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.
PHYSICAL
FAILURES:
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 are underestimated.
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 connector.
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 be
disconnected.
ELECTRICAL
FAILURES:
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) when 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 resistance.
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 will not
"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 vice versa.
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 water?
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 success.
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.
Revision
2
Stabilants
are a product of Dayton Wright research & development and are made in
Canada
NATO Supply Code 38948
15 mL of S22A has NATO
Part # 5999-21-900-6937
The
Stabilants are patented in Canada - 1987; US Patent number 4696832. World-wide
patents pending. Because the patents cover contacts treated with the material,
a Point-of-sale License is granted with each sale of the material.
MATERIAL
SAFETY DATA SHEETS ARE AVAILABLE ON REQUEST.
Stabilant,
Stabilant 22, and product type variations thereof are Trade Marks of D.W.
Electrochemicals Ltd.
© Copyright
1987, '88. '89, '90 - D.W. Electrochemicals Ltd. This note may be reproduced or
copied, provided its content is not altered. The term "contact
enhancer", © 1983 Wright Electroacoustics.
NOTICE: This Application
Note is based on customer-supplied information, and D.W. Electrochemicals is
publishing it for information purposes only. In the event of a conflict between
the instructions supplied by the manufacturer of the equipment on which the
Stabilant material was used, and the service procedure employed by our
customer, we recommend that the manufacturer be contacted to make sure that
warranties will not be voided by the procedures.
While to our
knowledge the information is accurate, prospective users of the material should
determine the suitability of the Stabilant materials for their application by
running their own tests. Neither D.W. Electrochemicals Ltd., their
distributors, or their dealers assume any responsibility or liability for
damages to equipment and/or any consequent damages, howsoever caused, based on
the use of this information.
Stabilant,
Stabilant 22, and product type variations thereof are Trade Marks of D.W
Electrochemicals Ltd.
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