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Stabilant 22 Contact Enhancer Application Notes |
App. Note 21 - Gold Plate to Solder Alloy
Are there potential problems when gold-plated connectors are mated to solder-alloy-plated units?
If the connectors are designed so that there is a low contact to
contact pressure, and if the connector is being operated in a high
temperature, high humidity environment the mating of a gold-plated
contact to a solder-alloy plated contact is no recommended under any circumstance;
either with or without Stabilant 22/22A/22E. This combination, under
conditions of high humidity, temperature, and high frequency operation
seems to speed up the formation of a hard black deposit consisting of mixed
oxides of tin and lead. This coating is a poor conductor at best and will
cause contact problems.
The exception occurs where the connector is designed to maintain sufficient
pressure at the point of contact so as to exclude the entry of oxygen.
This condition is typically met on good-quality IC sockets. Providing the
contact pressure is in the range of 500 to 800 lb./in 2 this will usually
exclude oxygen from the junction and stop the problem under most circumstances;
nevertheless the use of Stabilant 22/22A/22E will enhance the contact's
reliability over a longer period of time.
A better solution is of course the use of gold-plated to gold-plated
contacts, or eve solder-alloy-plated to solder-alloy-plated contacts, either
of which may be made more reliable by Stabilant 22/22A/22E.
Can you give an example of this problem?
Yes, many of the first enhancement boards made to fit the Maclntoshâ„¢ (Appleâ„¢) computer used a "connector clip" which snapped down
over the 68000 microprocessor chip on the main board. The initial design
had gold-plated contacts which exerted only a low pressure over a fairly
large area of the corresponding solder-alloy plated IC contact. The design
of the "clip" together with the shielding effect of the board to which
it was attached ensured that a high-temperature environment was created
in the vicinity of the contacts. As the plastic housing for the "clip"
was molded of polycarbonate plastic, which will absorb small amounts of
moisture when cool, releasing it when heated up, It was not unusual to
find that the enhancement board became erratic after two to three weeks
of operation. Examination of the hard black film that formed at the junction
of the two metals revealed that it was substantiate composed of oxides
of tin and lead.
Redesign of the "clip" has significantly reduced the problem, nevertheless
a number of companies servicing this equipment have still found it prudent
to apply Stabilant to the contacts, and they report that this has
taken care of the marginal cases.
Where the older graphic accelerator boards have still been encountered,
the
application of Stabilant 22 alone increased the mean time between
failure from the two week period to about seven weeks; but this order of
magnitude of MTBF is not acceptable in the application, and additional
contact cleaning was necessary.
On the basis of solving similar problems with other types of connectors,
our recommendation was that the gold-plate be changed to solder-alloy-plate,
and that the contact pressure be increased either by the use of a thicker
contact stock or the use of a small dome protrusion on the contact.
We have found that a solder-alloy to solder alloy contact under these
circumstances, treated with Stabilant 22, will have a MTBF well
in excess of several years.
Are there any inhibitory chemicals which would retard the formation of
the oxides?
Yes there are, but the bulk of them are hardly the type of materials
that could be used where a good contact had to be maintained.
Can any generalizations be made about the use of dissimilar metals?
It is not good engineering practice to employ a design using contacts
where the surfaces are materials lying at grossly different potentials
on the galvanic scale. It is an open invitation to "Galvanic corrosion".
(An example of this would be a low contact pressure gold to aluminum
contact.) Even if the corrosion products are conductive they often exhibit
semi-conductor or rectifying properties and will often demodulate any RF
signals present on the lines as well as cause rise-time problems that can
"crash" the computer or peripheral using the connector. Contact pressures
should be high enough to reduce the entry of oxygen into the contact pairs.
While the use of Stabilants will reduce the problem by negating
most of the thin film rectification problems, it is still better practice
to employ connectors where the contacts are of identical materials or are
close in their galvanic potential.
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Revision 3
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Stabilants are a product of Dayton Wright research & development and are made in Canada
NSCM/Cage Code - 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 2003 - 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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All prices in US dollars.
Sales tax will be added to all purchases delivered in California.
Prices, products, and specifications subject to change without notice.
All trademarks and registered trademarks are owned by their respective companies.
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