Doug,
Well stated.We use DC 3145 for all legacy and momentum products for the
exact reasons you described:tons of test data;Qual data; and years of
trouble free flight use.We have since developed an epoxy adhesive,which is
more in line with today's DFM practices.We also had some failures with the
RTV at higher G levels of vibration.
We had to do a lot of testing and Qual work,because a lot of the bigger
adhesive manufacturers make more money selling to the commodity market and
don't want to bo bothered meeting and testing to our more stringent
requirements.
Properly chosen RTV's meet those requirements and are reworkable,but impose
certain restrictions and complications to the manufacturing process.Some
epoxies meet these requirements, but are potentially less reworkable.
It is imperative that you understand the enviroment of use and the
compatiability of the materials and processes.Is the assembly to be
Conformal Coated?Is the material amine free and is the by-products of the
cure process a potential source of corrosion,contamination or an electrical
migration initiator?

PS:I owe you two Mountain Dews now.
Dewey

> -----Original Message-----
> From: [log in to unmask] [SMTP:[log in to unmask]]
> Sent: Thursday, February 14, 2002 7:28 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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