Accelerated Corrosion of Printed Circuit Boards due to High Levels of Reduced Sulfur Gasses in Industrial Environments

Paul Mazurkiewicz, Ph.D.

Hewlett-Packard Company, Fort Collins, Co, USA Paul_mazurkiewicz@hp.com

Abstract

Accelerated corrosion leading to system failure has been observed on printed circuit boards present in industrial environments that contain abnormal levels of reduced sulfur gasses, such as hydrogen sulfide (H2S) and elemental sulfur. The problem is compounded by the fact that elemental sulfur is regulated by OSHA as a nuisance dust, and is allowed in a human working environment at the parts per thousand levels. Anecdotal data shows clearly that elemental sulfur gas present at the parts per million level can cause computer systems to fail within 2 months of use. Newer technologies such as immersion silver plating are especially susceptible to this type of corrosion. With the rapid growth of organically coated copper (OCC) and immersion silver platings, the number of failures due to reduced sulfur gasses in the environment has risen substantially.

Introduction

Elemental sulfur can come from many sources. For example, the Kaloin (China) clay used in the prototype modeling of vehicles and consumer products contains over 50% elemental sulfur. During its use, the clay is heated to temperatures that liberate large quantities of elemental sulfur. Other sources include paper mills, where sulfur is used in the bleaching process and power plants where geothermal sources are used to turn steam turbines. Oil refineries produce copious amount of sulfur gasses during the processing of crude oil. Hydrogen sulfide, another common form of reduced sulfur can come from waste treatment plants, automobiles and oil refining. With the advent of the RoHS legislation in Europe, which forbids the use of lead in electronic products, HASL boards became obsolete. BGA related failure modes on ENIG based boards also made this technology undesirable. As a result, alternatives such as immersion silver (immAg) and organically coated copper (OCC) are currently used as board finishes. Due to inherent processing difficulties with OCC boards, immAg boards are quickly becoming the standard PC board finish in the electronics industry. With this change to immAg, failures relating to high reduced sulfur levels have increased dramatically.

Corrosion of PC Boards

The reason for the dramatic increase in corrosion failures due to the use of ImmAg involves the propensity of silver and copper to react with sulfur. This is further complicated by the fact that there are areas on immAg circuit boards that contain both exposed silver and copper in contact, such as the inside of via barrels. The purpose of the silver coating is to protect the copper beneath. If the coating was complete, corrosion would not take place as rapidly. It is well known that when a metal is coated with a more noble metal that galvanic corrosion can occur in the presence of a suitable electrolyte [1]. Silver is more noble than copper, so this system, when in the presence of atmospheric water, forms an electrochemical cell. The water from the atmosphere is present in monolayer quantities, which is enough to promote the reaction [2]. Ag+ + e- ���� Ag 0.800 V standard electrode potential Cu2+ + 2e- ���� Cu 0.340 V standard electrode potential The equation above shows the galvanic cell potentials of both copper and silver metal reduction. Based on this data, the copper, due to being the more active metal, will always be the anode when electrochemical reactions are possible with these two metals. In addition, copper corrodes significantly faster than silver in a sulfur based oxidizing environment [3]. This results in a very high corrosion rate. The corrosion product contains metallic material that is highly conductive. The concentrations required to cause this corrosion appear to be high. The normal level of hydrogen sulfide, a reduced sulfur compound is around 0.3 ppb [4]. The amount of elemental sulfur in a typical environment is considered to be undetectable. PC technology is normally designed to some standard, such as the IEC environmental guidelines, which suggest a level of around 4 ppb for reduced sulfur gasses [5]. Measurements at facilities which have had sulfur based

Proceedings of the 32nd International Symposium for Testing and Failure AnalysisNovember 12-16, 2006, Renaissance Austin Hotel, Austin, Texas, USA

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corrosion issues suggest the level is in the parts per million range.

Examples of Field Failures

Example 1: Modeling Facility Some of the earliest examples of this phenomenon are from design facilities where Kaolin based clays are used to model products, such as automobiles, while they are being simultaneously designed on a computer based CAD program. This first example is from 1999. The purpose of building the models is to allow the artists and designers to fully experience the product of interest and to model physical characteristics such as airflow, which can be difficult to model computationally at the present time. Kaolin clays used to build the models are made of organic components, minerals and about 15-50% elemental sulfur. At one facility, the failure rate of computing systems was found to be significantly higher than the typical annualized failure rate. Inspection of the board at our failure analysis facility showed severe corrosion on the surface of the PC board. An image of this corrosion is shown below in figure 1.

Figure 1 – Corrosion found on an electroplated gold finish PC board after exposure to a high elemental sulfur environment. Note that this board was based on an electroplated gold over electroplated nickel technology, which is not used in current products due to spacing limitations on modern fine pitch components. In this board metallurgy, the plating left significant amounts of exposed copper. This copper then reacted with the elemental sulfur in the design environment, causing bridges between the leads of components, and system failure. An EDS (energy dispersive spectrogram) of the corrosion product is shown below.

