What makes solder stick? - Page 1
Forum section: electronics / general

author: rexxar
author_rank: Frequent Contributor
timestamp: October 24, 2013, 08:11:22 pm
content: I realized recently that I don't really know how solder works. Does anyone here know? I looked up the Wikipedia page, but it didn't seem to talk about that, or if it did, I didn't see it in the wall of text. I would presume that there's some sort of molecular bonding going on, but I don't know what that would be, or how it works across such a wide range of metals.

author: free_electron
author_rank: Super Contributor
timestamp: October 24, 2013, 08:14:11 pm
content: electron exchange

author: bmwnomad
author_rank: Contributor
timestamp: October 24, 2013, 08:16:28 pm
content: Part 1 of a set of soldering tutorials by Pace. Has that 70's/80's vibe to it

author: Fsck
author_rank: Super Contributor
timestamp: October 24, 2013, 08:28:31 pm
content: look up wetting.

author: walshms
author_rank: Regular Contributor
timestamp: October 24, 2013, 10:15:54 pm
content: Mostly I think it's related to two things: mechanical friction and alloying.Solder solidifying into tiny imperfections in wire/component leg/pad, etc. provides the mechanical bond and you can think of that as being something like "friction." Since solder conforms to the surfaces it bonds to, a lot of the strength of the bond comes from this.There's also a bit of alloying, as copper readily alloys with tin/lead solder. That alloying occurs at the surface and doesn't penetrate more than a few atoms deep, but it does add a lot of mechanical strength to the solder joint, and ensures good conductivity. It also helps to keep the joint from acting too much like a thermocouple, i.e. dissimilar metals generate a tiny voltage when bonded together.Other metals, like in lead-free solder and gold-plated pads, etc. also alloy a bit when soldered, and the result is much the same.

author: IanB
author_rank: Super Contributor
timestamp: October 24, 2013, 10:37:57 pm
content: There are two ways to join metals together with heat: welding and brazing.Welding causes the metal parts themselves to be melted and to fuse together. Sometimes some extra filler metal is added to help make a stronger joint. The strength of a welded joint is about as strong as the original metal.Brazing does not melt the actual parts, but uses a molten filler metal of lower melting point to bridge the gap and join the two pieces together. The strength of a brazed joint depends on the strength of the filler metal.Soldering is a kind of brazing usually reserved for low melting point filler metals where the working parts do not reach red heat.In a soldered or brazed joint the filler metal or solder forms an alloy with the parts being joined so that they become continuous with no boundary. In a properly made solder joint you cannot "unstick" the solder from the surface. The solder will give way first as the solder is weaker than the joint.(The purpose of flux in soldering or brazing is to remove any oxide layers from the surfaces so that the alloy can properly form between the two metals.)

author: olsenn
author_rank: Frequent Contributor
timestamp: October 24, 2013, 10:39:38 pm
content: Flux

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 24, 2013, 10:43:37 pm
content: Quote from: rexxar on October 24, 2013, 08:11:22 pmMetals don't have molecules, strictly speaking. They form a crystal lattice with a sea of shared electrons.A good solder joint requires that the base metal be oxide-free, so that the solder can share its electrons with those of the outer layer of the base metal. That's why we need flux, to reduce the oxide layer.It's not like glue, where the solder would simply melt into the crevices of the base, sticking by mechanical friction. If that were the case, we wouldn't need flux; and we'd be able to solder to ceramics and other strong nonmetallic materials. No, the solder bonds to the base metal in the same way the solder bonds to itself, and the same way that the metal bonds to itself. Adjacent atoms are sharing their electrons with one another.

