Thanks Doug! Another knowledge nugget that can be filed away.
Phil Nutting
-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Thursday, February 14, 2002 9:28 AM
To: [log in to unmask]
Subject: Re: [TN] Adhesive Recommendations? Silicon??
Peter Duncan stirs the pot with:
I'm still trying to reconcile all the continued use of silicon adhesives
(as recommended by several learned members of this forum) with all the
learned discussion about how terrible it is to put silicon anywhere near
boards because of contamination issues. Is anyone prepared to put his/her
neck on the line and try for a definitive statement on this issue - is
silicon on boards good, OK or bad? If it's bad, why are so many still doing
it? Is it Force Majeur, are there no alternatives, or is there a mysterious
land out there where the use of silicon is not detrimental and the benefits
are worth having? Which land is this?
**Doug Pauls, never one to turn down a challenge, responds:
So Peter, you want someone to make a definitive statement that any single
material is absolutely good, or absolutely bad, for all applications for
all products for all end use environments. I don't think that could be
said of any material. Every material has engineering tradeoffs. Take gold
for example. Excellent for RF. Plates well. Nearly corrosion proof.
BUT, a soft metal, can cause solder joint embrittlement and is damned
expensive.
So look at silicones. From what has been said in recent days, do we say
they should never be used in electronics? I'm sure that the good folks at
Dow Chemical and GE would beg to differ. Like any material, you have to be
aware of the material properties and weigh the pros and cons. Do the
advantages of a material outweigh the disadvantages? Is the benefit worth
the pain?
Rockwell has been using silicone adhesives for several years and we build
very high reliability stuff. When our stuff fails, airplanes fly into the
sides of mountains, so we take reliability VERY seriously. If it is a
plane, it has some of our equipment on it somewhere and since I fly alot, I
REALLY take the issue to heart.
We are trying to get away from using silicone adhesives for two reasons:
(1) most of our customers don't want silicone adhesives on the product
because they outgas. The outgas products can tend to cross-contaminate
other hardware, especially switches and relays which can develop open
circuits. So our customers, especially the Europeans, want us to minimize
or eliminate silicones soon. (2) the stuff causes problems in our
manufacturing flow. The silicones can cross contaminate wash solutions, it
takes a long time to cure them fully (24+hours), especially in thick cross
sections (3-5 days), and acrylic conformal coat does not stick to
silicones. That is why we have been looking at thermally cured epoxies and
urethanes lately.
You ask if there are no alternatives and why are we still using this stuff.
Rockwell Collins makes thousands of different products, all with a drawing.
To change from a silicone material to anything else is a HUGE undertaking.
Our products were qualified using silicones. Many of the products were
designed around the properties of the silicones. To change from a silicone
to an epoxy or urethane means that we would have to change thousands of
drawings. A single process initiative, or some form of blanket change, is
not really an option because each product needs to be reviewed by the
design engineer (some of whom are no longer living) to see if the new
material properties change the intended function or reliability. That's a
few thousand reviews and that ain't cheap. So, to answer your question
about why we use them, it is too painful to change, or we have to be able
to show that the reliability and cost savings are sufficient to go through
that change. I have yet to see a large company with diverse product lines
and high reliability requirements implement such changes without a major
investment.
But, if you are making products where outgassing is not a concern, or you
don't conformal coat, the silicones may be just fine. You don't need an
oven to cure them and they get to at least a tack free state in under an
hour and you can keep processing them while they cure.
There are ALWAYS alternative materials. The organic material field is very
diverse. You just have to make the determination of the engineering
tradeoffs with each material and then determine if the benefits of change
outweigh the pain of change.
As to the land where silicones have only advantages and no disadvantages, I
suggest either Oz, second star on the right and straight on till morning,
or the marketing department of a large company in Midland, Michigan.
Doug Pauls
Rockwell Collins
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