bond. The damage was determined to be a gold spike (as in ESD) that punched the junction shorting the diode.

zener-eds-2

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Severe corrosion on PCBA

Activated solder flux residues left on a PCBA can cause severe corrosion problems. The leads on the component shown below are severely corroded.

pmic-corroded-1

The elemental spectrum suggests that the needle like corrosion product is lead chloride & lead bromide.

pmic-corroded-2

Conclusion – avoid inadequate cleaning of PCBAs.

pmic-corroded-3

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PCBA Corrosion

Corrosion due to residual solder flux and cleaning process chemistries left on PCBAs can be a real problem for reliability. The corrosion can cause both open circuits as it eats through metal runs and short circuits due to electro-chemical migration.

Pixel_Size=1.25003E+1 µm Mag=13.890 × Display_Mag=158 Zoom=6.33000 Stage_X=-11.299 mm Stage_Y=-8.815 mm Stage_Z=0.000 mm Stage_R=0.00° Stage_T=0.00° Stage_B=0.00° Spot_Size=29.3% Accelerating_Voltage=15.0 kV Working_Distance=21.9 mm Operator=SEM Lab Instrument=PSEM II Analysis_Date=1/6/2011 Analysis_Time=02:10:10 PM Sample_Number= Client_Number= Client_Name= Project_Number= Caption= RasterBox=[119,233,375,284] Live_Time=78.00 Acquisition_Time=91.00 Dead_Percent=11.0% Detector_Tilt=35° Peak_Label=Cr@5.414 Peak_Label=Cr@5.946 Peak_Label=Ni@7.477 Peak_Label=Ni@0.851 Peak_Label=Fe@6.403 Peak_Label=Fe@7.057 Peak_Label=Fe@0.705 Peak_Label=Sn@3.904 Peak_Label=Sn@3.443 Peak_Label=Sn@3.662 Peak_Label=Br@1.481 Peak_Label=Pb@2.345 Peak_Label=Cl@2.622 Peak_Label=Si@1.741 Peak_Label=C@0.277 Peak_Label=O@0.525

This (above) is the elemental spectrum of the end of a through-hole connector solder joint. The chlorine in the spectrum is likely due to chloride activator from the solder flux.

Pixel_Size=1.24996E+1 µm Mag=13.891 × Display_Mag=160 Zoom=6.41000 Stage_X=-11.169 mm Stage_Y=-8.048 mm Stage_Z=0.000 mm Stage_R=0.00° Stage_T=0.00° Stage_B=0.00° Spot_Size=29.3% Accelerating_Voltage=15.0 kV Working_Distance=21.5 mm Operator=SEM Lab Instrument=PSEM II Analysis_Date=1/6/2011 Analysis_Time=02:07:24 PM Comment= RasterBox=[250,245,273,286] Live_Time=65.00 Acquisition_Time=78.00 Dead_Percent=14.0% Detector_Tilt=35° Peak_Label=Sn@3.904 Peak_Label=Sn@3.443 Peak_Label=Sn@3.662 Peak_Label=Br@1.481 Peak_Label=Ba@5.156 Peak_Label=Ba@4.827 Peak_Label=Ba@4.465 Peak_Label=Pb@2.345 Peak_Label=Cl@2.622 Peak_Label=Si@1.741 Peak_Label=C@0.277 Peak_Label=O@0.525

This (above) is the elemental spectrum of some corrosion product associated with the through-hole connector solder joint, which appear to be Pb and/or PbO crystals growing out of the surface of the solder joint.

pcba-corrosion-3

These are the Pb and/or PbO crystals growing out of the surface of the solder joint.

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Shorted FET

Electrical continuity tests suggested that the gate was shorted to the drain and the source was intact. SEM analysis revealed a suspected damage site at the end of a gate finger (see below).

shorted-fet-1

An elemental dot map at the breakdown site showed some slight disturbance of the aluminum metallization at the site.

shorted-fet-2

Voltage contrast imaging confirmed that the suspect damage site was indeed the location of the short.

shorted-fet-3

These results suggested that the device failed due to a voltage transient on the gate signal. The small dimensions of the damage site suggest that the transient was very fast (rough estimate 1E-9 to 1E-6 seconds). The damage could potentially be related to an ESD event. It could also potentially be caused by accumulated damage at the tips of the gate contacts due to excessive turn-on or turn-off dV/dt.

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Dendritic Growth – Shorts on PCBA

Here is another example of electro-chemical migration that resulted in shorted signals on a printed circuit board assembly.

dendritic-growth-1

These ECM residues can often be hard to see under an optical microscope because there are optically transparent metal oxides present with very small metallic dendrites.

dendritic-growth-2

Backscattered electron SEM images show contrast related to atomic number, so for example lead appears very bright (Pb z=82) and carbon appears dark (C z=6). The dendritic structure is a clue that this was ECM related.

dendritic-growth-3

The elemental map below shows that the dendrites are mostly lead but there is also some tin. Both lead and tin are likely to be involved for Sn-Pb solder joint assemblies.

dendritic-growth-4

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ECM of PWB Battery Contacts

ECM (electro-chemical migration) can occur when moisture, voltage bias, and ionic contamination come together, such as was the case for the PWB battery contacts shown in the BSE SEM image below.

battery-contacts-1

Elemental analysis suggested that the ECM residues were primarily copper, but also zinc (from brass?) and nickel (from ENIG finish or nickel under-plate on the connector contact?).

battery-contacts-2

The feature that identifies this as ECM versus plain chemical corrosion is the dendritic structure of the residue as shown in the next image.

battery-contacts-3

Historical data from this laboratory shows that the most significant factor is often moisture, followed by voltage (or local electric field), and finally ionic contamination. The lesson seems to be “keep it dry”.

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Bad Board Design

Design rules suggest that MLCCs should be placed away from board edges and panel break out zones for reasons illustrated in this example.

bad-board-design-1

The MLCC was placed far too close to the breakout feature during the design phase for this product.

bad-board-design-2

This is a higher magnification image of the flexure fracture that caused the MLCC to short circuit some time after the board was placed into service.

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Solder Joint Failure

This example is a SMT connector where the solder joint appeared suspect.

creep rupture 1

Below is a BSE SEM image of a microsection through the suspect solder joint and its neighbors. Note the coplanarity issues for these connector leads.

creep rupture 2

This appeared to be a creep rupture failure of the solder joint where the lead that failed was under stress that caused creep (time dependent plastic deformation) of the solder joint. The vertical displacement of the lead after the solder joint fractured is the key feature that suggests this was a creep rupture failure.

creep rupture 3

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Failure Analysis Images and Discussion