author: walshms
author_rank: Regular Contributor
timestamp: October 24, 2013, 11:16:05 pm
content: Quote from: AG6QR on October 24, 2013, 10:43:37 pmWell... sort of. I think what you really have is a lot of relatively free (as in not tightly bound) valence electrons that can jump around from atom to atom with minimal electromotive force.QuoteCorrect on needing to remove the oxide layer, but that's probably because the oxides generally are insulators because the electrons are much more tightly bound to their respective atoms. That also impedes the alloying reaction you're looking for in soldering.QuoteIf that were true, then simply touching solder to something without heat would cause that to occur; that doesn't happen. You wouldn't need to solder at all; just put a surface mount resistor on a pad and it would stick if electron bonding alone were in play. A good resource to understand this would be Donald Sadoway's lectures on solid state chemistry at MIT's Open Courseware site (ocw.mit.edu). Let me see if there's a YouTube copy... yep.. Right here:

author: walshms
author_rank: Regular Contributor
timestamp: October 24, 2013, 11:27:16 pm
content: A more direct link, to a more recent lecture, specifically on bonding:http://ocw.mit.edu/courses/materials-science-and-engineering/3-091sc-introduction-to-solid-state-chemistry-fall-2010/bonding-and-molecules/8.-ionic-crystals-born-haber-cycle/

author: walshms
author_rank: Regular Contributor
timestamp: October 25, 2013, 12:23:29 am
content: And the lecture on metallic solids here:http://ocw.mit.edu/courses/materials-science-and-engineering/3-091sc-introduction-to-solid-state-chemistry-fall-2010/electronic-materials/13-band-theory-of-solids/

author: rexxar
author_rank: Frequent Contributor
timestamp: October 25, 2013, 01:44:48 am
content: Wow, you guys have posted some great info! Now that I think about it, it does seem obvious that it forms an alloy with the metal you're soldering to.

author: Stonent
author_rank: Super Contributor
timestamp: October 25, 2013, 02:01:45 am
content: Sometimes the two metals that you are trying to alloy together do not get along, like this example:

author: G7PSK
author_rank: Super Contributor
timestamp: October 25, 2013, 09:33:49 pm
content: As well as alloying with the copper lead expands a bit on solidifying so will tend to push anything inside a blob of solder together, thats why lead was used to fix iron railings into stone walls at one time.

author: grumpydoc
author_rank: Super Contributor
timestamp: October 25, 2013, 09:46:43 pm
content: QuoteSpontaneous bonding doesn't occur when you do that because of the oxide layer on the surface and the fact that the surface irregularities are huge on the atomic scale so you don't get the surfaces close enough together to bond.In theory if you machined two metal surfaces sufficiently flat and stripped the oxide layer then bringing them together should actually bond.

author: Mr Pig
author_rank: Contributor
timestamp: October 25, 2013, 09:55:20 pm
content: Quote from: rexxar on October 25, 2013, 01:44:48 amSeconded. I watched all of the videos in the series, really clear and instructive. Hopefully my soldering will be less rubbish now.

author: walshms
author_rank: Regular Contributor
timestamp: October 26, 2013, 02:15:03 am
content: In theory... but in practical application, it will probably never happen. At least not during our lifetimes.

author: sub
author_rank: Regular Contributor
timestamp: October 26, 2013, 11:50:44 pm
content: Quote from: grumpydoc on October 25, 2013, 09:46:43 pmCold welding would be an example of this.

author: ivan747
author_rank: Super Contributor
timestamp: October 27, 2013, 12:43:21 am
content: Quote from: Fsck on October 24, 2013, 08:28:31 pmdon't.Unless you have a good web filter.

author: Fsck
author_rank: Super Contributor
timestamp: October 27, 2013, 01:13:35 am
content: Quote from: ivan747 on October 27, 2013, 12:43:21 amsolder wets by embedding itself in the topological imperfections and later forms an alloy when solidgoogle's filter is your friend. (I just googled it and I only get the chemical definitions)

author: John Coloccia
author_rank: Super Contributor
timestamp: October 27, 2013, 01:39:18 am
content: Quote from: grumpydoc on October 25, 2013, 09:46:43 pmOr just use pressure. It's well documented that a proper wire wrap and proper crimp causes cold welding between the parts. There's enough pressure to mush the oxide out of the way and to bring the base metals into contact, welding them together.

