Where to source strong, corrosion-resistant, oscilloscope-tip-type wire? - Page 1
Forum section: electronics / projects

author: rs20
author_rank: Super Contributor
timestamp: December 13, 2014, 02:08:45 pm
content: The tips of oscilloscope probes aren't a particularly thick gauge, but they seem to be made of much sterner stuff than your typical 0.7mm tinned copper wire -- I'm sure there's more to it than just gauge (open to correction). I would like to know what these probe tips are made of, and what keywords I should be searching for if I want to buy a small length. BeCu comes to mind, but I'd like to invite more knowledgeable minds to make other suggestions.I found some piano wire in the hobby store that had ideal mechanical properties, and the bulk conductivity of piano wire is more than good enough for the application, but I'm concerned about soldering/making a decent bond at the permanent end, and (even worse) corrosion issues at the pogo end. Since someone will ask, the context is having a go at making my own pogo pins, including the mechanical aspects of designing and winding a spring. This is very much a project-for-the-journey-not-the-end-product kind of deal; I am well aware that there are off-the-shelf pogo pins but I'm keen to do it by hand nevertheless

author: Andy Watson
author_rank: Super Contributor
timestamp: December 13, 2014, 02:37:54 pm
content: I would not have thought that the conductivity would be an issue over such short distances.How about stainless steel ? Can be had in various sizes of welding filler rod.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 13, 2014, 03:00:20 pm
content: Stainless. Not ideal (insulating oxides and all), but stronger and more corrosion resistant than others.Piano wire is impressively strong, which is an interesting subject all its own. Bare iron / steel (which it is) is also readily solderable (which stainless is not -- you need special fluxes to do it well, if at all). But, as you know... rusts..Phos bronze might be good?For exotica (breaking the "for the journey" part, most likely), something with a sometimes-conductive oxide, like niobium, might be a way to go. (Lower Nb oxides are conductive, which is why Nb capacitors are trickier to make than Ta or Al parts.)Tim

author: robrenz
author_rank: Super Contributor
timestamp: December 13, 2014, 04:54:15 pm
content: Harris Stay-Brite solder and flux is great to have around for soldering SS or when you need higher strength soft solder (around 14000psi). A trick for electronic applications since the Stay-Brite flux is corrosive. Is to tin your stainless part with the Stay-Brite flux and solder and then thoroughly clean to remove any corrosive flux residue. Now your regular electronic solder and flux will readily wet to the Stay-Brite solder film on your stainless part.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 14, 2014, 03:20:41 am
content: http://www.ebay.com/itm/New-50Pcs-P50-B1-Dia-0-68mm-Length-16mm-75g-Spring-Pressure-Test-Probe-Pogo-Pin-/371077892134?pt=LH_DefaultDomain_0&hash=item5665f74826

author: TerraHertz
author_rank: Super Contributor
timestamp: December 14, 2014, 07:06:33 am
content: For relatively stiff pins, I use gold plated pins from 0.1" headers. They are easy to push out of the plastic backbone. Or cut them free with side cutters.If you need thinner stiff pins, try gold plated wire-wrap IC socket pins.For super hard, pointy pins, I use old steel gramophone needles. I inherited a big box of them, but they can still be bought new. They are cheap too; $5/100, $11/300 on ebay.You can solder them easily as well.Edit: Oh, and for small 'pins' with rounded ends, there are gold-plated male pins from D-shell connectors. Cut the connector metal shell apart, and the pins fall out. The solid machined pins with solder termination have many uses. For instance you can make short jumper leads with one on each end, and use female D-shell connectors to make miniature patch panels.

