Flat-flex cables *not* copper?

[ejeffrey]

I would like some flat-flex ribbon cable with the conductors made of constantan or brass or some other copper alloy with a medium resistance.  Basically anything but pure copper.  I haven't been able to find anything like this, which I guess isn't surprising, but has anyone else heard of such a thing?  Right now I am using bundles of individual copper-nickel wires, but I am interested in a mass termination option.

[mikeselectricstuff]

Why not copper?

[Alex Eisenhut]

Wow, that's an odd request, but I'd try:
Go to a real hardware store and find the 1 mil brass shim. Cut and glue to Kapton tape. Well the tape does the gluing. Glue another strip of Kapton tape on top to seal.
Just for testing, y'see.

[Marco]

Is this for probe cables or something?

[T3sl4co1l]

Membrane panel manufacturers often use something like silver ink... not the most conductive stuff.  But if you're looking for stable resistance...

Tim

[ejeffrey]

This is for cryogenic use.  Copper has too high a thermal conductivity, which is directly related to electrical conductivity.  What is worse, copper changes resistance (electrical and thermal) by a huge and unpredictable factor at low temperature due the exact quantity and type of impurities, as well as work hardening or annealing..  Pretty much any sort of alloy will work -- the tempco doesn't have to be exactly zero -- I just need a nice resistance of about 0.5 - 1 ohm that doesn't have huge changes with temperature.  Eventually I would like to use this for high frequency as well (a few GHz), so a controlled impedance would be nice (I don't mind a loss), but at the moment this is strictly low frequency (kHz).

I can't think of any other reason someone would want this, so it isn't surprising it doesn't exist off the shelf.  I just thought I would ask because you never know what some aerospace or medical application might have created that I could piggy back off of.

I've got some DIY ideas, but probably if it comes down to that I will probably just look for a custom cable assembly place and keep using the wiring we are using now.

[T3sl4co1l]

Does it have to be flex?  Coax I would think would be more common for cryo / vacuum applications.  Not sure where you go shopping for physics lab toys, but such a supplier would be the first place to check I think.  It's going to be $$$ either way.

I think I'd be pretty surprised if flex can be made in the same way as usual, from metals or alloys other than fairly pure copper.  The ductility and electroplatability of elemental copper is perfectly suited to its use in printing, which is a wonderful coincidence for being the second most conductive metal.  One option would be vaporized or sputtered metal (thin films), which can still be etched like copper circuits, and is suitable for many elements, alloys, and even nonmetallic compounds, but the deposition process is very different and probably expensive, especially if you need a thick layer.

For any connection at all, you might settle for thermocouple wire, which of course is available in various special alloys... typically in matched pairs (though you probably only want just the single strand..).  You might well be better off making your own from wire strips and Kapton tape.

Why is the resistance a problem?  Is it not simply sufficient to put a 1 ohm resistor in series, so that that 0.1 to 0.001 ohm variation from the cable disappears in the same way?  You literally gain nothing by going to the trouble of getting better (or worse, really) alloys.  In the extreme case, a current loop has no regard whatsoever for the resistance, which is why it's been so popular in low-rate signaling for the last.. century and a half or so.   Likewise, for the transmission line, if it's normally losing 0.5dB, and it loses 0.05dB when cold... why do you really care at that point?

Anything you can do to arrange the measurement as a ratio, or with Kelvin leads, or a locally matched difference, or so on, will not only improve such problems as tempco and noise, but will only improve the measurement of whatever physical phenomenon you're trying to measure.

Tim

[ivaylo]

A friend fixes cryostats. But he says he deals with ribbon cables, but not flatflex. If you can use ribbon there is plenty of constantan ones - http://usa.cryospares.com/Cryogenic_Ribbon_Cable_p/a8-301.htm

[ejeffrey]

Does it have to be flex?  Coax I would think would be more common for cryo / vacuum applications.  Not sure where you go shopping for physics lab toys, but such a supplier would be the first place to check I think.  It's going to be $$$ either way.

We use a lot of coax now, but we use a lot of lines (currently about 100 and would like to do more).  This gets expensive fast, and also can be a real bitch to work with because it is too crowded.

I think I'd be pretty surprised if flex can be made in the same way as usual, from metals or alloys other than fairly pure copper.  The ductility and electroplatability of elemental copper is perfectly suited to its use in printing, which is a wonderful coincidence for being the second most conductive metal.  One option would be vaporized or sputtered metal (thin films), which can still be etched like copper circuits, and is suitable for many elements, alloys, and even nonmetallic compounds, but the deposition process is very different and probably expensive, especially if you need a thick layer.

Interesting, thanks. 

Why is the resistance a problem?  Is it not simply sufficient to put a 1 ohm resistor in series, so that that 0.1 to 0.001 ohm variation from the cable disappears in the same way?  You literally gain nothing by going to the trouble of getting better (or worse, really) alloys.  In the extreme case, a current loop has no regard whatsoever for the resistance, which is why it's been so popular in low-rate signaling for the last.. century and a half or so.   Likewise, for the transmission line, if it's normally losing 0.5dB, and it loses 0.05dB when cold... why do you really care at that point?