author: Mr Pig
author_rank: Contributor
timestamp: October 27, 2013, 09:29:25 am
content: Quote from: John Coloccia on October 27, 2013, 01:39:18 amI think that's best left to the industry pros. If you don't know what you're doing you could apply too much pressure and damage the structure of the materials or not enough and get a poor connection. Like you say, works great if you do it right though.

author: John Coloccia
author_rank: Super Contributor
timestamp: October 27, 2013, 11:51:22 am
content: Quote from: Mr Pig on October 27, 2013, 09:29:25 am??Electrical crimps and wire wrap is best left to pros? This is normal, everyday stuff.

author: Mr Pig
author_rank: Contributor
timestamp: October 27, 2013, 01:09:29 pm
content: QuoteSure, but I think it's easier to get a reliable joint with solder than crimping if you have the choice, that's all. Nothing wrong with a good crimped joint.Last year my son did some Saturday work in a Hi-Fi shop. One of the things they had him doing was making up speaker cables including crimping the plugs on the end. I've used crimped speaker plugs before. It can be hard to be sure you've got it right and over time they can oxidise and degrade.Inside our shower are about six micro-switches and it kept eating them! Seemed like every other week some function of the shower would pack in and every time it was a bad micro-switch. The cables were connected to the switches with 4mm spades, which were crimped onto the cables. Over time the cables had oxidised in the joint and, although they were still working, the resistance had gone up, the joint was heating up and the heat was cooking the micro-switches! I soldered up the joints and its not killed a switch since.I'm not saying that crimped connections don't work, of course they do, but I think it's easier to be confident that you've got a good, solid connection if you solder instead. If you've got the option.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 27, 2013, 05:16:28 pm
content: try wetting, fluxing or whatever for solder into aluminum or somekind of cast iron, good fun!

author: SeanB
author_rank: Super Contributor
timestamp: October 27, 2013, 07:19:58 pm
content: Silver solder..........

author: rexxar
author_rank: Frequent Contributor
timestamp: October 28, 2013, 04:27:44 am
content: Quote from: Mr Pig on October 27, 2013, 01:09:29 pmWhat kind of shower has six microswitches in it?

author: Mr Pig
author_rank: Contributor
timestamp: October 28, 2013, 05:06:24 pm
content: Quote from: rexxar on October 28, 2013, 04:27:44 amIt's a Mira Supreme 9Kw electric shower. Thing is ancient but I'm very reluctant to replace it, as I tiled around it! ;0)It's a very old, simple shower. The current for the heater tank goes through two 20-amp microswitches. I have replaced every working part in the shower except the inlet valve and the wires! The trickiest thing I had to do was repair the water pressure sensor. The diaphragm perished and it's a sealed unit. The part was no longer available so I had no choice but to prise open the casing, make a new diaphragm and close it back up. Worked fine :0)I'll keep it going as long as I can as I'll have to re-do the whole shower cubicle if I ever were to replace it.

author: GreyWoolfe
author_rank: Unknown
timestamp: October 29, 2013, 02:37:58 am
content: I am going to go with walshms's explanation. Now I know why hot solder sticks to my bare leg when I am soldering in shorts!Tom

author: IanB
author_rank: Super Contributor
timestamp: October 29, 2013, 02:45:48 am
content: I do know there is no vocabulary adequate for the feeling of hot molten solder landing on your bare flesh, especially if you tip over a pool of the stuff!

author: Fsck
author_rank: Super Contributor
timestamp: October 29, 2013, 02:56:18 am
content: Quote from: IanB on October 29, 2013, 02:45:48 amin english, it's a vocabulary built almost exclusively with 4 letter words

author: SeanB
author_rank: Super Contributor
timestamp: October 29, 2013, 06:11:27 pm
content: Quote from: GreyWoolfe on October 29, 2013, 02:37:58 amDo not try welding using Aussie safety shoes...............