The "reaction" between the silver particles in the adhesive and the
tin in the solder is accelerated by temperature so the 150C cure temp
required by the epoxy to cure it is not helpful, however with a normal
conductive the joint will fail anyway. I wouldn't like to quantify the
exact difference the cure regime makes but the orders of magnitude are
going to be a few 10s of hours off a life of 1000s.
Turning to Ag migration so far as I know this has not been an issue.
Certainly if you look at the main historic uses of Ag conductives in
electronics - die attach and component attach in thick film for
example - the essential conditions which would lead to migration
[moisture and volts] would cause the bond to fail anyway.
These historic applications are conventionally sealed or encapsulated
for device protection so the conditions don't really arise. In an
unprotected bond you would additionally be looking at dissimilar
metals in the presence of moisture and volts and any ionics in the
adhesive or immediate environment being carried to the bondline as
well. So yes some encapsulation or coating would be beneficial in that
sense. The basic incompatibility would not be
addressed whether encapsulated or not.
It would be useful to hear from a manufacturer and/or user of solder
replacement adhesives, to get an additional perspective on this.
Mike
----- Original Message -----
From: "Creswick" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, January 25, 2001 1:29 AM
Subject: Re: [TN] silver epoxy & solder coated parts
Jim,
Like Gregg Klawson we have never found conductive adhesive attachment
to
solder coated devices to be electrically stable, but all of this work
was
with cure temepatures in the 150-165°C range (ceramic-like
substrates).
I can make myself believe that lower cure temperatures would be better
by
getting away from the plastic range of the solder.
I know there are multiple adhesive manufactures out there selling
materials
for these applications, but have not been called upon to use them.
Question back at you all - with the readily exposed silver, seemingly
just
waiting to migrate, doesn't anyone worry about this? Is conformal
coating
percieved as the 'fix' to migration??
I would seem that board flexure during subsequent processing, or use,
would
be catastrophic for polymer attachments.
I would appreciate enlightenment by the forum.
Steven Creswick
CTS RF Integrated Modules
At 03:34 PM 1/23/01 -0500, you wrote:
>Hello, Technet...
>
>Here is an unusual situation. We have to assemble a surface mount
assembly
>using silver epoxy. Some of the components will be solder coated. I
know,
>don't bother asking... this is the way it has to be. The good thing
is that
>this assembly is a prototype for bench-top laboratory use only.
There is no
>harsh environment and needs to operate 1, maybe 2, years. The
question is
>does anyone foresee any problems? What happens when one mixes toe
two
>technologies?
>
>Thanks,
>Jim Marsico
>EDO Electronic Systems Group
>[log in to unmask] <mailto:[log in to unmask]>
>
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