author: tggzzz
author_rank: Super Contributor
timestamp: December 14, 2014, 09:59:30 am
content: This is a interesting thread since it applies tobothcontacts on high-speed scope probes. Apart from the centre signal contact, you also need a good ground contact (doh!).Usually this is a 150mm wire with croc clip, but that's useless at medium and high speeds due to its inductance.The traditional accessory supplied with scope probes is a coiled short piece of spring wire that slips over the end of the probe. This is fine for circuits where you can make contact with a ground plane at a fixed distance from the signal being probed. But that can be a problem with solder masks.A good solution is a "bayonet" or "spear" ground connection, particularly on "low-impedance Z0" probes. Commercial versions work to several GHz, and homebrew versions work to 1GHz. I've tried one pogo pin for a spear, but it was too fragile for general-purpose use.Here's picture of a 500MHz high-impedance probe; the low impedance probes have spears that are roughly half the length.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 15, 2014, 12:29:41 am
content: On the topic of high speed probes, this is the head end of a HP N1020A TDR probe. Typical use is connecting to test points on PCBs, to do trace impedance measurements.The center pin is signal, while the other holes are for a pogo pin, at different radii from the signal pin. Obviously only one of the two pins needs to be a pogo pin.And yes, dammit, one of the holes has a broken-off pogo pin in it. Was like that when I got it; I haven't yet figured out how to extract it without damaging the gold plating in the hole.

author: rs20
author_rank: Super Contributor
timestamp: December 15, 2014, 02:03:57 pm
content: Thank you everyone, much appreciated! The knowledge that plain steel solders well enough is news to me, and will be the basis of v1. But all the info provided is very interesting and helpful!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 15, 2014, 02:38:32 pm
content: Ah, yes. Be careful it's not, like, chrome plated (= as bad to solder as stainless), and fresh and clean. So, bright shiny paperclips probably not the best. Zinc plating also doesn't solder all that well (it dissolves and/or forms intermetallics). Rosin flux will work, and don't overheat it (iron oxidizes faster than copper).Typical example: the antennas on my Theremin are coat hanger wire, bent into reasonable shapes, and cleaned at the ends. Scrape, file or sand away the paint or varnish, and it should tin just fine.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: December 15, 2014, 03:17:25 pm
content: Quote from: T3sl4co1l on December 15, 2014, 02:38:32 pmFor the purposes stated, it might not even be necessary to make solder connections. Some of the connections in expensive professional probes rely on having a good oxide-free mechanical compression connection. See, as one example, the ground spear on the 500MHz HP probe I uploaded earlier.

author: SeanB
author_rank: Super Contributor
timestamp: December 15, 2014, 05:19:44 pm
content: Quote from: TerraHertz on December 15, 2014, 12:29:41 amAre you on good terms with your local dentist? Take a microscope there and the probe, and use one of his drills to carefully counterbore the piece of probe for about 2mm, then take a small pice of spring steel wire about the hole diameter and grind it to a triangular shape, then gently tap into the hole and twist it out.Is it writ large.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 15, 2014, 09:04:13 pm
content: A tiny resistance shouldn't bother anything in a probe tip, so how about a piece of heavy gage constantan (thermocouple wire)? It's stiff, corrosion resistant, but you can solder to it with only mild difficulty.

author: Muttley Snickers
author_rank: Unknown
timestamp: December 16, 2014, 02:34:06 am
content: To rs20,Some good suggestions here by all, you could also use the centre copper coated steel core from Belden 9725 RG-59 or RG-6 Quad.If you heat it up you can get a tight radius if needed, solderable as well.Muttley

author: TerraHertz
author_rank: Super Contributor
timestamp: December 16, 2014, 04:22:31 am
content: Quote from: SeanB on December 15, 2014, 05:19:44 pmWell, I have tiny drills and such. But this thing isn't screwed in, it's a sliding pogo-pin. So the remaining center pin can rotate and flop around freely in the hole . And the outer sleeve is just a press fit in the hole. AND thegold platingsurface finish of the lining sleeve in the hole near the surface must be retained, or that won't be a contact point suitable for high precision TDR measurements any more.See pic below, which is an intact pin. The broken one has the thin shaft snapped off near the shoulder of the end contact (left end.) The spring is gone, leaving the hollow stainless sleeve (right hand end in the pic) pressed in the bottom of the hole, and the remaining pin part loose. It falls down into the bottom, but even at the end of it's free travel 'out', the tip is below the surface plane so can't be gripped.This thread made me have a look at it under a binocular enlarger, and I now see the holes appear to have amazingly tiny stainless steel press-fit lining sleeves. Making them even more delicate.If I ever really need that particular hole for use, I think side drilling near the bottom, and pushing it out that way might work. Maybe.God only knows how some jerk managed to break the pin originally. They also lost the spare surface impedance calibration plate that is supposed to be with this kit. But that's ebay for you. Can't complain about previous owner being an idiot, when the result is you get something nice, cheaply.Btw, for rs20, when soldering steel the best flux is a zinc chloride solution. Here in Oz it's commonly available in hardware stores as 'Baker's Soldering fluid'. It's just zinc dissolved in hydrochloric acid. Clean the steel surface of all grease and oxides, then paint on the fluid with a brush. Use crappy big old soldering iron, not your nice fine point electronics one. After soldering, clean the joint area well with water & detergent, to remove any remaining flux. It really makes everything rust if you leave any. Also the brush you use better be kept for just this use, as the fluid ruins it. Use one with no metal or a steel shell, not aluminium.

