The Reliability and Failure Analysis group at ERA Technology has recently diagnosed failures in electronic devices due to interaction with low levels of gaseous sulphides - failures that caused both a financial impact to the manufacturers and safety issues with their customers.
Hydrogen sulphide is well known as the classic "bad egg gas" of the school chemical laboratory, with its pungent, unpleasant and easily recognised smell, even at very low concentrations. The odour threshold varies between individuals but is normally around 150µg/m3. With such a low concentration, you might expect that if there is no odour then there will be insufficient hydrogen sulphide to represent a threat to electronic component reliability. However, we have found that failure can occur in situations where there is no odour or obvious source of sulphur. The following are examples of failures in electrical systems that have been attributed to gaseous sulphides.

Examples of sulphide corrosion: Resistor Network for steel works

Fault: Corrosion to internal silver/palladium thick film.
Nature of Corrosion: Formation of silver sulphide dendrites. Device manufacture leaves a small flaw between ceramic dielectric layers. Internal corrosion of thick film layer is sufficient to cause an open circuit.
Sulphide contact: Devices were used in steel plant, adjacent to blast furnaces that are certain to raise ambient levels of hydrogen sulphide, although no local measurements of its concentration were carried out. The devices were likely to have seen several months of levels above 100 µg/m3 and transient peaks of several milligrams per cubic meter.

Relay Contacts in sub-surface application

Fault: Excessive formation of silver sulphide causing high contact resistance.
Nature of corrosion: Very fine needles of silver sulphide on contact surface.
Sulphide contact: Contacts in an underground train system were not sealed against the environment and had operated for several years since the last service. The level of sulphide in this underground location was likely to be elevated in comparison with normal background values, but no odour of hydrogen sulphide was noticed by staff, so was unlikely to be above 100 µg/m3.

Copper wire from water treatment plant

Fault: Corrosion to copper windings on relay
Nature of corrosion: Crack and pit corrosion to unprotected copper wires forming part of relay windings. Layer of sulphide on wire surface and internal corrosion along grain boundaries leading to an eventual open circuit failure.
Sulphide contact: Hydrogen sulphide, from water treatment processes, was known to be present. Levels experienced by the failed units are probably very variable but recorded measurements were below the minimum values of the meter used at 1ppm. Hydrogen sulphide is heavier than air and the basement location of the equipment may have increased exposure levels. Other gases such as ammonia may also have been evolved during the water treatment process and these may have assisted the corrosion process.

Surface Mount Resistors

Fault: Open circuit resistors
Nature of corrosion: Corrosion to conductor layer below end-cap to body junction - small silver sulphide crystals on the surface.
Sulphide contact: None known. Board mounted in protective housing with small breather hole. Sulphur detected in rubber seal. First failures occurred after several months. Attempts to measure the hydrogen sulphide concentration within the units showed values less than 200µg/m3. There is evidence that the rate of corrosion is faster below a silicone anti-vibration sealant.

Flexible circuit connection to LCD

Fault: Low resistance between tracks
Nature of corrosion: Copper sulphide growth at end of tracks below silicone seal and below tracks in adhesive layer. There is a small amount of corrosion to externally exposed copper but the heaviest corrosion is below the silicone polymer sealant used to cover the copper to ITO layer junction.
Sulphide exposure: None known. Connection was located inside a sealed housing, with no apparent source of sulphur. First failures occurred after a few months. The manufacturer of the component stated that failures are restricted to one user and suggested inappropriate cleaning chemicals as the primary cause. This problem is still under investigation.

How much sulphide is required?
Copper and silver are widely used in electronics because of their excellent electrical and thermal properties. Unfortunately both have very low activation energies towards the formation of sulphides with hydrogen sulphide. The sulphide corrosion product is porous so hydrogen sulphide continues to reach the metal surface. As long as the gas is present there is no mechanism to stop the corrosion process. In contrast tin, another metal commonly used in electronic assembly, forms an impervious sulphide layer, which prevents further reaction.
Anyone who has polished silver will know that a bright silver finish never lasts. It will eventually tarnish, no matter how clean or odour free the house may appear. Recent work on the preservation of silver artifacts in a museum environment showed that tarnishing could occur even with measured hydrogen sulphide levels as low as 0.2µg/m3. Another reference describes the formation of a 20 nanometres thick layer of silver sulphide in 100 hours at a concentration of just 100µg/m3.

Copper is probably less at risk at these low levels due to the thicker layer of surface oxide, but clean copper surfaces can still be at risk. Synergistic effects due to the presence of other gases like hydrogen chloride, sulphur dioxide or ammonia may play a part in accelerating the corrosion process.

Where does hydrogen sulphide come from?
Sulphur is a common element in the environment and reduced sulphur species like hydrogen sulphide can be expected whenever there is organic matter containing sulphur and oxygen is depleted. There are also many industrial sources of sulphide that can cause temporary or more permanent elevation in sulphide concentration; values regularly exceed 100µg/m3 with transient rise to a hundred times this level. Background levels of sulphide that an electronic component could experience in a normal working environment are much lower, probably less than 1µg/m3. Actual levels can usually only be guessed at, as quantitative measurements are rarely carried out at these low levels.

Sulphides from anaerobic bacteria activity can also produce a range of other reduced sulphur species including methyl and ethyl sulphides, dimethyl disulphides and some thiols. The degradation of sulphur containing proteins such as wool and hair can also produce carbonyl sulphide. It is likely that these forms of reduced sulphur gases can contribute or accelerate the corrosion process but their role in this effect appears to have been little investigated.

What environments represent high risk?
The examples shown above, indicate that threats to reliability can arise from external industrial or naturally occurring sources, as well as from hydrogen sulphide generated by outgassing or decomposition of organic materials containing sulphur. Industrial sources may cause large fluctuations in the level, with a more modest long-term average. Hydrogen sulphide from outgassing or decomposition will probably be at a lower but more constant level. Both may constitute a significant risk to vulnerable components.

What does this mean for reliability?
Bare copper or silver surfaces may be unavoidable in relays and other electrical systems. When these are intended for use in an environment where sulphides are known to be present then fully sealed units may be required. Otherwise bare copper or silver are rare in electronic systems and are not normally considered to be at risk. However a small molecule like hydrogen sulphide can diffuse into the finest crack or flaw and may also diffuse through some polymer layers. So unless a component is known to comply with a gas corrosion test such as EN 60068-2-60 (Flowing gas corrosion testing) it could be at risk in even modest levels of hydrogen sulphide.

One clear risk factor highlighted by the above examples is outgassing of sulphide from seals, gaskets and other polymers within closed housings. It would appear even low concentrations of sulphide gas can exploit the smallest flaw to access the metal surface and start corrosion. It may only take a few months before this type of failure becomes obvious. As components become ever smaller, with increased area to volume ratios, the risk of such failures will increase.

In two of the examples, corrosion appears to be accelerated by the presence of a silicone polymer layer. Although hydrogen sulphide is weakly ionic it is in fact more soluble in some organic solvents than it is in water. It appears that some polymers increase the absorption rate on to surfaces, in turn increasing the local corrosion rate. This should be borne in mind when considering applying protection layers to electronic components and may even affect the selection of conformal coatings.