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Soldering wire is very soft and pliable, but solder on a circuit board is hard. Why? I haven't been able to find a definitive answer, but some ideas that come to mind are:

  • Some kind of chemical reaction that takes place when the solder is heated and then cools. If so, what is this reaction? Maybe with flux reacts somehow, but what about solder that doesn't have flux?

  • The solder wire is less dense, either through being hollow or having a flux core, making it seem easier to bend. This seems less likely, because tin whiskers seems much harder than solder wire despite being thinner.

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    \$\begingroup\$ Whiskers are much harder and stiffer. Whiskers are single-crystal structures. Soldering wire is polycrystalline. Solder on a circuit board is polychrstalline, but where the physical size is comparable to the crystal size, the bulk properties are highly dependant on the physical arrangment. \$\endgroup\$ – david Apr 22 '15 at 4:54
  • \$\begingroup\$ The solder on a PCB shouldn't be any different to the solder 'wire' it came from unless that PCB has been subjected to some extreme conditions and the solder has "dealloyed". \$\endgroup\$ – brhans Apr 22 '15 at 12:40
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    \$\begingroup\$ You should probably ask this again on the physics and/or chem SE site. The answers below, while heavily upvoted, are seat-of-the-pants. I can't claim a deep understanding of metallurgy, but do note that solder forms an alloy with Cu at the surface. That's why it sticks to it. The other issue of [re]crystallization etc., I don't know much about. What I can tell you for sure is that why solder does or doesn't get harder when [re]melted isn't a topic normally taught in an EE class. \$\endgroup\$ – Fizz Apr 23 '15 at 19:22
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    \$\begingroup\$ Doesn't anyone have a hardness tester handy? \$\endgroup\$ – Spehro Pefhany Oct 20 '16 at 13:01
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@Kaz & @LongStrokinYerMomma are near to the right explanation.

When you talk about mechanical properties of a metal/alloy we have to consider lattice structures. And in this case not much chemical reaction is of our concern.

You see, two phenomena are responsible for this observation:

1. Re-crystallization

Ability of metal/alloy to be drawn into wire is called ductility. When solder wire's billet is drawn through various dies of reducing diameters - it undergoes a process called strain hardening which makes it more resilient (i.e. to bend repeatedly without fracturing easily) to shear/deforming forces as compared to the initial cubical billet of same alloy. Hence when you melt it, it loses strain hardening & undergoes re-crystallization which makes it appear more brittle.

2. Perfection of lattice structure

Diamond is the hardest material, not just because of its bonds but because of its perfect lattice structure. If you compare degree of perfection of lattices PER unit mass of a small cube, say 1mm3 & a large cube, say 20mm3 of chemically identical alloy/metal/mix, you will find the smaller cube to be more perfect hence stronger/harder than the larger cube, even though their chemical compositions are exactly same (this is what user @LongStrokinYerMomma pointed out in his Abstract from that paper)

To get a simpler everyday feel of it, think of breaking a stick, you can easily break a 2-feet long stick but not 10-cm long stick, yes in this case lever-action/torque-arm plays a role, but you get the idea.


Your logic:

The solder wire is less dense, either through being hollow or having a flux core, making it seem easier to bend. This seems less likely, because tin whiskers seems much harder than solder wire despite being thinner.

is perfectly valid, it partly explains why soldering wire is pliable. But note that the assertion Solder on a circuit board is just as soft as the wire solder it came from is definitely incorrect.

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    \$\begingroup\$ You seem to be onto something (unlike top-voted answer here). For instance, the book by Tu Solder Joint Technology: Materials, Properties, and Reliability has a chapter on ductile to brittle transition in solder, toward the end of the book. This is after the more commonly known stuff on the alloys solder forms etc. The copper-tin reaction does appear to be a factor in the said transition. However, some parts of your post make rather tenuous inferences based on analogies (with the diamond etc.) that may not actually hold. \$\endgroup\$ – Fizz Apr 23 '15 at 19:38
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Solder on a circuit board is just as soft as the wire solder it came from, since it's the same material. However, the wire solder is not supported by anything, so feels much more bendable. Note that softness isn't the same as bendableness. Wire solder can also feel softer to something like pinching with your fingernails because most solder is hollow with a soft flux core, and you are collapsing it by pinching.

Solder on a circuit board is usually a thin layer that is well supported by a the board itself thru a thin layer of copper, and pins of whatever device is being soldered down. This makes it feel a lot harder than the unsupported wire.

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    \$\begingroup\$ Same material... so is pencil lead, charcoal, and diamonds, but they are not equally hard/soft. And that's not even getting into cold work or annealing of materials like copper. \$\endgroup\$ – Passerby Apr 22 '15 at 2:23
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    \$\begingroup\$ And solder that has been cut off from underlying support like copper is still harder than solder before it is melted. And op made a point about tin whiskers. \$\endgroup\$ – Passerby Apr 22 '15 at 2:27
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    \$\begingroup\$ @Passerby Charcoal and diamonds are not the same material, even though one can be made by rearranging the atoms in the other. \$\endgroup\$ – immibis Apr 22 '15 at 6:53
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    \$\begingroup\$ @Passerby: I just did an experiment and cut 2cm of thick 60/38/2 solder from a heavy current trace on my board, formed it roughly to 1mm thickness, put that one and one 1mm thick solder of the same type (should be even the same roll it came from) into my 3rd hand, hung a fat power transistor onto it, and both bent down like wet spaghetti. No surprise to me as I would guess in the factory to form the alloy the solder was melted once too. \$\endgroup\$ – PlasmaHH Apr 22 '15 at 8:30
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    \$\begingroup\$ @Passe: Charcoal and diamond contain the same atoms, but they are very different materials. How the atoms are arranged matters. In the case of solder, both are frozen the wire and a blob on a board are the same frozen alloy of tin and lead. There is will be some difference in hardness due to how exactly it was melted and worked, but both are still basically the same material, unlike graphite, diamond, and various fullerines all made from carbon atoms. \$\endgroup\$ – Olin Lathrop Apr 22 '15 at 15:09
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It's all about shape. A small bead of nylon is hard. A nylon fiber (such as a fishing line) is flexible. Ditto for glass and other materials. Glass can be a rigid crystal ball, a somewhat flexible window pane, cloth, or soft and fluffy insulation in your walls.

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There is one more thing I do not see in the answers:

Most solder on reels has the flux in the core. This flux can be as much as 45% of the solder wire by mass, and is burned away as part of the soldering operation. The flux is far more flexible than metals, so the real amount of metal in the solder wire is actually less than the base weight, thus making the overall wire more flexible.

The purpose of the flux is to clean the surfaces to be soldered and is the substance we see burning off during soldering.

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I'm going to go out on a limb here and say that there is a basic relationship between the solder's metallic crystal structure and it's mechanical performance. This paper says that:

ABSTRACT

With the continuing increase of the integration density in electronics, dimensions of interconnections for electronic components have been miniaturized to a scale that is comparable to those of their crystallographic structure. For instance, a SnAgCu solder joint in the flip chip package can contain only one or a few grains. In this case, the mechanical behaviour of the micro-joint is expected to shift from a polycrystalline-based to single-crystal one.

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  • \$\begingroup\$ Welcome to EE:SE! It's a good idea to add some information from any links you post here so that your answer is still useful if (when) the link dies. I've suggested an edit to add part of the abstract to your answer - feel free to edit it again if you think you have a better excerpt. \$\endgroup\$ – Greg d'Eon Apr 22 '15 at 2:17

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