TECHNET Archives

August 2013

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
Inge Hernefjord <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Inge Hernefjord <[log in to unmask]>
Date:
Mon, 12 Aug 2013 15:45:19 -0700
Content-Type:
text/plain
Parts/Attachments:
text/plain (108 lines)
The fear of silicon in electronics has its roots in a Swedish Defence
report 50-60 years ago. At that time a military component test lab
performed tests of small signal relays. Thousands of relays were tested
with numbers of conditions.  A few relays had increasing contact resistance
after thousands of operational elapsing time. Analysis gave that the
contact had small spots of some silicon product, which was never decided in
detail. The team made an anticipation, that the silicon somehow had
'migrated' from a silicon grease that was used for some reason. The grease
was not on the PWBs, but had been creeping all over the equipment, and had
finally found its way on inside of the relays. It was called 'silicon
migration'. Relay failures were the most feared in the military boxes at
that time. Note that the relays were low contact force constructions, so
called 'tongue' relays. Power relays were not involved in the risk group.
Personally, I have analyzed hundreds of reported relays during decades, and
have never seen contact issues caused by silicon. We use silicon products
of all sorts since the 60's and have no silicon issues. I talk about
millions of boards. The academic high priests reported horrible facts
,that  increased the fear even more, e.g. a paper that concluded, that just
a few drops of silicon oil could spread over an area of a football arena in
short time! I have demonstrated my scepticism lots of times, and assured
them that the risks are pretty exaggerated.

Inge



The use of silicones for electric and electronics protection is as old as
some military application from the late 1940's. Since then the silicones
have become one of the most relevant alternatives for electronics
protection for reliable applications, and its share continues to growth due
the increase demand of current applications and the limits of organic
alternatives.

The term "Silicone Migration" is also rather old, the first articles
talking on the subject date on early 1950's. Why then, if this phenomenon
is known and recognized for over half a century the use of silicones is
well accepted, recognized and in some cases proven indispensable?

The "Silicone Migration" term is one almost every electrical engineer has
heard, yet very few actually know its true meaning. Often this term is used
to simply restrict or discourage the use of silicones without further
explanation and most of the times also unjustified/unnecessarily.

One of the reasons is that this term is equally used to describe any of the
three contamination mechanism for silicones (which by the way are the same
for epoxy, acrylics and urethanes yet then is simply called
"contamination").

The three contamination mechanisms are: Transfer by contact (something
touching a wet silicone and then touching any other surface), Transfer by
atomization (by fine spraying silicone that can be then move airborne and
collect in any surface) and Transfer by Low Molecular weight species than
can volatilize and condensate at normal operation conditions.

The first two account for most of the surface contamination experiences,
being the last one the least observed and the most difficult to generate
since it requires strictly several conditions to be present to occur.

One of the easiest way to prevent the volatile silicone migration is simple
to select a Controlled Volatiles product. Since one of the elements that
must be present is the SVCC (Silicone Volatile Condensable Compounds)
selecting a CV product eliminates the problem.

Krayden offers CV products of the Dow Corning product line that include
Adhesives, Conformal Coatings, Encapsulants and Thermally Conductive
products. These CV products don't have the hefty price tag of space grade
materials and are the right answer against this potential "Silicone
migration."

The Volatile Migration is relevant only if all the conditions are present.
Not all the volatiles are relevant, only the ones that can condensate at
operational conditions. Also it is required to have the electronics in a
hermetically confined volume for this to be relevant; if the volatiles can
escape their high diffusivity will prevent them to condensate again in
relevant amounts.

Also an intermittent electrical contact is required for the migration to be
relevant like a switch, relay or a motor; an already plugged connector is
not an issue if it is not in and out constantly.

Finally the volatiles will show it's presence in a certain range of voltage
and current intensity. The SVCC are fairly inert and will not dissolve,
oxidize or react with any other element on the board.

There are means to calculate the amount of volatiles present in an
application if all the data is available. Krayden can help via Dow Corning
to get this information if required.

One important thing to clarify that the risk of the silicone volatile
migration has to be estimated on its proper terms, one average a person
carry far more silicones volatiles due the use of antiperspirants, hair
products, clothes, make up that an average coated PCB!

Silicone migration is a far misunderstood and feared topic, yet if you
allow us we can help you take the right approach in your material selection
for your electronics applications.
[image: image] *Wayne Wagner*<http://www.circuitnet.com/experts/panel_331.shtml>
*President*
*Krayden Inc.* <http://www.ipcoutlook.org/directory/49103.shtml>
Wayne Wagner has over 25 years in the conformal coating industry and is the
president of Krayden Inc., a leading distributor of engineered materials.


______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________

ATOM RSS1 RSS2