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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!

Thanks, Theo

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Theo Markettos                 theo@markettos.org.uk
Clare Hall, Cambridge          atm26@cam.ac.uk
CB3 9AL, UK                    http://www.markettos.org.uk/
Reply to
Theo Markettos
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Hasn't this question been addressed in your industry already? A month in the lab will save you a day in the library. Beware ionic additives and impurities! Free halide is the universal corrosive.

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Look up light- and UV-cured adhesives, especially optical bonders for lenses. They are literally used by the drop and 100% cure within seconds but have almost unlimited pot life in the dark. Shrinkage can be reduced by adding micronized filler, but that can boost the viscosity.

There are a number of resins whose cure is based upon opening of spyrocyclic monomers (that expand on curing) instead of linear concatentation of small monomers (that shrink on curing). You can thus get essentally zero volume change - for a price - in several cure system mechanisms. Beware ionic additives and impurities!

One cute possiblity is to take a low viscosity epoxy, urethane, photocure, or whatver and add a modest loading of dense micron-radius glass beads used to make reflective coatings. You pot your device with well-mixed filled resin and let the potting settle, maybe with a thump or three to close-pack the beads at the bottom by the chip. Cure. Now you have a physically robust potting with little local shrinkage, the whole shrink-wrapped by overlain unfilled epoxy.

Google adhesives -epoxy cure 31,500 hits

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Uncle Al
http://www.mazepath.com/uncleal/
 (Toxic URL! Unsafe for children and most mammals)
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Reply to
Uncle Al

That won't work. Glass will contaminate the encapsulant with sodium, which eats up aluminum bond pads.

Best solution is to pretend to be working for a semiconductor manufacturer, and request samples from encapsulant suppliers. Pure epoxy would be nice for its low viscosity, but it's usually supplied with lots of silica filler and formulated for injection molding (an exception would be Thermoset, who offers semiconductor-grade pure epoxies). Silicones tend to be higher viscosity, but are widely available in semiconductor grade (Dow-Corning, Wacker, Shin-Etsu). Polyurethanes are available, but combine the worst features of both silicones and epoxies.

What you are asking for is commonly called a "glob top" encapsulant.

Before selecting a UV-cure system, make sure your chip can tolerate it. Flash memory is permanently disabled by UV light (it erases critical data held in flash memory cells outside the memory array, such as read threshold voltage).

Glob tops are generally filled with carbon black to make them opaque. Is that a problem?

Reply to
Mark Thorson

Experiment with plain parafine, which fuses at low T.

J.J.

Reply to
jacques jedwab

Paraffin wax has an amazing shrinkage upon crystallization. It continues to ripen over time.

--
Uncle Al
http://www.mazepath.com/uncleal/
 (Toxic URL! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?"  The Net!
Reply to
Uncle Al

What about a drop of 'superglue'? Then you can apply some other material over it to complete your seal.

Reply to
Greysky

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

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