What we really care about is the heat conducted from room temperature into the cryostat.  We actually already add RC filters on the low frequency lines and attenuators on the RF lines -- the variation in electrical resistance/loss doesn't matter, and gets calibrated out anyway.  The problem is just the thermal conductivity of the copper, especially the low-temperature conductivity of pure copper.

[quote user=ivaylo]
A friend fixes cryostats. But he says he deals with ribbon cables, but not flatflex. If you can use ribbon there is plenty of constantan ones - http://usa.cryospares.com/Cryogenic_Ribbon_Cable_p/a8-301.htm
[/quote]

Thanks!  That will probably work for the low-frequency wiring.  We have made cables similar to that before by hand, but a professional made cable off-the-shelf will be a lot nicer.

[ConKbot]

Omega and others make flexible kapton heaters and strain gages with a resistive layer that's not copper, so the process exists. Where to get it done? Not sure. If youre looking at more then a small quantity, try contacting Omega and seeing if they do the heaters themselves and would maybe be willing to do them for you, or refer you to a manufacturer possibly.

[mikeselectricstuff]

What we really care about is the heat conducted from room temperature into the cryostat. 

Long FFC, folded up a few times?

[penfold]

Have a word with Allectra ( http://www.allectra.com/ ), they make a number of specialist cables for both vacuum and low temperature applications, they make cables in all sorts of configurations of insulators and conductors you wouldn't want to use in any other situation.

[tggzzz]

A friend fixes cryostats. But he says he deals with ribbon cables, but not flatflex. If you can use ribbon there is plenty of constantan ones - http://usa.cryospares.com/Cryogenic_Ribbon_Cable_p/a8-301.htm

Thanks!  That will probably work for the low-frequency wiring.  We have made cables similar to that before by hand, but a professional made cable off-the-shelf will be a lot nicer.

Ribbon cable can be satisfactory for high frequency wiring, depending on the frequency and distance; the classic example is computer IDE cables. To a first approximation, a single-layer flex cable would be neither better nor worse in this respect.

Main points are to have gnd-signal-gnd-signal-gnd, and to measure the characteristic impedance so that it can be terminated correctly.

[Marco]

Ribbon cable can be satisfactory for high frequency wiring, depending on the frequency and distance; the classic example is computer IDE cables. To a first approximation, a single-layer flex cable would be neither better nor worse in this respect.

Main points are to have gnd-signal-gnd-signal-gnd, and to measure the characteristic impedance so that it can be terminated correctly.

At a GHz that's going to need to be a very short cable ... or he'll need a lot more grounds in between conductors. Ultra-ATA had 10% crosstalk at 66 MHz on a 12" cable according to this.

How about going differential? Ribbon cable of twisted pairs seems relatively cheap (12 times cheaper than coax on that page).

[tggzzz]

Ribbon cable can be satisfactory for high frequency wiring, depending on the frequency and distance; the classic example is computer IDE cables. To a first approximation, a single-layer flex cable would be neither better nor worse in this respect.

Main points are to have gnd-signal-gnd-signal-gnd, and to measure the characteristic impedance so that it can be terminated correctly.

At a GHz that's going to need to be a very short cable ... or he'll need a lot more grounds in between conductors. Ultra-ATA had 10% crosstalk at 66 MHz on a 12" cable according to this.

How about going differential? Ribbon cable of twisted pairs seems relatively cheap (12 times cheaper than coax on that page).

Whoops, I missed the frequency since it was in a later post!

Clearly differential would be highly beneficial at those frequencies; there are many semi-standard standards/protocols to choose from. I'd also look closely at that cable's electrical characteristics as a function of frequency, since it has a highly regular non-uniform structure.

[ivaylo]

Also if you want this stuff to be flexible at those temperatures (the flatflex part of the requirement kinda suggests it) the physical properties at low temp. of the insulator are as important. Especially if you go the DIY path. The everyday stuff we use gets stiff, and if you keep flexing it just crumbles when deeply frozen.

[Ton]

Just a thought

You could try to hunt down one of those company's that make heater foils for automotive use (for side mirrors, screens  and the likes) I am pretty sure that they among other things use brass for the conductors.
they might be able to use polyimide as a carry foil, this could do -270 to +270 deg C

I recently stumbled upon the web page of a German company (Thermo.de they do have plant in US also) doing this (for automotive, medical.....).

Ton

[ejeffrey]

It doesn't actually need to flex at cryogenic temperatures, that is mostly to make installation easier.  The insulation is still important -- some insulation will crack after repeated temperature cycling even if you don't explicitly flex it, mostly due to thermal contraction.  Kapton is good, which is one reason why I thought flat-flex cables would be a good idea.  I was also assuming that because FFC is used for things like hard drives that it would be fine up to a few GHz, but I may have to reconsider that.

Anyway, thanks everyone for the help.  That heater foil from thermo.de looks pretty promising, too.