Dendritic Growth – Shorts on PCBA

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

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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.

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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.

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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.

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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.

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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?).

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

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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.

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

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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.

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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.

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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.

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Tombstoning of SMD Inductor

Tombstoning is a well known issue for SMT soldering and is usually caught at post solder visual inspection. However, at times the effect is a subtle lifting at one end of the component as was the case in this example of an SMD inductor. The failure was a high resistance in the associated signal net that wasn’t detected until final assembly and testing of the product.

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This is a BSE SEM image of a microsection of the device as soldered on the PCBA.

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The high resistance was caused by a failure of the original solder reflow process to wet the termination, which caused “tomb stoning” of the inductor.

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Silver Sulfide Crystals

Silver is a common material used to connect resistor terminals to the resistive element within the resistor. This example shows some unknown crystals growing out of the termination area.

Silver corrosion

These crystals are shown in the BSE SEM image below.

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EDS analysis showed that these are silver sulfide crystals.

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These crystals most likely formed as a result of gaseous sulfur corrosion of the silver thick film conductor near the termination of the resistors. Corrosion such as the type seen here is typically due to atmospheric sources of sulfur such as diesel fuel fumes or out-gassing from cardboard.

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IC Bond Pad Corrosion

Sometimes IC bond pad corrosion is caused by residual process chemistry from the die fabrication or packaging process as was the case in this example.

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The decapsulated IC showed corroded aluminum bond pads.

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EDS results showed a clear chlorine peak associated with the corrosion.

The corrosion caused signals to open circuit resulting in device failure.

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Crystal Frequency Shift

Crystal frequency shift

This is a BSE SEM image of a quartz crystal that reportedly shifted in frequency. The problem was apparently caused by tin-lead solder on the spring mounts that wicked out onto the silver contact metallization on the crystals, which would be expected to change the mechanical response and therefore the frequency of the crystal.

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Damage in BGA Device

BGAs are susceptible to damage from thermal-mechanical warpage stresses as well as mechanical bending stress.

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The damage tends to be subtle and isn’t visible at low magnifications such as the image above of a microsectioned device.

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At higher magnification a fracture became apparent in the upper layers of the die.

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In the image above, the die attachment appears to have failed and there is a void in the molding compound due to a processing flaw at molding.

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The bottom of the die separated from the molding compound and a fracture radiates away through the molding compound.

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However, the PWB laminate under the BGA pads also showed fractures suggesting that the assembled PCBA was bent out of plane. It doesn’t take a great deal of bending to induce this type of damage, so the problem may not be apparent at the moment it is caused.

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Manufacturing flaws in ceramic capacitor

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This is an optical image of the capacitor showing the part number.

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This is the section after final polish. Note the lack of a solder fillet between the right side termination and the lead.

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This is a BSE SEM image of the section. These anomalies are parallel in the plane of the capacitor plates (i.e. knit line defects) raising a question about the “as-sintered” strength of the structure.

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This fracture shows characteristics of thermal shock damage, possibly during attachment of the device leads to the capacitor end caps.

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This fracture traverses capacitor plates of opposite polarity, which typically results in a shorted capacitor. The fracture appears to propagate from a knit line defect suggesting the root cause is related to the original firing of the multilayer ceramic element.

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Failure Analysis Images and Discussion