Figure 2 – EDS OF copper sulfate corrosion product from board in figure 1. The corrosion product is clearly composed of a copper-sulfur compound. The literature suggests that the material is a mixture of copper sulfates, CuS and Cu2S, with the majority of the material being Cu2S [6]. Conductivity experiments verified that the corrosion product was conductive, probably due to small amounts of metal present in the crystalline matrix of the residue. This example is important, as it shows the propensity of copper to react with sulfur in the environment when not present with silver. Presumably, the nickel layer on the copper could also act in an electrochemical cell, but nickel has little or no propensity to react with the elemental sulfur in the environment. Example 2: Power Plant Another example of an electroplated gold board corroding in a high sulfur environment was experienced at a geothermal power plant in late 2001. Our computer systems are used in this environment to regulate the cooling towers in a process identical to that used in nuclear reactors. An extremely high failure rate was observed at this facility, and boards that were returned showed a high level of corrosion. See figure 3 below for an example of the corrosion.

Figure 3 - power plant board failure due to corrosion

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An investigation of the customer environment showed that several environmental controls that were originally designed into the building were not being used. For example, carbon filters present on the air handling system had never been replaced, and were no longer present in the system. The function of these filters was specifically to filter out elemental sulfur gas from the environment. Double doors (airlocks) had been disabled, and the outside air, which contained a very high level of sulfur gas, was allowed to enter the facility.

Outside the facility, 4 foot high mountains of elemental sulfur were present. One operator at the facility indicated that during hot summer months, the piles spontaneously burst into flame, which would lead to very high levels of sulfurous gasses in the local environment. As a solution, all engineering controls were fixed and implemented properly. In addition, all PC technology was placed in NEMA type enclosures with active carbon filtration. The failure rate dropped to within normal AFR levels after these engineering controls were put in place. Example 3: Modeling Facility II For several years, the failure rate of mainboards due to high sulfur environments appeared to tail off considerably. At this time, HASL finishes became popular. HASL finishes contain only tin-lead eutectic solder completely covering the copper of the mainboard, and were therefore much less susceptible to this corrosion mechanism. With the advent of the ROHS legislation in Europe (Restriction of Hazardous Substances) [7], HASL finishes on PC mainboards became obsolete, since they contain eutectic tin-lead solder, and lead is forbidden in this legislation. The new finish of choice appears to be immersion silver. This finish involves a very thin layer of silver over the copper of the printed circuit board. The silver is treated with organic chemicals in order to retard corrosion. With the introduction of immersion silver as the finish of choice for most PC based technology, the instances of sulfur based corrosion failures increased dramatically. One of the first failures was at a clay modeling center where PCs and clay models were being used to design transportation products. PC technology was failing within weeks of being installed in these environments. An example of the corrosion levels on returned boards is shown below in Figure 4. The corrosion level is significantly higher than what was observed with gold electroplated boards. Via corrosion was not observed on gold electroplated board. This board was in the customer environment for approximately 1.5 months.

Figure 4 – Example of immersion silver corrosion failure During the next several years, other examples of this failure in Kaolin clay based modeling facilities were observed. An investigation based on the location of the failures, customer usage models and the design facilities’ floor plans indicated that it is most likely the elemental sulfur gas that is the main corrosion agent, and not the elemental sulfur dust particles present in the environment. Again, the highest rates of failure were observed in facilities where the operators had deliberately sabotaged the engineering controls that were present to eliminate sulfurous gasses. As an example, a hood system over a clay firing oven had been disabled because it was “too noisy”. The door to the room had been propped open in order to provide easy access, and the door to the computing area beyond that had also been permanently wedged open. The average lifetime of modern PC technology in this environment was about 1 month. Example 4: Sulfur refining and distribution center A recent failure occurred at a major sulfur refining and distribution facility. This facility uses our PC technology for the control of the highly automated systems used to package and move thousands of tons of elemental sulfur generated from the petroleum industry. Within feet of the computing areas, thousands of metric tons of elemental sulfur lay piled, ready for shipment to agricultural regions around the globe. Prior to the use of immersion silver technology, this facility had not experienced PC failure rates above the normal AFR levels. After the installation of PC technology that contained immersion silver plating, failure levels rose dramatically. An example of the corrosion found on the boards from this site is shown below in figure 5.

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Figure 5 - corrosion at sulfur production facility.

Discussion of Failures

Immersion Silver Board Technology As was mentioned above, it is well established that copper corrodes very rapidly in the presence of reduced sulfur gasses such as elemental sulfur and hydrogen sulfide. This corrosion is accelerated by the presence of a more noble metal and atmospheric moisture due to the formation of a galvanic cell. The susceptibility of immersion silver boards to this type of contamination can be understood when one examines both the failure pattern and a cross section of the failed regions. A typical sulfur based corrosion failure on an immersion silver board is shown below in figure 6.

