Soldering Chemistry

[haddow]

A discussion (debate with Grinder about solder versus deans plugs) came up about soldering chemistry the other night....here is a little note describing the pitfalls of soldering..



Chemistry and Metallurgy of Soldering

When metal atoms associate, they form metallic bonds. Electrons are delocalized from individual atoms and they form an “electron gas”. This allows electrons to migrate easily, resulting in high conductivity. The goal of soldering is to join two different pieces of metals (often of different metal types) while maintaining the high thermal and electrical conductivity.

There are two main enemies of a good solder joint: oxidation and structural defects. When metals oxidize, the electrons are tied up in metal oxide molecules, unable to flow from place to place: i.e, they become insulators. A second requirement is that the metallic atoms of both pieces to be joined come in very close proximity to each other. In fact, the solder will permeate a short distance into each piece forming an alloy with a smoothly changing composition across the transition area. In order for a solder joint to form this alloy, the solder and pieces to be joined must all be heated. If one piece is not adequately heated, the solder will not diffuse into it and there will be an abrupt interface between two metal types. Like a crack in a crystal, this will ruin the joint: it won’t conduct well and it will be prone to mechanical failure.

The metals used in electronics oxidize in the presence of oxygen all the time, forming a thin layer which has poor conductivity properties. When soldering, this layer must be removed. The solution is flux, an acid which reduces the metallic oxides to metallic salts and water which can then migrate away from the joint. Most solder comes with flux mixed in it. Flux can be any of a number of chemicals, but is often an organic acid like carboxylic (R-COOH) acid. The “smoke” you see when soldering is actually a result of the flux burning, and contains very little metal. The burning of the flux has a side effect of consuming the oxygen immediately around the joint, which helps to combat further oxidation while soldering. Fluxes which use organic acids leave organic residue on the PCB which, depending on severity, may need to be removed using a solvent. “No-clean” fluxes are available which typically perform quite poorly, but do not require cleaning. All of this typically boils down to a couple rules of thumb during soldering: the joint must be heated uniformly so that a good alloy joint can be formed, and the joint must be free of oxidation. The first can be a bit hard to check for, since a poor interface layer can be hidden. But poor joints will be mechanically weak—in some cases, a little tug will be revealing. In the case of oxidation, it’s much easier to see: if the joint is shiny and lustrous, there’s not much oxidation. If the finish is dull, or even blackened, there is oxidation and the joint should be cleaned by adding flux and reflowing the joint.

stolen from somewhere else with no contact or author info but it sounds scientifically sound to me.

peace out

[Grinder]

'Tis true and much of what I was saying is that by soldering your leads directly to the battery each additional soldering causes that solder to oxidize.  Solder itself is a poor conductor with a conductivity of 11% of copper (copper is the baseline).  The only metal that has a better conductivity than copper is silver.  Contrary to popular belief Gold isn't a better conductor.  The main reason you see gold contacts is MOSTLY for asthetic ands and to make more sales.  Gold is used because it is very hard to oxidize.  The problem that most people don't know is that copper plated contacts will actually have the copper molecules migrate through the gold to the surface and oxidize.  Now you have a poor contact!  Its easily remidied by cleaning the contact with a very mild abrasive like a pencil (NOT pen) eraser.

Ok, enough of the Gold tyrate....  Silver... yes if we could use pure silver for our wires we would get more power to the ESC/Motor, etc.  Since that's not a viable option there are two ways we can take advantage of silver.  First is to use silver plated copper.  Silver is quite heavy (not that copper is light) and weight is usually an issue with our cars.  In electronics there is a phenomenon known as the "skin effect".  It is mainly seen in high frequency applictions where most of the energy travels along the outside or "skin" of the conductor.  The effect is smaller in DC applictions but is still present.  By using multi strand conductors and silver plating we can get the advantage of the skin effect.  The second way we can take advantage of the conductivity of silver is to use it in our solder.  The higher the silver content the higher the temp required to solder.  1% is a very common silver content in solder.  This raises the conductivity from 11% to 14% which may not seem like a lot but is about 30% better than typical solder.  You can get higher silver content but it requires more heat and even 3.5% silver content still only has 14% conductivity.  The other advantage to silver solder is that it make a mechanically stronger joint as well.

So to sum up all the geek speak...  If you solder directly to your batteries you will save yourself two extra solder joints from a Deans connector.  The problem is each time you heat the solder up you reduce its effectiveness in conduction because the heat oxidizes the solder and also changes the crystaline structure of the alloy.  You also add destructive heat to your batteries destroying the nylon vents.  If you MUST solder each and every time to your batteries you'll have to remove the solder and re-apply - DON'T REFLOW!!!  Some of the solder will wick up the wires so you should replace those every so often as well.  Use flux - I perfer organic flux - DON'T use and "acid" flux like plumber's solder paste.  Clean the joints with alcohol before and after.

Alternatively you can use the Dean connectors.  Make a good joint on the connectors and they will remain good for quite some time.  The contact resistance on a Deans is next to nil (which is why you can pull 80+ amps through a Deans no problem and no heat).  This is a no-maintainance method.  If you solder and reflow you will be the same as a Deans after a couple of reflows because of the oxidization plus your batteries have taken a hit from excessive heat.

