App. Note 29 - Outdoor Environments
What are some of the environmental conditions that cause connector failure?
Typically, these are those found in coastal regions where a combination
of salt-spray and high winds combine with fog, rain, and large temperature
variations to produce an environment which is very hard on electrical connectors.
What are some of the reasons for connector failure under these conditions?
There are two areas of failure, physical failure of the connector,
and electrical failure of the connector. While there are some designs in
which only one or the other failures will occur, normally a combination
of failures will take place.
The physical failures generally result from the use of connectors which
are not designed for the environmental conditions encountered. Unfortunately
even with all the thousands of designs on the market today, it is often
very difficult to find a connector which will survive some severe environments,
and when one is found, the limited market for such a design usually results
in very high costs. The engineer specifying the connector has to face the
decision of first cost vs. replacement/maintenance costs and it is not
unusual for the initial cost of the connector coupled with an apparently
low replacement cost to account for the choice of connectors. Bu sometimes
environmental effect costs due to increasing pollution levels are underestimated
in the same way that long term maintenance costs due to rising labor costs
The most common physical failure results from corrosion eating
away the parts of the connector holding the male and female components
together. The result is that the connector simply disconnects. The cause
of the corrosion is usually a lack o resistance of the connector body material
and/or finish to the combination of salt/water and/or chemicals in the
environment. The corrosion can be hastened by the use of materials which
are incomparable galvanically, or by the use of the connector shell as
both a physical coupling and a current carrying element.
Assuming that the connector cannot be replaced with a type more suitable
for the climate, and that the connector is still functioning, the usual
solution is to clean all traces of salt/corrosion from the connector, and
re-assemble it, finishing by enclosing it in such a way as to prevent the
entry of the contaminants. This may consist o "buttering" over the connector
body with a high viscosity silicon grease and enclosing it in some sort
of protective sleeve which will prevent the grease from being eroded away.
A second vulnerable area is the connection between the wire or
cable and the connector itself. Many physical failures take place because
salt water has penetrated into that part of the connector where the wires
are crimped or soldered onto the pins. Quite apart from the electrical
effects of the corrosion products, these products frequently occupy much
more volume than the metal upon which they feed. Occasionally enough pressure
can be generated to rupture a shell that has already been weakened by corrosion.
Sometimes it is necessary to use a supplementary means of cable/wire
strain relief in order to minimize the effects of wind-induced vibrations
in the cable in causing a "grinding" action between the two halves of the
A third type of failure is the exposure of the connector to unusual
corrosive liquids or gasses not normally encountered in the environment
but which might be generated, from time to time, by other failures in the
operating plant. This can also extend to corrosion-inducing chemicals being
liberated from connector components subject to over-heating due to thermal
runaway of contact-wire junctions.
This latter also suggests that we must also consider the possibility
of other types of electric-current-induced physical failures. If the shell
is being used as a conductor is there going to be sufficient heat rise
caused by the combination of electrical resistance and current flow across
the mechanical connection to cause physical failure of any of the other
connector components, such as strain relief boots, seals, thermoplastic
insulation's, etc.? Could there be enough heat rise to cause failure of
the electrical wire/cable connection to the connector?
It must be remembered that there are two elements involved in the electrical
failure of a connector: the insulation, and the conductor/contacts/connections;
in other words, the non- conductive parts and the conductive parts.
Dealing for the moment only with the part of the connector that is meant
to disconnect and reconnect (and not with the parts that are attached electrically
or otherwise, to the cable/wire; any connector may be broken down into
a male-female or hermaphroditic component which is designed to mate with
an equivalent part, and thereby pass electricity, and the parts needed
to hold the former in alignment. The latter are usually insulators. In
many connectors carrying AC signals it is necessary to have the latter
parts dimensioned so that the electrical impedance of the connector
is the same as that of the wire otherwise there will be a discontinuity
and a reflection will occur in the transmission system.
Where the AC electrical impedance is not a consideration we have to
deal with losses in or on the electrical insulation. Excessive leakage
across the insulation will result in heating in high-power applications,
insulation breakdown in high-voltage applications, or signal leakage in
multiple-pin control circuits. None of these are acceptable, and the consequences
could be destruction of the connector and a fire hazard to consequential
damages due to failure of a process-control, alarm, or communications system.
