On 2/28/2018 8:43 AM, Ed Huntress wrote:
On Tue, 27 Feb 2018 17:01:24 -0800 (PST), "
wrote:

On Tuesday, February 27, 2018 at 10:38:26 AM UTC-5, 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.



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 citrus 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 pressure on. But one of the gov engineers and I ran some tests and the damn grounds would just fall off after a long time.

My manager said to stop doing tests and that he could have me fired for running 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 results are." Eventually the factory engineers gave up and we did not use the citrus based solvent in that one case. I do not know if it was just that the silver filled epoxy had less strength or if the silver and citrus based solvents were not compatible.

Dan

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.


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





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.