Chemical Engineering January 2017 - 42

was performed.
Mill staff recognized that the resistance
of a choke installed on the
base unit of the I/O station to eliminate
signal noise was increasing,
and on failed units was a high or
open circuit. Ironically enough, this
choke was only installed to meet
other E.U. legislation pertaining to
radio-frequency interference
(RFI)
emissions. The critical issue identified
pertaining to this choke was that
it was an electronic chip design with
a very isolated thin layer of silver in
compliance with the lead-free regulations
of RoHS. Corrosion of this
silver layer destroyed the choke's
connection to the printed circuit,
resulting in the high resistance.
Consultation with the manufacturer
confirmed the presence of silver on
the chokes.
The mill was already in the process
of installing air-cleaning equipment to
remove corrosive gases from the air
and reduce the rates of copper corrosion.
However, the extremely short
time to failure for these silver-containing
components accelerated this
effort in order to prevent additional
failures. The staff was confident that
these failures were purely the results
of RoHS compliance and the use of
silver. The lack of information and
warnings by the supplier regarding
potential issues related to their RoHS
compliance programs is troubling,
considering that even when presented
with this evidence, they still
concluded these failures were due to
" installation errors. "
Problems exist in many locations.
CCC data from several additional
mills (Table 2) show that many locations
exhibiting an ISA Class G1
environment for copper, including
air-handling units (AHUs) and motor
control centers (MCC), have corresponding
silver corrosion rates
that would now cause the environment
to be classified as G2, G3 or
even GX. This would indicate serious
cause for concern for any electronic
equipment with an ImmAg
surface finish specifically or any
silver or silver-plated components
in general.
Another contributing factor to
concerns over the increased use
42
of silver in industrial applications is
that even with tightened control requirements
for other environmental
parameters (including temperature
and humidity) and the positive effect
this has on the rate of copper corrosion,
silver can still exhibit high rates
of corrosion, even in well-controlled
environments. Closer examination of
CCC data shows that in locations
reported as ISA Class G1 for copper
corrosion, the corresponding silver
corrosion rate can be up to 10 times
higher [4]. Furthermore, every CCC
analyzed shows evidence of sulfur
contamination (as Ag2S). On average,
the amount of silver corrosion
measured is double that of the copper
corrosion reported.
Contaminant gases containing
sulfur, such as SO2 and H2S, are
the most common gases that cause
hardware corrosion in paper mills,
petroleum refineries and chemical
plants, and corrosion control
is acknowledged as a requirement
to assure electrical and electronic
equipment reliability. One example
of component failure is from sulfur
gases entering a component package
and attacking the silver, resulting
in the formation of Ag2S. The
mechanical pressure created by the
Ag2S formation inside the package
damaged its mechanical integrity
and caused the device to fail. This
and other failure mechanisms are
becoming common
occurrences
when using RoHS-compliant electronic
equipment and components
produced using the ImmAg process,
and to a lesser degree, the
electroless nickel-immersion
gold
(ENIG) process.
To maintain a high level of equipment
dependability and availability,
it should be understood that a
control room is a dynamic environment
where many maintenance
operations, infrastructure upgrades
and equipment change activities
occur on a regular basis. Airborne
contaminants that are harmful to
sensitive electronic devices can be
introduced into the operating environment
in many ways in addition
to the ventilation system. For
instance, chlorine can be emitted
from PVC insulation on wires and
cables if temperatures inside the
DCS cabinets get too high. However,
it is still the outdoor ambient
air used for cooling and pressurization
that remains the primary
source of corrosive contaminants,
and this air should be cleaned before
its introduction into the controlroom
environment.
With the changes to process control
equipment due to the RoHS
directives, plant managers and operators
should include an environmental
contamination monitoring
and control section as part of an
overall site plan, as well as plans
for risk management, mitigation and
site improvements.
Looking forward
RoHS regulations, along with the
continuing reductions in circuitboard
feature sizes and miniaturization of
components necessary to improve
hardware performance, makes today's
electronic hardware more
prone to attack by airborne contaminants.
Increases in corrosion-related
electronic hardware failures have led
to new electronic equipment warranties
that require environmental
corrosion (reactivity) monitoring and
control of airborne contamination
where necessary.
Manufacturers have to maintain
the reliability of their equipment, and
therefore the need to control airborne
contaminants and to specify
their acceptable limits are now considered
to be critical to the continued
reliable operation of processcontrol
equipment. ANSI/ISA Standard
71.04-2013 now includes silver
corrosion monitoring as a requirement
in determining environmental
severity levels. Most manufacturers
of process-control equipment currently
reference this standard in their
site planning and preparation guidelines,
as well as their terms and conditions
for warranty compliance. The
addition of silver corrosion rates as
a required metric serves to bridge
the gap between ambient environmental
conditions and the reliability
of RoHS-compliant (lead-free)
electronic equipment.
Ongoing research will serve to
further refine Standard 71.04 both
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