Where AC electrical impedance is a factor, insulation leakage can cause
loss of signal strength and or unacceptable modification of the signal
caused by line reflections. A good example of the latter is ghosts or detail
blurring in cable TV.
Failure of what patent attorneys like to call the "connector means"
can range from a simple erratic connection which could be called intermittent,
to terminal failure of the contact pair.
The introduction of corrosion products into the gap between the connecting
pair o connecting means can also result in problems ranging from rectification
effects (most corrosion products can act like crude semiconductors) which
can produce strange modulation distortion of the signals or even
introduce spurious signals derived from the rectification of whatever RF
(conventional RF of even fast rise time) signals ma be present in the environment.
Generally this is characterized as excessive sensitive to "electronic smog".
As before, the corrosion products can completely break the contact means
or through its increased volume, lock-up the connector so that it cannot
If a mated contact were potted in a clear material, sawn along at right-angles
to, the connection plane polished, and examined under a microscope, it
would be seen that what we think of as smooth contact surfaces are really
almost mountainous, and that as a consequence, the contact area is far
from continuous. One of the benefit of gold plating in the days when gold
was much less expensive, derived from the fact that gold is soft and malleable.
Under the action of making the connection, the gold deformed, producing
a much larger total contact area. The plating, being thicker, was much
less likely to be porous, and so corrosion was also prevented. In addition,
the closely-mated surface prevented the intrusion of oxygen and other contaminants.
At the present cost of gold, where gold is used, it is applied in as
thin a "flash" a possible consistent with porosity. Even then special processes
are used to try t minimize porosity as will be evident upon reading almost
any connector manufacturer's brochures.
Where gold is not used on both surfaces, the question becomes one of
the compatibility of the connecting surfaces both with one another and
with the intrusion environment. Often when less noble metals than gold
are used in a contact pair and combined with sufficiently high contact
pressure, they perform with greater reliability than gold to gold, or gold
to ? at lower pressure. The key here is to have enough pressure to exclude
oxygen and other contaminants.
Stabilant 22 (or its isopropanol diluted form, Stabilant 22A)
used on a con tact needs only be present in a film thick enough to fill
the interstices (or gaps) between the contact surfaces. Because of its
switching ability, it will become conductive there without becoming conductive
between adjacent pains or causing leakage across insulating surfaces.
Now the conductivity of a new connector will not be substantially improved
by the Stabilant for the reason that there will probably be sufficient
contact already so that any added contact area aided by a material which
has a higher volume resistance than the contacting metal will be of minor
consequence. However the Stabilant's presence will help to exclude
oxygen and corrosive materials from the contacts, and its surfactant action
will keep existing contaminants in suspension.
The action of Stabilant on an aging or older contact is somewhat
different. Here the contact will not be as good; thus the conductivity
of the Stabilant (once switched on) will appreciably lower any contact
On high current applications, the lowered resistance well may be enough
to stop thermal runaway of the contact means, a situation where the heating
of a joint causes expansion which by stretching the clamping means beyond
their elastic limit results in a reduced contact pressure, increasing the
resistance of the contact area, and further increasing the heating. In
extreme cases this can literary cause a high current connector to explode.
Because of the "switching threshold effect", Stabilant
"switch" to a conductive state between adjacent contacts and its "off"
resistivity is high enough to prevent signal leakage.
In an environment where it is impossible to guarantee the exclusion
of contaminants, silicon grease can be used on the insulation to keep its
surface resistance as high as possible. The problem them becomes one of
preventing the contamination of the Stabilant by the Silicon, and
On low frequency connector applications it is sometimes possible to
take a thin sheet of soft silicon rubber (with a Durometer of about 30
to 40 Shore A) and make a washer which is perforated with holes for the
male contacts and which will fit inside the connector shell. The material
should be thick enough such that when the connector is screwed or clamped
together, the silicon will deform and form a water and gas tight seal between
the adjacent contacts.
This solution will work only where the connector design allows the silicon
washer to be compressed.
