Why
do silicone films cause intermittence in contacts?
While
the water-repellent characteristic of silicones make
them potentially beneficial for use on connector assemblies exposed to
excessive moisture, there are other characteristics which, under certain
circumstances, can cause problems. Silicone films can, when the proper
conditions exist, combine with metal ions to form restive films such as sodium
silicate. As some potentially reactive silicones can be polymerized by metal
such as lead or sodium, the potential for the formation of very thin, high
resistance films can exist.
Where
does this problem exist?
The
most obvious location is where silicone dielectric greases are used to
waterproof connectors, but it can also exist where silicone mold-releases were
employed in the manufacture of either molded-rubber or molded-plastic parts
used in the connector assembly. While the service departments of some companies
are aware of the potential problem, some well-meaning customers may cause the
problem by employing cleaning-sprays that contain silicones. Several of these
were very popular as TV-tune cleaners and are still available from electronic
parts houses.
There
are also a number of pieces of equipment which use silicone oils as part of
their design. The most notable of these are encountered in the copier industry
where silicone-bearing oils are often used in the ink-fusion section of the
equipment in order to ensure that the fusion rollers will not stick to the
melted ink particles. As the surface energy of the melted ink is often quite
low, there is a need for a very low surface-energy on the heated roller in
contact with the paper. Under these circumstances it is not unusual to
encounter contact problems in the vicinity of the ink fusion section. In
addition, the cooling airflow may carry the volatile silicone to other
connectors downstream of the fusion unit. There is even a potential problem for
connectors near the paper path that follows the fuser. Although the amount of
oil that is actually carried by the paper itself is probably negligible, there
is usually some small amount of airflow caused by the motion of the paper and
it may well b this that carries the silicones to the
paper-exit areas of the equipment.
Because
the deposition of silicones on the contacts in copiers takes place over a long
period of time, there is a greater potential for those films to react to other
atmospheric contaminants than would occur in the case of contacts where a silicone oil was used as a moisture barrier.
This
phenomenon can be divided into two classes, those where the material acting to
catalyze the silicone is found in the contact itself, and those where the
catalytic material is external to the contact. In the former case, the metal
connector pins, especially when plantings such as solder-alloys, will be the
source of the catalyst; while in the latter, atmospheric metal ions such as the
salt-laden air found in coastal areas can initiate the problem.
Can
these problems be prevented?
Yes,
most cases can be prevented. The ideal solution is the elimination of the
silicones from the connector and/or the equipment. For obvious reasons the
latter may not be possible. In some cases it may be feasible to enclose a
wiring-harness-mounted connector-pair in a long-sleeve of plastic tubing in
order to minimize the exposure of the connector to the silicone-bearing
airflow. While this might not prevent the problem from occurring in the
long-term it might delay the time when a critical film thickness was reached.
It may also be possible to re- position the connector, or even to alter the airflow
so that the connector lies in a stream of clean air.
The
ideal situation is where the problem can be anticipated in the equipment design
stage.
What
can Stabilants do to stop these problems?
If
the Stabilant can be applied during
manufacture of the equipment, the material will act as a barrier to deposition
of the silicone.
Where
the condition already exists, liberal application of the isopropyl alcohol
diluted Stabilant 22A can usually purge
the collector of most of the silicones, albeit at the cost of wasting Stabilant. A liberal washing of the convector, first with
kerosene and isopropanol, ad then with isopropanol
alone, using several alternations of the (fresh) solvent pair, should help.
But
if the silicone has already reacted to form, say, a film of sodium silicate, it
may be necessary to use abrasion to remove the film. Where doubt as to the
actual state of the silicone contamination exists, it is probably worth while to replace the connector and have the surface
of the contact analyzed in a laboratory equipped for micro-analysis. In such
cases, the laboratory may well be able to advise the service department as to
the most efficient and/or most environmentally acceptable solvent to use in the
removal of the contaminant film.
In
any event, the Stabilants could prevent the problem
from recurring.
Revision
4
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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