Chemical Engineering January 2017 - 36

Copper corrosion rate distribution guidelines
100,000
Reported failures
Unacceptable
range
10,000
5,000
Marginal
range
1,000
830
Corrosion creep across insulation
and PCB traces, 4-6 months
Wire wrap failures, 2-3 months
IC socket failures, 6-8 months
Edge card intermittents, 6-8 months
Dielectric failures, 2-3 years
Edge card intermittents, 1-3 years
Edge card failures, 4-5 years
The RoHS 2 directive (or RoHS recast)
is an evolution of the original directive [2].
It became law in the E.U. in July 2011 and
took effect in January 2013. RoHS 2 addresses
the same substances as the original
directive, while the new scope has been
extended to all electrical and electronic
equipment, including monitoring and control
instruments, with full implementation
required by July 2017. However, even with
this extension, experience has shown that
printed circuitboards (PCBs) made using
lead-free materials can be more susceptible
to corrosion in high sulfur environments,
which typically have been associated with
industrial applications.
100
Recommended
range
10
99.99
99
90
FIGURE 2. These corrosion
guidelines were developed by
various collaborating companies
to protect control equipment
against corrosion [3]
50
20
% of composite sites
sation and sea salt mist
* Solids, including grit, sand and dust
* Gases, including active sulfur, sulfur oxides,
gaseous chlorine, nitrogen oxides,
hydrogen fluoride, ammonia, ozone and
strong oxidants
One complicating factor that has arisen
in the last several years has been the passage
of the European Union (E.U.) Restriction
of Hazardous Substances (RoHS)
directive regarding the use of certain hazardous
substances in electrical and electronic
equipment [1]. The RoHS directive
restricted the use of lead (Pb), mercury
(Hg), cadmium (Cd), hexavalent chromium
(Cr6+), polybrominated biphenyls (PBB) and
polybrominated diphenyl ether (PBDE) in
the manufacturing of certain electrical and
electronic equipment sold in the E.U. Although
a misnomer, these regulations are
most commonly referred to as " lead-free "
manufacturing, due to the fact that essentially
all electrical and electronic equipment
to this point contained lead. This was the
first of many regulations passed to control
electronic waste by eliminating lead
and other hazardous substances in electronic
products. In its first iteration, RoHS
provided exemptions for several types of
critical electrical devices, including process
measurement and control equipment, due
to lingering reliability concerns when these
regulations took effect in 2006.
36
5
1.0
0.01
Causes of corrosion
Corrosion of metals is actually a chemical
reaction caused primarily by attack of
gaseous contaminants, and is accelerated
by heat and moisture. Rapid shifts in either
temperature or humidity cause small portions
of circuits to fall below the dewpoint
temperature, thereby facilitating condensation
of contaminants. Relative humidity
(RH) above 50% accelerates corrosion by
forming conductive solutions on a small
scale on electronic components. Microscopic
pools of condensation then absorb
contaminant gases, which become electrolytes
in the pools, where crystal growth
and electroplating occur. Above 80% RH,
electronic corrosive damage will occur, regardless
of the levels of contamination.
In the context of electronic equipment,
corrosion is defined as the deterioration of
a base metal resulting from a reaction with
its environment. More specifically, corrosive
gases and water vapor coming into contact
with a base metal result in the buildup
of various chemical reaction products. As
the chemical reactions continue, these corrosion
products can form insulating layers
on circuits, which can lead to thermal failure
or short-circuits. Pitting and metal loss can
also occur.
Corrosive gases. Specifically, we are only
concerned with the three types of gases
that are the prime culprits in the corrosion
of electronics: acidic gases, such as
hydrogen sulfide, chlorine and hydrogen
fluoride, as well as oxides of sulfur and nitrogen;
caustic gases, such as ammonia;
and oxidizing gases, such as ozone. Of the
gases that can cause corrosion, the acidic
gases are typically the most harmful. For
instance, it takes only 10 parts per billion
(ppb) of chlorine to inflict the same amount
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JANUARY 2017
Copper corrosion rate (Angstroms/yr)
Composite process control
Composite indoor business
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Chemical Engineering January 2017

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