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Epoxy for electronics

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New for September 2012! This page came about by request of Dusan. Thanks!

Epoxy is one means of attachment that is used in electronics, for many different problems. There really aren't a whole lot of electronics' epoxy suppliers out there, and we are not going to recommend any unless they sponsor this page. Other forms of attachment include welding and soldering. Even explosion-welding has been used in the quest to join two dissimilar substances together!

The definition of epoxy is a two-part system where a resin is mixed with a hardener, which starts a reaction and produces a strong polymer. Most types of epoxies found in electronics take some heat in order to cure (anywhere from 100 to 200 C). After cure, they can be weakened by heating above the TG or glass transition temperature.

Storage temperature of epoxy can be at room temperature, but many types must be kept in a freezer or refrigerator.

The beauty of epoxy is that you can attach some components, then later attach some more, at the same temperature. This would be a real problem if you were restricted to solder attachment.

Electrically conductive epoxy

Electrically conductive epoxy is used in attaching all manner of parts in a hybrid microcircuit, including MMICs, silicon ICs, passives such as resistors, capacitors and inductors, and thin film networks. The conductivity is created by adding globs of silver to the mix, which touch each other and establish electrical connections after curing.

Note that sometimes gold is used in conductive epoxy. Gold is more expensive, and has less conductivity than silver, so why would you want to use it? It turns out that gold-based epoxies are at least partially conductive before curing (silver-loaded is not) and this property can provide a means for tuning microwave circuits with a tiny paint brush. Don't throw out expired gold epoxy, just ship it to Microwaves101 and we will dispose of it for you....

Conductive epoxy is most often used to attach electrical components, often is part of the critical back-side path to RF and DC ground as well as heat sink.

Many MMICs are attached with epoxy, but before you do this you need to consider the thermal implications. This is especially true with new gallium nitride power amps, where that first critical interface should be a low thermal resistance. If you are at a design review and someone is suggesting that you attach a Gan power amp with epoxy, raise your hand and say this: "are you out of your mind?"

To quote just about any power amplifier MMIC data sheet:

Use U.S.N. (80/20) solder with limited exposure to temperatures at or above 300°C (for 30 sec. max).

If you want to create some rework ability in a power amplifier assembly, you should consider soldering the power amps to thermal tabs, which can then be attached with conductive epoxy.

Thermally conductive epoxy

Most electrically conductive epoxy is sold as thermally conductive. That's because the silver chunks that make the mix conduct electrical signals also provide a much better path for heat than the polymer itself ever could.

The problem with thermally conductive epoxy is that quite often the manufacturer provides thermal conductivity data that don't hold up in practice. If it seems to be too good to be true, it is.

Nonconductive epoxy

NC epoxy is often used as the final staking for big parts such as electrolytic capacitors. It has very poor thermal conductivity, and often has high thermal expansion.

Author : Unknown Editor

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