Sulphur induced corrosion of electronics

Morten S. Jellesen, Vadimas Verdingovas, Svava Davidsdottir, Rajan Ambat Materials and Surface Engineering, Department of Mechanical Engineering, Technical

University of Denmark, DK-2800 Lyngby, Denmark, msj@mek.dtu.dk

Summary Environmental conditions such as humidity, temperature, contamination and gases affect the reliability and lifetime of electronic products. Sulphur polluted environments include regions near volcanic activity, pig farms, rubber manufacturing plants, oil refineries, coal-generation power plants, paper and pulp industry. Failure analysis of electronic products exposed to an environment containing sulphur gas typically show uniform corrosion and/or migration of silver and copper. Conformal coating is not always the adequate countermeasure against sulphur induced corrosion since gases can permeate through conformal coating and react with metals at the printed circuit board especially silver, copper and alloys hereof. This paper describes the corrosion of silver in sulphur gaseous environment and investigates the effect of hydrogen sulphide gas concentration and humidity on corrosion of a ceramic hybrid thick film substrate with conducting lines of silver and various contact pads, resistors, capacitors and integrated circuits. Experiments are carried out in a chamber where sulphur gas concentration is adjusted from mixtures of hydrogen sulphide and artificial air. The corrosion of a silver metallized quartz crystal is monitored online by having the quartz crystal exposed to the sulphur environment. The silver sulphide formation on electrically conducting silver lines and a silver coated quartz crystal and chip resistors are analysed using optical microscopy and SEM/EDS. The results show the relationship between sulphur gas concentration and humidity on the corrosion of silver. Silicone gel used as coating only had minor effect on silver sulphide formation. Keywords: Silver, sulphur, electronics, corrosion, gaseous corrosion.

1.Introduction The exposure of electronics to environments where temperature, humidity and atmospheric pollutants such as chlorides, NOx, SOx, and hydrogen sulphide (H2S), which are not controlled, can result in atmospheric corrosion of the metallic components on the printed circuit board assembly (PCBA). In order to minimize the risk of corrosion failure, it is important to be aware of corrosion risk during the design stage, reliability evaluations and qualifications, assembly processes, storage, shipping, and in the final use of the electronic devices. 1,2

Silver is used as a surface finish in electronic assemblies as e.g. hybrid circuits and if service life conditions contain H2S or SO2 gas such surface finishes will suffer from sulphur corrosion if humidity levels are high or even intermediate levels. Failures can occur in as little as a few weeks in industries such as rubber manufacturing, water treatment, paper mills or fertilizer production, among others. 3 In a field failure return from a pig farm sulphide corrosion of silver surfaces was observed on a ceramic hybrid due to permeation of corrosive sulphur gasses through the silicone coating. Three types of corrosion features were reported in this case: uniform distribution of silver sulphide, dendritic growth, and creep corrosion of silver on gold wires.4 Another example of dendritic growth of silver caused by sulphur gas ingress during electronics service life is given in figure 1.

Figure 1. Field failure return showing silver dendrites formed between conducting lines under an integrated circuit chip with several mm thick layer of silicone potting gel. A study of silver corrosion at indoor conditions in an electronics production site, dedicated to assembly and functional testing of microelectronic devices, show how silver plated copper lead frames showed sulphur corrosion due to presence of hydrogen sulphide because of vapour emissions from a nearby geothermal power plant.5 The concentration of H2S in the tested electronics plant atmosphere was 0.9 ppm and in some areas of the assembly process reaching 1.5 ppm, due to the operation of ovens used for curing epoxy resins as part of the manufacturing process. Temperature was reported to vary between 19-27 °C and relative humidity from 32 to 64 % relative humidity (RH). After 60 days silver coupons showed clear color change into royal blue coloration in the centre surrounded by purple at the edges. 1.1 Sulphur gas sources Factors that can cause accelerated corrosion of silver and silver sulphide formation in the service life of electronic products include industrial polluted areas or near geothermal/volcanic activity sites with high concentrations of elemental sulfur, sulphide and/or sulfur dioxide in the environment. Also paper, drywall or paperboards (e.g. used for transportation of electronic parts) is known to be a potential sulphur source. Especially at elevated temperatures aggressive substances from materials like foam pads, rubber sealing, elastomers vulcanized with sulphur, anti-vibration pads, or thermal conductive pads can evaporate and cause sulphur induced corrosion. In case of rubber, peroxide cross-linked materials are available on the market as alternative to sulfur cross-linked versions.6

