Electrically conductive epoxies

On 2/27/2018 8:38 AM, Ed Huntress wrote: > Another off-the-wall tip from my old manufacturing editor's resources: > > Electrically-conductive epoxies are showing up in a lot of industrial > applications, replacing solder. They may solve some problems for > hobbyists, too. > > At the rist of sounding like a shill for MasterBond (again), I'll > suggest the following info from this epoxy specialist: > >

Thermally conductive epoxies are also very useful for small heat sinks in odd applications. Arctic Silver comes to mind.

Here is another off the wall tip. When you clean the surface before using the epoxy' do not use a citrus based cleaner. I can not remember it Very well, but we used silver filled epoxy to glue a ground to the rocket motor. Very thing worked well when we used 1-1-1 Trich. But in a change to a cit rus based solvent we ran into problems in just that one case. Every seemed good but about 24 hours the ground would become unattached. The factory people that had selected the citrus based solvent essentially said use the damn citrus solvent. It is a universal solvent. And really put the press ure on. But one of the gov engineers and I ran some tests and the damn gro unds would just fall off after a long time.

My manager said to stop doing tests and that he could have me fired for run ning tests like that. And I said go ahead and fire me. My manager and I really got along well so he softened that to " well tell me what the test r esults are." Eventually the factory engineers gave up and we did not use th e citrus based solvent in that one case. I do not know if it was just tha t the silver filled epoxy had less strength or if the silver and citrus bas ed solvents were not compatible.

Dan

I worked at a small company that built ultrasonic transducers. The transducers were a 2" ceramic piezo bonded with epoxy to a 1/16th inch aluminum disc. After lapping the two surfaces together using a figure 8 lapping motion the pieces were rinsed in water, lapped some more, rinsed in acetone, this was repeated a couple times and then the pieces were left in the acetone tray to prevent any oxidation of the aluminum, as prep was made to install the epoxy. We drove these at 660kHz with 1000 watt or pulses 250 watts continuous. We drove never had a bond separation, but we did burn up a few. Mikek

That's a good point. The problem in using epoxy to bond metal has been the same problem for the past 50 years. Actually, it's two problems.

One is the difficulty of getting chemical bonds on metal, and the other has been mechanical compatabilty of the adhesives and metals, starting with the thermal expansion coefficients between metals and epoxies, and including the behavior of epoxies when joints are loaded in peel and cleavage.

It's a long story but the conclusion is that both problems have been solved. The trouble with it in a practical sense is that you have to understand the factors involved if you're going to make strong, long-lasting joints. Either that, or you're going to need experts to engineer the bonds for you. The latter is what the aircraft companies, and now the car companies, have done. The high-end Buicks and Ford F150s are half glued together now, and the joints are stronger than the welds and mechanical fasters used before. When you're bonding high-strength aluminum, there's almost no way to maintain the strength of the parent metal in a welded joint.

These electrically conductive epoxies available now are engineered to be much more versatile and reliable than anything available in the past. If you follow instructions, you're very likely to have successful applications, without engineering each joint individually.

That's interesting. By keeping the aluminum piece covered with acetone, you're accomplishing something similar to the "scratch-in" method of prepping metals for adhesive bonding. Aluminum, stainless steel and copper alloys are a bear to bond because they form oxides literally within seconds after they're exposed to air.

I was familair with several other Arctic Silver products (Thermal paste/greases for use with CPUs/heat sinks - having used several of them over the years; but I had somehow completely missed that they also made an epoxy product (different application than thermal grease - but good to know for instances where epoxy is indicated) Thanks for pointing it out.

I was told the epoxy we used was used in the making of IC chips, this was 25 yrs ago. It came in 12" x 12" sheets and had to be kept on dry ice. The thickness, I'm hazy on, 0.004" to .008" thick. When the aluminum, epoxy and ceramic were assembled they were put on a large solid block of steel that had been preheated to (Hazy again) 155*F and clamped with 18 psi. The heat expanded both the aluminum and the ceramic, (by different amounts). When the transducer cooled you could see the aluminum was concaved a slight amount because of the different expansion rates and being bonded and then cooled. Heating or cooling one of the transducers would give it enough charge to draw a good spark or give someone an unexpected shock!

Mikek

Thick and thin film modules have been using forms for decades. Now with the complex issues of solder silver, gold and copper in epoxy does a good job. Spot welding is still in use on tiny stuff.

Mart> Another off-the-wall tip from my old manufacturing editor's resources: >

I use starting fluid (OUTDOORS..) for cleaning that leaves no residue.

I don't know what the different brands may contain (I use straight ether for starting), but avoid anything that contains acetone.

Pure acetone is great for adhesive prepping, but getting your hands on pure acetone is not easy. The stuff they sell in cans in hardware stores is almost always recycled from industrial users, and it usually contains some oil. It has been the source of many bonding failures by home boat builders.

Final wash with methyl hydrate or ethanol - or as the previous poster noted - ether.

BUT some starting fluids ALSO contain an oil to reduce damage from "wash down" when starting an engine -- -

After reading your post Ed, I looked at the can of acetone that I have. It does not have an ingredients list, insinuating "pure" acetone. There's no statement of purity either. I've had some paint failures that I blamed on poor prep techniques. Using acetone for prep often allows me to wipe the old coat of paint away without scratching the part. I've gone back to using alcohol to wipe the part down before painting. Oil in acetone could explain it. I'm going to contact the manufacturer on the can for comment since I bought it at the Borg. Stay tuned...

Steve

Here's an interesting claim from TAP plastics -- "Our Acetone is virgin acetone and is not made from recycled acetone. It does not contain any residual waxes or oils."

Acetone sold in hardware stores is almost always "industrial grade," which is the stuff that contains small amounts of contaminants.

Apparently there are ways to recycle it to remove all of the non-volatiles, but it's pretty well known in the boatbuilding business that hardware-store acetone is corrupted with non-volatiles, especially lubricating oils, and that it can't be used to prep cured polyester resins for laminating. When surfacing polyester cures (and all of the resin sold in hardware stores is surfacing resin) a waxy layer rises to the surface to seal it from the air, and that gets into acetone, too, when you wipe the layer down to laminate on top of it.

(There's no need for it with epoxy prep; just wash with warm water and detergent, like real TSP.)

It's too bad, because acetone is a great cleaning solvent, and safer than a lot of others. It's just that the common grade can present problems when you need a really clean surface.

Paint shop I worked at ran a "still" that was designed to reclaim the acetone we used during production. Worked OK but you never got 100% acetone out of the system. It worked OK for thinning and such but you could tell it was a blend.

I had good results with this on the front surface mirror in a copier.

Bob

Another English incongruity: naphtha = nap-tha, not naph-tha

How did you use acetone such that it was reclaim-able? I've always used it as wipe-on or spray-on, wipe-off. I.e., ending up with the acetone in rags that were discarded.

That's quite a hydrocarbon soup:

Hey, if it works, it works.

I was thinking exactly the same thing. "Naptha" covers quite a range of volatility. "Light," which they claim this one is, is more volatile that most.