Don't know if this is making any good to you, Jim, but we found that it
wasn't the oxides themselves that caused problem, instead the degradation of
silver particles was just one physical change. There was also developments
of weaker zones in the whole, so any mechanical change or temp change caused
galvanic interrupts. Thus the DC res became a roller coaster. Another source
of trouble was if there was aluminium present, a complicated chain of events
then inhibited the katalyst to work good on inside of the epoxy. We did not
spend time to dig in this, just bought other component with more suitable
finish. Again, lots written e.g. Sean X. Wu, Kai S. Hu, Chao-Pin Yeh
'Contact Reliability Modelling and Material Behaviour of Conductive
Adhesives under Thermomechanical Loads' and James Morris at Binghamton
'Conductive Mechanisms and Microstructure Develoment in Isotropic,
Electrically Conductive Adhesives'. James seem to be kind of guru here.
Of special interest here is why there are sometimes DC res change AND bad
joints in extremly small adhesive joints, talking 50-100 microns diameter or
so. Seems as such small dots don't exhibit same internal structure and
composition as normal and larger amounts. Former member Paul talked about
much influence from type of pump, screw, needle and other dispensing
attributes. We have daily or weekly chats, but nowadays P is more interested
in bio and philosophy. I don't like to move his thoughts back to such things
as glue and syringes. Former XO sniffer Mike may have something, but he is a
laserguy nowadays.
Ingemar
-----Original Message-----
From: Marsico, James [mailto:[log in to unmask]]
Sent: den 26 januari 2001 15:09
To: [log in to unmask]
Subject: Re: [TN] silver epoxy & solder coated parts
Thanks for all the responses regarding the use of silver epoxy and solder
coated parts... obviously, not the way to go!
I'm still trying to understand the mechanism for increased DC resistance,
though. From what I have learned, silver oxides are conductive, so this
isn't it. The tin seems to migrate into the epoxy, I assume leaving a
lead-rich zone. Is it that the lead oxidizes? Are lead oxides conductive?
Can anyone explain?
Thanks again,
Jim Marsico
EDO Electronic Systems Group
[log in to unmask] <mailto:[log in to unmask]>
-----Original Message-----
From: Mike Fenner [SMTP:[log in to unmask]]
Sent: Wednesday, January 24, 2001 1:52 PM
To: TechNet E-Mail Forum.; Marsico, James
Subject: Re: [TN] silver epoxy & solder coated parts
I would avoid it.
Conventionally:
Epoxy has poor adhesion to tin and its alloys
The tin/silver interface will give an inter-reaction at the bondline
leading to electrical opens although you may retain mechanical
adhesion. One/Two years is a long time in this situation especially
if
you have mechanical stress/vibration or any temperature swings in
service.
This is very well documented in the literature and you should see
something in the Technet archives.
More open mindedly:
There are some companies working on this and there are now products
which claim to have solved this problem, either by using a sort of
halfway house product (adhesive with a fusible metal filler
basically)
or by special undisclosed ingredients/formulations.
Contact suppliers for the latest
Mike
----- Original Message -----
From: "Marsico, James" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, January 23, 2001 8:34 PM
Subject: [TN] silver epoxy & solder coated parts
> 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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