500 µm

1.2 The effect of gas concentration and humidity on sulphur corrosion of silver Sulphide corrosion of silver surfaces has been studied by Graedel,7 stating that silver will readily form silver sulphide by the reaction:

The presence of water is believed to increase the reaction rate,8,9 nevertheless discrepancy in literature on the effect of humidity on silver sulphide formation exists on this point, e.g. Rice et al. reports no dependency on humidity on silver sulphide formation.10 1.3 Countermeasures against sulphur corrosion Countermeasures against sulphur corrosion involves the control of several atmospheric factors (wherever possible), including lowering of air humidity and temperature and the level of contaminants in order to avoid silver corrosion. Silicone gels are frequently used as encapsulation of e.g. hybrid circuits and light emitting diodes to protect dies and wire bonds against service life contaminants and humidity. Silicones possess a high optical transparency, favourable mechanical properties, superior thermal and radiation stability. Nevertheless, many examples are given in literature no silversulphide formation under silicone gel as well as the permeability of gases through silicone has been described.11 1.4 Sulphur corrosion and flux residues The purpose of the flux is to act as a wetting agent and surface activator during the soldering process. Although the idea of using ‘‘no-clean” flux is to leave minimal residue after soldering, in practice large amounts of residues are left on the PCB surface, depending on the skills of the technical personnel performing the soldering work. 12 The ‘‘No-clean” solder flux systems often contain weak organic acid (WOA) and/or halogenide (activator) in order to facilitate the chemical reactions that remove oxides on the metal surface. Resin is added to prevent re-oxidation of the cleaned metal. The effect of temperature on the degradation of flux residue has showed that a temperature above 250 °C is needed for complete decomposition of WOA components in a no-clean wave solder flux.13 It is known, that some of the WOAs have more detrimental effect on the surface insulation resistance (SIR) and corrosion than other acids, when tested under a variety of temperature-humidity-bias conditions. A number of investigations reports the effects of flux residues on the leakage current and corrosion as a function of humidity, temperature, flux type and its concentrations.14 Investigations of leakage current due to WOAs on the SIR pattern as a function of RH have shown that for succinic acid significant increase in current is observed at 98 %RH, which is also the deliquescence RH for succinic acid.15

2.Materials and Methods A part of a ceramic hybrid thick film substrate was used in this study to investigatethe effect of hydrogensulphide gas concentration and humidity on silver sulphide formation. Several areas of the hybrid were investigated before and after sulphur gas exposure.In this paper, one selected area consisting of pure silver lines, and silver lines coated with gold for gold-gold wire bonds are presented. The hybrid was exposed to sulphur environment in

unbiased conditions. Precontamination with succinic acid was made by adding 1 ml of a solution of 10g/l succinic acid in isopropanol. The precontaminated hybrid was dried at ambient temperature and humidity for 1 hour prior to exposure to hydrogen sulphide. Some of the hybrids were silicone potted with a conventional used silicone gel by adding 10 ml to a glass dish with an area of 44 cm2 resulting in a layer thickness of 4 mm. Curing of the gel was carried out in an air circulating oven at 120 °C for 30 minutes. A QCM200 Quartz Crystal Microbalance (QCM) from SRS Inc. with 5 MHz AT-cut silver coated contact quartz crystal was used for weight gain measurements in H2S environments. The crystals were metallized with 500 nanometer silver by International Crystal Manufacturing Company, Inc. The quantitative relationship between the change in frequency of the piezoelectric crystal and a change of mass caused by the mass loading on a piezoelectric crystal surface is given according to Sauerbrey equation:16

