electrically conductive epoxy

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zotdoc
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electrically conductive epoxy

Post by zotdoc » Wed May 02, 2007 11:15 am

I want to make fireworks ignitors using a double sided pc board chip and nichrome wire, gluing them together using electricaloly conductive epoxy. Anyone with experience out there? Will this work and will the epoxy actually conduct properly?

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Chris Smith
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Post by Chris Smith » Wed May 02, 2007 11:47 am

As much as the conductivity may work, I would doubt the current capabilities especially after repeatedly shunting a heavy current needed for the pulses to fire the wire?

Between reliability and the shock of the current, It may not stay reliable?

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haklesup
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Post by haklesup » Wed May 02, 2007 12:44 pm

Not sure I understand, you want to glue the NiCr wire to the pads on the PCB then subject that mechanical joint to high current. It might work but conductive epoxy doesn't necessarily have low resistance though some might.

Why not solder, won't stick to NiCr, I think an acid flux might help. Why not a receptacle pin. Solder that to the board, slip the NiCr wire in the hole for a friction fit (see www.mill-max.com ).

A ways back I worked in a lab where we used a modified spot welder to weld wire to the pins of IC's to protect them from ESD during other physical testing.

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GoingFastTurningLeft
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Post by GoingFastTurningLeft » Wed May 02, 2007 12:45 pm

Don't want to use regular ignitors and alligator clips, huh? Or are you trying to make some sort of reusable one?

zotdoc
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Post by zotdoc » Wed May 02, 2007 1:55 pm

I want to mass produce them, make about 1000 or so. They are destroyed when ignited. The nichrome and lead wires will be glued to a tiny piece of double sided pcb board. the nichrome end of it is dipped in nitrocellulose acetate and gunpowder after the wires are glued in place. I wanted to try the glue to see if it will be easier than soldering /sockets on a mass scale. I thought maybe someone had tried this before and could give me a go no go on the idea. I will post later once I have tried it on a few prototypes and let everyone else know how it worked out. Thanks.

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jwax
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Post by jwax » Wed May 02, 2007 2:37 pm

Conductive epoxies have been used and refined for ages. Most of the good ones are NASA (space) certified, as well as military specs.
For your application, give a call to a few of the biggies and talk with their applications engineers for advice on which to use.
Epoxy Technology, and Ablestik are two biggies.
Best of luck, I think it'll work fine!
BTW, the epoxy is not cheap!

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dacflyer
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Post by dacflyer » Wed May 02, 2007 6:43 pm

JB weld has steel in it...perhaps this might work ?

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jwax
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Post by jwax » Thu May 03, 2007 3:22 am

Doubtful, dacflyer. Too high a resistance to let a few amps get to the nichrome.
Zotdoc- are you mixing your own ignitor composition, or are you using a mix off-the shelf? If you're mixing your own, you'll have to be meticulous about it's chemistry and quantity to get consistent results.
A big blob will have less resistance than a small blob, firing differently.
You can check resistance after drying to see if they all fall "in-range".

rshayes
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Post by rshayes » Thu May 03, 2007 5:04 am

Have you considered Constantin? It is about half the resistance of Nichrome, but it solders easily. Omega sells it in fine gauges for thermocouples.

Constantin may cost a little more than Nichrome, but the cost of conductive epoxy may make Nichrome as expensive.

The cooling time for solder (seconds) is also much less than the setting time of epoxy (hours).

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Janitor Tzap
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Post by Janitor Tzap » Thu May 03, 2007 3:40 pm

zotdoc,

Are you trying to make these for model rocketry?:?

If this is the case.......

Some years ago, I found that I could use ordinary picture wire for ignitors.

I would take the stranded wire, and cut into 3 inch lengths.
Then I untwisted the strands, and bend the single strands into a U-shape.
I would push the bent end into the rocket engine nozzle,
making sure it was right up against the propellant.
Then use masking tape to hold the wire in place.

Worked great.

I saved several dollars on cost of the store bought rocket ignitors.


Signed: Janitor Tzap

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Chris Smith
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Post by Chris Smith » Thu May 03, 2007 4:35 pm

If your making your own ignitor’s then grind up several red head matches, add in a drop of [watered down] white glue until its real tacky, then use a single strand of copper wire in the shape of the ignitor adding in a tiny strip of masking tape to keep it stable.

[simalar to store bought ignitors]

Place a small gooey drop of the match and glue compound at the bend of the wire and let it dry.

When the wire heats up, the match compound goes off first, and aids in the ignition.

You can make thousands of them for pennies.

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haklesup
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Post by haklesup » Fri May 04, 2007 10:43 am

Some years ago, I found that I could use ordinary picture wire for ignitors.
Actually you could use any wire you want, what changes is the amount of current needed to heat (~500F min) or fuse the wire.

NiCr is desireable due to its high resistance and high streagth and is bendable and can be formed etc. The high R (not real high but compared to most metals it is) allows a decent wattage to be concentrated in a small area with not too much current (voltage you need for that I depends on the resistance you end up with or vice versa).

But other wire could certainly be substituted based on your design requirements, you could find a material with suitable characteristics. I can even imagine reuseable ignitors.

Ideally the wire would neck down in the center so only that portion gets hottest but a wire of continuous diameter could work as well, just need a bit more power to heat the whole wire. Might try heating the wire with a torch then pinching or rolling the middle in a press to get the necking without a complicated wire pulling thing.

mikeu
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Post by mikeu » Fri May 04, 2007 3:00 pm

Zotdoc,

You can purchase electric match heads, nichrome wire already soldered in place from two sources.

www.oda-ent.com

or

www.skylighter.com

Note: You will still have to dip them in a composition.

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jwax
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Post by jwax » Sat May 05, 2007 6:15 am

mikeu- Seems like you are one who has "smelt the smoke". :grin:
Do you have any recipes for the pyrogen that does NOT require a nichrome element?
The leads are simply immersed in a conductive composition that ignites with current flow through itself- no heating element needed.

Dean Huster
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Post by Dean Huster » Mon May 21, 2007 5:36 am

Rather than expensive conductive epoxy (and the cure time), why not simply stake the wires onto the PCB using eyelets? Cheap and reliable.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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