Another potential problem in connectors is the area where the wire and
or cable is connected to the contact means. Frequently the wire may be
of solder or tin plated copper, while the rear of the contact body could
be anything from gold-plate, through silver or tin plate, to an as-machined
alloy. The introduction of solder itself on a bare copper wire can provide
a potential problem of galvanic corrosion while some of the fluxes themselves
can cause problems if they wick up into stranded wires. Then too there
is the possibility that breakdown products from the cable jacket can cause
corrosion of the copper.
Multiple point crimps, made with properly designed tools such that there
is sufficient pressure on the conductors are often much more reliable than
soldered joints besides having greater consistency. The Stabilants can
be used to enhance the operation of such joints.
What procedures can be followed on complete connectors?
Once a connector is assembled, it may be necessary to protect it against
the environment by somewhat (in the eyes of the connector manufacturer)
less orthodox means. One of the simplest of these is the use of a heat
shrinkable polyolefin tubing with an internal low-molecular weight polyolefin
(or equivalent) materials that literally melts when the outer tube layer
is being shrunk. This provides a much more intimate seal when a length
is used that is long enough to stretch from the wire jacket over the connector
and on to the wire jacket. A problem with this material is that it looses
its elasticity and gets stiff at low temperatures, and if leaks will occur,
they will do so when its cold. We have seen this material used with rubber
splicing compound (as used on high voltage connections) where a single
layer of stretched splicing compound is used over the wire-connector-wire
area before the heat shrink tubing is used, The elasticity of the splicing
compound under compression is certainly better than that of any of the
heat shrinkable materials and the resultant "booted joint" is much less
messy to open up.
Another treatment is to use a thick long-fiber-equivalent silicone dielectric
grease such as vacuum grease applied in a layer over the connector and
wire. This can be used inside heat shrink tubing. Enough should be applied
so that the shrinking of the tubing extrudes the excess from the ends of
the tubing. A possible problem here is that the grease may also be forced
into the connector with degradation of the metal to metal contact. In an
attempt to resolve this latter problem I have seen the same treatment used
with an external wrapping of kitchen wrap being used to the point that
the silicon is covered. Ordinary vinyl tape can be used over this providing
the silicone has not got on the surface of the plastic.
Yet another technique is to use one of the low-temperature-melting tool-protection
coating material such as the buterates. These are an oil bearing plastic
material normally used in the tool-room to protect sharpened milling cutters
against damage. The material melts easily and connectors can be dipped
into the liquefied butyrate. It is not easy to apply in the field, but
it can be readily cut and is easy to strip away from the connector.
All of these solutions are, of course, designed to exclude the salt
and moisture from the connector and a choice of which treatment to use
will be based on the location of the connectors, the ease of application
of the treatment, and the life of the materials used. Consideration must
also be given to possible degradation of heat shrink materials themselves
by ultraviolet, ozone, or chemical contaminants.
In what forms is Stabilant available?
The Stabilants come in different forms. The basic material or
concentrate is called Stabilant 22, while the isopropanol diluted
form is designated Stabilant 22A. This is a 4:1 isopropanol dilution
(by volume) and is much easier to apply. When used a normal room temperatures
or higher, the isopropanol will evaporate after the application, leaving
a thin film of the concentrate in place. In some applications such as socketed
IC's it is not even necessary to unplug the IC to treat the connection.
The dilute form should be used for treating existing crimp-type joints
between multiple stranded wire and the contact.
Where isopropanol is not allowed, consider using Stabilant 22E
with a diluant of ethanol!
What are the names of some of the materials that can be used to exclude
Silicone greases are manufactured by several companies including Dow
Corning and General Electric Silicones Division. Besides the usual silicone
dielectric greases, which are available from many manufacturers there is
a much stiffer silicone grease called High-Vacuum grease which is somewhat
easier to handle as an external moisture barrier due to its higher viscosity.
If there is a restriction banning silicones from use we have been told
that Apiezion Grease type T and the more rubbery Apiezion- type N grease
(both generally used in laboratory glassware applications) have had some
Heat Shrinkable tubing is manufactured by such companies as Alpha. The
surface irradiated type with the soft inner core is their type FIT-300.
Their standard shrinkable polyolefin tubing is type FIT-221.