( ) mCmA

ff fqq

∆−=∆

−=∆ 2/1

202

ρµ (1)

where f0 (Hz) is the resonant frequency of the crystal; µq (g⋅cm-1⋅s-2) is the shear modulus of quartz; ρq (g⋅cm-3) the density of quartz; A (cm2) is the surface area of the electrode; ∆m (g) is the change in mass on the crystal. The sensitivity factor Cf for the crystal used in this work at room temperature was 56.6 Hz µg-1⋅cm2. The chamber used for sulphur gas exposure is a desiccator box with a size of 45cm• 40 cm•25 cm. H2S gas in artificial air was added the dessicator box from certified 10 l cylinders with a nominal pressure of 200 bar from British Oxygen Company, BOC a member of the Linde Group. The certified gas concentrations have an uncertainty of < 2%. In this study gas concentrations of 100 and 2.5 ppm were used. Inside the desiccator box a ventilator is placed inside the box in the center front to secure homogenous distribution of the gas. A flowmeter adjusts the flow inlet set to 15 ml/min. An outlet leads the gas to a water trap and for safety reasons the whole setup is installed in a fume hood. The humidity is controlled by 4 beakers placed below a perforated plate in the bottom of the box, filled with solutions to adjust relative humidity, i.e. deionized water (99% RH) or saturated solutions of NaCl (~75%RH) or CaCl2 (~35 %RH). All experiments were carried out at room temperature.

3.Results and discussion

A part of a ceramic hybrid thick film substrate was selected for observation before and after exposure to hydrogen sulphide. The investigated area consists of silver lines with plated gold and gold wire bonds. The selected area prior to hydrogen sulphide exposure is shown in figure 2.

Spectrum wt.% O Al Si Ca Ti Zn Sr Zr Ag Sn Ba Au 1 34.0 7.0 14.2 1.6 1.8 10.5 9.5 6.4 14.9 2 4.4 0.9 94.7 3 6.5 1.5 3.1 88.9

Figure 2. SEM micrograph and EDS analysis of part of a ceramic hybrid thick film substrate selected for observation before and after exposure to hydrogen sulphide. Since silver and gold are both noble metals corrosion does not happen at an unbiased hybrid high humidity without hydrogensulphide gas despite contamination with succinic acid prior to humidity exposure (figure 3).

a) (b) Figure 3. Exposure for 7 days of PCBA to (a) 75% RH and (b) 99 % RH. Both hybrids were contaminated with 1 ml 10 g/l succinic acid prior to humidity exposure.

1

2 3

3

days

7

days

10

days

12

days

Untreated Succinic acid precontaminated Silicone gel coated

Figure 4. PCBA as is, PCBA precontaminated with succinic acid and PCBA coated with silicone gel and exposed to 100 ppm H2S at 99% RH for 3,7,10 and 12 days. Severe sulphur corrosion is seen for the succinic acid contaminated hybrids (figure 4). The silver sulphide mobility across the hybrid surface can result in bridging of circuit features to occur. The corrosion product film is still electrically conducting, however as the corrosion continues the silver line film thickness decreases, ultimately to a point where it is broken and electrical failure will occur. Figure 4 also shows that silicone gel does not decrease the rate of silversulphide formation, actually the silicone gel coated hybrids appear more homogenously discoloured black than untreated hybrids. After 12 days of exposure dendrites are formed for the hybrids that are silicone gel coated (figure 4 lowest right).

35 %

RH

75 %

RH

99 %

RH

Untreated Precontaminated with succinic acid Figure 5 Untreated and hybrids precontaminated with succinic acid at 3 different humidity levels and exposed to 2.5

ppm H2S gas in artificial air at room temperature for 72 hours.

