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.

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.

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.

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

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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).

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

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

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

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.

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.

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 (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.

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?).

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

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

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.

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 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.

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

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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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.

The decapsulated IC showed corroded aluminum bond pads.

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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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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