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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 Analysis
November 12-16, 2006, Renaissance Austin Hotel, Austin, Texas, USA
Copyright© 2006 ASM International® 469
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
470
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.
471
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
472
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
473