35 %

RH Wt..% O Si S Ag Au

1 1.2 1.1 97.7

2 7.5 2.3 7.2 83.4

Wt..% O Si S Ag Au

1 13.6 1.5 2.2 82.9

2 11.5 0.9 1.5 86.1

75 %

RH

Wt..% O Si S Ag Au

1 1.1 1.9 97.1

2 6.0 1.6 2.2 13.2 77.0

Wt.% O Al Si S Zn Zr Ag W Au

1 45.9 0.8 52.9

2 34.7 2.73 62.6

3 58.6 6.4 11.0 8.4 5.9 9.5

4 42.4 8.3 14.1 13.6 7.9 13.6

99 %

RH

Wt..% O Si S Ag Au 1 1.2 1.8 97.0

2 5.1 1.9 3.3 24.2 65.5

Wt.% O Al Si S Zn Sr Ag Ba W Au

1 10.2 2.3 2.7 81.2 3.6

2 5.9 1.9 3.9 24.5 63.7

3 27.8 3.5 10.4 4.2 5.0 7.7 22.0 11.0 5.1

Untreated Precontaminated with succinic acid

Figure 6 SEM micrographs and EDS analysis of untreated and hybrids precontaminated with succinic acid, exposed to 3 different humidity levels at 2.5 ppm H2S gas in artificial air at room temperature for 72 hours.

Figure 5 and 6 show the effect of succinic acid on silver sulphide formation. For the untreated hybrids discoloration increases with humidity and at 75 % RH also the gold coated silver lines are attacked by hydrogen sulphide. SEM investigations revealed porosities of the gold layer leaving reactive silver sites exposed for hydrogen sulphide reaction. The hybrids that were succinic acid precontaminated showed similar response until 99% RH where more evident dark coloration can be seen as well as spreading of silver sulphide on the ceramic hybrid surface. This result fits with previous investigations showing increase of leakage current above 98% RH for succinic acid contaminated SIR patterns. 15

0 24 48 72 96 120 144 168 192 216 2400

2

4

6

8

10

12

∆m

(µg/

cm2 )

Time (h)0 24 48 72 96 120 144 168 192 216 240

2020

2030

2040

2050

2060

2070

Rm

(Ohm

)

Time (h)

Figure 7. Change of mass (a) and motional resistance (b) of a silver coated QCM crystal during exposure for 10 days to 100 ppm H2S, at 99 %RH. The QCM crystal surface has been added 1 µl silicone gel and cured for 30 min at 120 °C prior to hydrogen sulphide exposure. Exposure of a silver coated QCM crystal to 100 ppm H2S at 99% RH shows an increase of mass with time (figure 7). A homogenous black coloration of the QCM crystal was seen also under the area covered with silicone gel. More work is ongoing to study the effect of H2S gas concentration and humidity on silver coated QCM crystals.

4.Conclusion The use of silver in electronics can result in climatic reliability issues if the service life environment contains sulphur containing gases. Investigations of silver sulphide formation on a ceramic hybrid thick film substrate with conducting silver lines and a silver coated quartz crystal show the relationship between sulphur gas concentration and humidity on the corrosion of silver. Conventional silicone gel used as coating only had minor effect on silver sulphide formation, both for silver coated QCM crystals and for whole hybrids. • Sulfur gas permeate through conventional silicone potting gels and causes sulphide formation of silver as well as Cu and other metal elements used in electronics. • Contamination with active organic acid, as can be found in unreacted flux residues, increases silver sulphide formation rate and amount. • Present investigations of ceramic hybrids wit conducting silver lines showed that Increasing humidity increases silver sulphide formation rate and amount.

Acknowledgements Current research has been conducted as part of the ICCI project from the Danish council for Independent Research, Technology and Production (FTP) and IN SPE project funded by Innovation Fund Denmark. The authors would like to acknowledge the commitment and help of the industrial partners in the Centre for Electronic Corrosion (Celcorr) consortium. References

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