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November 1999

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Subject:
From:
David Douthit <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 4 Nov 1999 18:38:34 -0700
Content-Type:
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Hi Technetters,

The following is an abstract for a paper which was presented Oct. 19th,
1999 at the
ElectrChemical Soceity international meeting in Honolulu. The web site
is; "electrochem.org" The paper number is 650. It has not yet been
released for publication but the info in the abstract is pretty good by
itself.

D. A. Douthit
[log in to unmask]

-----------------------------
Reliability of Electronics in Harsh Environments:
Electrical Leakage and Corrosion Caused By
Hygroscopic Pollutant Particles

R. B. Comizzoli, R. P. Frankenthal, G. A. Peins,
L.A. Psota-Kelty, D. J. Siconolfi, J. D. Sinclair
Bell Laboratories, Lucent Technologies
Murray Hill, NJ 07974, USA

In many parts of the world, soft coal is the primary energy
source. Combustion of soft coal produces sulfur dioxide,
which oxidizes in the atmosphere to form sulfuric acid and
other oxidized sulfur-containing molecules. When
electronic equipment and devices are contaminated with
particles derived from these gases, subsequent exposure to
atmospheric moisture can cause failure due to electrical
leakage and arcing in the presence of electric fields.

The most important components of the environment with
respect to degradation of electronic devices are particles
and water vapor. Most of the mass of particulate matter in
the atmosphere exhibits a bimodal distribution. Particles
2.5 - 15 m are largely derived from natural materials and
are usually called coarse particles, while particles 0.1 - 2.5
m, usually called fine particles, are largely derived from
anthropogenic sources. The combined mass of fine and
coarse particles in the atmosphere is frequently referred to
as total suspended particulate (TSP). In the U.S. typical
TSP levels are 30-40 g/m 3 outdoors and 5 g/m 3 indoors.
Typical sulfate levels are 4-6 g/m 3 outdoors and 0.6-0.8
g/m 3 indoors. For the new work reported here, TSP
levels in excess of 200 g/m 3 have frequently been
measured outdoors in many parts of Asia. The sulfate
portion of this mass is frequently in excess of 15 g/m 3 .
Indoor TSP concentrations are frequently in excess of 30
g/m 3 .

Coarse particles form predominantly by abrasion
processes, e.g., construction activity or the action of wind
on soils. The main source of fine particles in populated
areas is fossil fuel combustion, though volcanic and
geological activity can also contribute. Due to their
different origin, coarse particles tend to have a lower
fraction of water-soluble ionic components (5 - 20%) than
fine particles (25 - 50%) , excluding H + , OH - , and CO3 = .
With any particle, the higher the fraction of water-soluble
ions, the higher is the corrosivity. As a result, fine particles
are more corrosive than coarse particles. Key to the
corrosive behavior of fine particles is their critical relative
humidity (CRH). CRHs at 24 degress C of some of the major
ionic compounds found in fine particles in most urban
environments are: NH4HSO4, 40%; NH4NO3, 65%; NaCl,
75%; (NH4)2SO4, 81%. The electrolyte films that form on
surfaces contaminated with fine particles when they absorb
moisture are corrosive to many metals and can lead to
electrolytic corrosion or leakage currents when an electric
field exists between conductors.

The indoor concentrations of fine particles in buildings
with central air handling systems can range from 20 - 50%
of the outdoor concentrations, depending on: (1) the
efficiency of air filtration systems, including both building
systems and equipment filters; (2) the rate at which
outdoor air is brought into the building; and (3) indoor
sources, e.g., smoking or human activity. Concentrations
within homes, sheds, and utility huts can range from 20 to
75% of the outdoor concentration due to: (1) the
penetrability of particles through air-leakage pathways or
open doors or windows; (2) the absence of efficient
particle air filtration; and (3) indoor sources like those
mentioned above, as well as aerosols derived from liquid
processes.

On circuit boards, the absorption of moisture by deposited
particles or other ionic contaminants may result in the
formation of an electrolyte around electrical leads.
Bridging of leads can result in leakage currents. The
current may lead to soft errors or cross talk. If the
electrolyte extends over a defect in the covercoat and the
defect sits above a conductor operating at a different
voltage from that on the lead, arcing can occur between the
lead and the conductor. Typically, the leakage current
increases approximately exponentially with RH.
Covercoats can be effective in preventing hazardous
leakage currents. However, device leads and electrical
contacts are not usually covered by covercoats.

The maintenance of the RH in electronic equipment locations
below 60% minimizes the risk of hazardous leakage
currents. For conductive contaminants (e.g., graphitic
carbon or metallic dust) high leakage currents may occur
even at low RH. The electrical leakage of dust from the
more polluted environments of the world tends to be
significantly higher at a given RH than dust from the U.S.
In addition to electrical leakage and arcing, dust and other
ionic contaminants can lead to direct corrosion of metal
conductors and ultimately to open circuits. A water-soluble
corrosion product may also migrate to the negative
conductor, where it can be reduced to form a metallic
deposit that grows toward the positive conductor,
eventually forming a short circuit. In some situations, an
arc may occur. Severe arcing can pyrolyze circuit boards
to form conductive carbon bridges. In the absence of
appropriate limitation of current, a fire could result.
Again, the combination of contamination, moisture, and
bias are required. To minimize leakage currents and
corrosion in field operations, it is essential that: (1)
covercoats are defect free, which can be evaluated by
appropriate dust exposure tests; (2) design rules and
specifications with respect to bond pad and line spacings
are strictly followed; and (3) the environment is
maintained within design specifications.

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