Figure 6 – via corrosion Notice that the majority of the corrosion appears to be coming from the via holes. An experimental investigation of the VIA holes on a typical silver immersion board shows that the silver coating does not typically extend all the way through the VIA barrel. See figure 7 for an example of this phenomenon.

Figure 7 – Cross section of a silver immersion via barrel showing exposed copper at one end of a blind VIA. In figure 7, the VIA is a blind VIA, meaning it is covered on one side by the solder mask. During the deposition of the silver, the VIA was not completely filled. This silver filling is also highly dependent on the aspect ratio of the VIAs, and vendors of this technology have indicated to this lab that many high aspect ratio VIAs would not be expected to be completely coated with silver throughout the VIA hole. In situations where the VIA hole is not completely coated, copper metal is vulnerable to atmospheric attack. Reference [1] indicates that this situation, which involves a more noble metal coating a metal that is prone to oxidation in the presence of an electrolyte (atmospheric water) is highly prone to galvanic corrosion. Indeed, this appears to be the case when one reviews the dozens of incidents and hundreds of failures in which PC technology is used in an environment high in reduced sulfur gasses. The majority of the failures show the heaviest corrosion mainly inside the VIA holes. The electrochemical theory behind this corrosion is well established [1]. One possible anode reaction is as follows: Cu � Cu +2 + 2e- The cathode reaction is as follows: S + 2e-

� S-2

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The electrolyte is provided by atmospheric water. The schematic diagram below shows what is occurring in the VIA barrel at a microscopic scale.

Figure 8 – Schematic of the electrochemical reaction occurring inside the VIA barrel at a microscopic scale

Mitigation

A solution to this failure mode in the customer environment can be difficult to implement. For example, the clays used in design facilities exhibit certain physical traits, such as viscosity, shrinkage and workability that are desirable in an artistic studio. The elemental sulfur present in these clays is a necessary ingredient to achieve these properties. Recently, Chavant Inc [8] has indicated that a sulfur free clay is available that appears to exhibit desirable qualities, and in several instances, this sulfur-free clay has been substituted for sulfur rich clay with positive results in the design environment. For environments that contain high levels of sulfur that cannot be removed, such as paper pulp plants, sulfur refining or power plants, engineering controls have proved effective. For example, NEMA Type 2 enclosures [9] have been used to completely eliminate the sulfur based damage on silver immersion boards. Conformal coatings have been discussed, but to our knowledge have not been implemented without extensive engineering controls due to the thermal considerations of coating a modern PC with a thermally non-conductive layer. One reference does

mention a light spray coating (vapor corrosion inhibitors) that could be effective in mitigating this corrosion mechanism [10]. Hewlett-Packard Corp. also sells a remote graphics package [11] that would allow customers to put sulfur sensitive electronics in a facility well away from sulfurous environments while using inexpensive ruggedized clients in the corrosive atmosphere.

Acknowledgments

Thanks to Chavant Inc, for many helpful discussions on clay chemistry and alternatives to sulfur based clays.

References

[1] Tullmin, M., “Tutorial Corrosion of Metallic Materials,” IEEE Trans Rel, Vol. 44, No. 2 (1995), pp. 271-278.

[2] Ibid. pp 274 [3] Graedel, T.E. “Corrosion Mechanisms for Silver Exposed

to the Atmosphere” J. Electrochem. Soc. Vol 139, No. 7, 1992 pp. 1963-1970

[4] Ibid. pp.1964 [5] IEC 721-3-3 “International Standard Classification of

Environmental Conditions Part 3” IEC 1994. pp 47 [6] Graedel, T.E. “Corrosion Mechanisms for Silver Exposed

to the Atmosphere” J. Electrochem. Soc. Vol 139, No. 7, 1992 pp.

[7] http://europa.eu.int/eur-lex/pri/en/oj/dat/2003/l_037/l_03720030213en00190023.pdf see also http://en.wikipedia.org/wiki/RoHS_Directive

[8] http://www.chavant.com/ in Farmingdale, NJ [9] http://www.nema.org/prod/be/enclosures/ [10] Chudnovsky, B.H. “Degradation of Power Contacts in

Industrial Atmosphere: Silver Corrosion and Whiskers” Proceeding of the Forty-Eight IEEE Holm converence on Electrical Contacts, 2002, p140-150.

[11] www.hp.com type “remote graphics” in the search box

Cathode (Silver metal

coating inside VIA)

Anode (copper metal

of the VIA wall)

Exposed copper

Electrolyte is

atmospheric water

e-

Cu � Cu +2 + 2e-

S + 2e- � S-2

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