author: SeanB
author_rank: Super Contributor
timestamp: December 16, 2014, 06:08:42 am
content: For stainless steel I use soldering paste, which originally was used by my neighbour in his work, which was making paper making machinery. He would solder a stainless steel fine mesh to make the paper web, 5m wide, and the only way you could see the join was that the metal web in that area was golden instead of metallic. You could not feel the join by touch. I cannot get my joins to be that good though, but the solder paste works well to hard solder stainless steel, brass or pretty much anything aside from aluminium, which melts first.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 16, 2014, 09:23:26 am
content: Quote from: SeanB on December 16, 2014, 06:08:42 amBut... there's ordinary solder paste, which doesn't give a golden join. Silver solder or brazing does, but I didn't know these came in paste form. Sigh... yes google, I'm an idiot, now I see that of COURSE they do.He must have made some kind of roller continuous process machine to make the mesh joins, to get them perfect. Otherwise the 'heating middle of a sheet' ruination effect would have been in play, you'd think.Pic below is of an air vent cover from a piece of military RF gear. That's the kind of golden colored join you mean?I would like to be able to make stainless mesh to metal joins like this, for a project. But I assumed it requires heating the part while in inert gas like argon, with a solder preform. Now I'll go watch some 'silver soldering paste' videos.Ha ha, there seems to be some terminology confusion due to ordinary solder looking silvery, while 'silver solder' (silver-copper alloy) is golden colored.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 16, 2014, 09:39:46 am
content: Yes, silver solder (a misnomer; it's a braze filler) is great stuff. Often stronger than the base metal (e.g., copper, brass). Though a little too hot for use with electrical components. Many silver braze alloys can also be tinned with ordinary soft solder.Tim

author: SeanB
author_rank: Super Contributor
timestamp: December 16, 2014, 11:54:07 am
content: He came to S Africa as a POW from Italy, loved the country and stayed after the war ended, and worked as a fitter for decades.He did this work with a set of basic tools, cutters, files, acetylene torch and little else, doing the join strand by strand with each one filed to perfection before and after. Then cleaned with acid and passivated again. The lifetime of the bed was typically about 6 months, making paper 24/7 during that time. There were parts that had to be replaced every 6 months as by then the running repairs had run out of metal to weld the patches to.One of his last jobs was making a paper pulp feeder out of titanium alloy, where each sheet was $10k plus, and he had to make his filler rods using a sheet guillotine to cut square section strips out of the one sheet offcut. With the price of the sheet he did about 20x the welding, instead of using extra sheets and having large amounts of scrap offcuts he welded them back into sheets. With the sheet cost that worked out cheaper in labour than the extra material otherwise. The scrap at the end was just enough to make 6 titanium alloy kitchen knives. Tough as anything but they do not hold an edge well.

author: robrenz
author_rank: Super Contributor
timestamp: December 16, 2014, 02:30:32 pm
content: Here is clip from my Kelvin clip 2 videowhere I am hard silver soldering a copper wire to a Tungsten carbide probe tip. The 56% silver solder is the lowest melting temperature of the hard silver solders.Here is clip from my Kelvin clip 1 videowhere I am using the Harris Stay-Brite soft silver slolder and flux on berylium copper and brass. Regular electronic solder and flux would have flowed here easily but the soft silver solder is much higher strength (14000psi)