Performance wise the difference is negligible.  If you race without EVER clipping a dot or going off the line then you probably want to go to those extremes for the microscopic advantage.  But don't take my word for it.  Get a precise multimeter that can measure milivolts or uV.  Measure the voltage droped across your connectors to see what is being lost.....  its negligible

[cyrrus]

man that a lot of reading when you first get up at midnight.  I think I just raised my IQ up by 2.  Wahoo,  I know have a IQ of 3 :lol: .   Serously great post guy's.   But.........  nah forget it.

[Tom]

There is more to hard-wiring than so-called performance gains.

I've always said that people who think they are getting a significant performance gain out of hard-wiring that matters are delusional.

However, I do hard-wire my 1/12 scale car.  And here's why:

1)  Connectors require a length of un-flexible conductor (the length of both halves of the connector).  Hard-wiring can have a flexible conductor the entire length.
2)  Hard-wired solder joints never come apart in a race.  Most people with connectors have had a connector come apart at some point in time when it wasn't supposed to.
3)  Many connector types (but not all) require a pair of wires to be close to one another at the connector.  In particular with a 1/12 scale car, this tends to require a fair bit more wire than the simpler hard-wiring wire routing.

And also, sane people that hard-wire do so on the end of a battery bar, and don't solder directly to the cell every time.

I've never hard-wired any of my 1/10 scale cars, but I do hard-wire my 1/12 scale car.  If I were doing it for performance, I'd do it the other way around, since 1/10 scale cars have much higher current draws, hence a more significant efficiency advantage than 1/12 scale cars.  Having said that, I do notice that my Deans plugs get quite warm, almost hot, when under a modest 30 amp load (I'd guess 5 Watts or so), so an old, worn-out, dirty Deans plug does have more of a loss than a similar length piece of wire.  I'm sure a fresh plug would stay stone cold.

It seems the Corally style bananna plug connectors some people are using are a pretty good idea; however I have seen these come out and short out batteries before.

[Grinder]

You'd have to have a pretty horrific crash or a Deans in very poor condition to rip it out.  I've seen airplanes flying with the battery hanging out with just the Deans holding it in.

Even soldering on the battery bar still sends destructive heat to the battery.  If you can solder the wire on while you hold you finger on the top of the battery then you don't need to worry about it  The main thing to remember if you are soldering to battery bars is to a) use as large a tip as possible b) use a high temp like 800 or 900 deg F.  This will allow you to heat the solder/wire/bar/flux quickly AND remove the heat quickly preventing it from traveling down the bar.

If you can - hardwired or not you - want the wires close to each other anyhow - weave them if you have three.  Cuts down on noise.  I do know what you are talking about though by keeping the wires short, etc.

I don't know what the specs on the Corally connectors is but I suspect they do have a large contact surface.  You could reduce the chances of shorting the battery out by insulating exposed contacts on the battery with a piece of electrical tape or heat shrinking the battery, etc.

[haddow]

I only hard-wire becaue plugs are ugly, I am all about a clean layout.

I also suck at soldering deans plugs and melt them if I dont first short the battery out I am putting a plug on! LOL

[Grinder]

I only hard-wire becaue plugs are ugly, I am all about a clean layout.

....quote]

What!  Last night you said I had nice plugs....

[haddow]

Dude, your boss called, he wants you to get back to work, slacker.

[Tom]

You'd have to have a pretty horrific crash or a Deans in very poor condition to rip it out.  I've seen airplanes flying with the battery hanging out with just the Deans holding it in.

And just last weekend, my battery came disconnected when I hit a bump after forgetting to put my battery hold-down in.  Not that hard-wiring could have allowed me to finish the race, but most people who have been racing for many years have had connectors come undone.

I don't know what the specs on the Corally connectors is but I suspect they do have a large contact surface.  You could reduce the chances of shorting the battery out by insulating exposed contacts on the battery with a piece of electrical tape or heat shrinking the battery, etc.

In the past, I've used tool-handle rubber to insulate the ends of my batteries.  Heat shrink, etc. can interfere with the way the batteries fit in the car, and further insulation is often not desireable.

[Grinder]

Heat shrink does have the disadvantage of keeping the heat in the batteries but it does add a lot of strength to them and helps keep them together.  Liquid plastic is a good method - there is also Liquid Electrical Tape that can be brushed on...

[Grinder]

Dude, your boss called, he wants you to get back to work, slacker.

I took the day off work to harass you....

[0000000000]

Very interesting.

Is there any information on actual numbers? For example how many milli-ohms of resistance there is between a deans connector vs a tamiya conector vs a brand new soldered joint vs a 20 times used solder joint?

I'm finding it all quite fascinating.

[Grinder]

Not that I know of off the top of my head.  The Tamiya is probably the only one you could measure the resistance with with a decent hand held meter.  Otherwise you'll need a sensitive bench meter that will measure milli ohms and less or milli or uV.  You would probably need a four wire setup as well to measure the resistance at that low a value which brings me back to the point that the differences aren't really worth mentioning.