Low viscosity potting compound

For various complicated reasons I have a chip in a ceramic PGA package with
direct access to the die and its bond wires. I wish to pot them with
something so that I can put moderate force on the bond wires without
snapping them (at the moment just touching a bond wire is enough to break it
from the pad at the package end).
So I think I need some resin to pot it. But because it's very difficult to
get potting compound between the bond wires which are probably 200um apart I
think I need some very low viscosity potting compound. I'd imagine trying
to put water on the bond wires wouldn't get through due to surface tension,
so we're probably talking something less viscous than water, or with a lower
surface energy (if I'm right in remembering this is the cause of surface
tension). Basically all I can do is introduce a drop and let it flow by
capillary action.
The other thing is that I'm worried that shrinkage during curing will be
each force to break the wires. Having something that is conductive is also
not an option, as I want the chip to work afterwards!
I think it's epoxy I need, but don't really understand the vicosity and
equivalent weight figures given the the manufacturers. Alternatively are
there other compounds that would do the same job (cyanoacrylate?). Whilst
I'm UK based, I'm mainly after information that would tell be where to start
looking. I only want a drop, not a gallon!
Reply to
Theo Markettos
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Experiment with plain parafine, which fuses at low T.
Reply to
jacques jedwab
What about a drop of 'superglue'? Then you can apply some other material over it to complete your seal.
Reply to
Not a chance, unless you only need the chip to survive for a few hours or days. The aluminum bond pads are sputtered aluminum, and they are so thin they will be eaten up by even low levels of ionics. They are not stable in the presence of (at most) 70 ppm or more of ionics (Na+, K+, NH4+, Cl-, etc.).
Very few materials that are not made for contact with a silicon chip are this ionically clean. Some silicones may be this clean, but other than that, forget it. And if you use silicone, use the platinum-catalyzed addition cure type, to avoid generating reactive species.
Reply to
Mark Thorson
Would suggest considering a low vis silicone potting compound from Dow Corning.
Reply to
Hmm... thanks everyone for your suggestions. Doesn't look like there's a particularly easy solution. I'll have a go with superglue or wax of some sort - as I don't actually want to pot the chip, just the bond wires, I can see if I can keep it clear of the die. Not that I need particularly good stability, as I'm only just using it as a hack for a few experiments. Plenty of things to think about though...
Thanks! Theo
Reply to
Theo Markettos

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