Why does Solder have a lower melting point than the parent metals

[JoeMK]

Is there a reason as to why an Alloy can have such a different melting point to its composing metals?

[YT2095]

damn good question!

 

one I`de like to know the answer to also

 

 

I suspect, but don`t know, that it`s something to do with the molecules of each metal not being Bonded chemicaly and so are easy to move around with the application of heat, it`s ONLY a guess though!

 

a bit like the difference between Dry sand and Wet sand in the wind, the wet sand has bonding energy (if that makes sense?).

[doomtiki]

According to the Wikipedia solder has a melting point of about 450 C and is made of mostly tin and lead . Tin mealts at 239.93 C. Lead mealts at 327.461 C.

[Thomas Kirby]

This article says that eutectic point solder, 63 percent tin, 37 percent lead, melts at 361 degrees Fahrenheit, which is 182 decrees Centigrade. That mixture melts completely at its melting point instead of having a phase where it is pasty, like 60/40 and 50/50 solder do.

 

The Wikipedia article describes the broader class of alloys that are called solders as having melting points up to about 450 C.

 

If it is true that solder joints are weaker with lead-free solder, if I had the choice I would balk at demands to use lead-free solder. Too many safety-critical systems depend on electronics. Anything that increases their failure rate can cause more danger to the public safety than the lead in the circuit boards. Solder joints fail all the time. They will fail more often if they are mechanically weaker.

[Lance]

You can make alloys with a melting point that is below room temperature from bismuth, indium, and gallium; none of these metals are liquid at room temperature.

[Bluenoise]

It's because the impurities disrupt the chrystal lattices of the parent materials.

 

So yeah YT you were basically right.

[Bluenoise]

According to the Wikipedia solder has a melting point of about 450 C and is made of mostly tin and lead[/url'] . Tin mealts at 239.93 C. Lead mealts at 327.461 C.

 

It says that solders have a melting point below 450. Those with the higher melting points aren't the typicaly solder that most people think of. They are not Lead/Tin alloys.

 

And then it goes on to say that typical electrical solder ( Lead/ Tin) has one below 190c Which is far lower then both lead or tin.

[JoeMK]

damn good question!

 

one I`de like to know the answer to also

 

 

I suspect' date=' but don`t know, that it`s something to do with the molecules of each metal not being Bonded chemicaly and so are easy to move around with the application of heat, it`s ONLY a guess though!

 

a bit like the difference between Dry sand and Wet sand in the wind, the wet sand has bonding energy (if that makes sense?).[/quote']

 

It's because the impurities disrupt the chrystal lattices of the parent materials.

 

So yeah YT you were basically right.

 

Thanks Guys, much apreciated

[H2SO4]

This article says that eutectic point solder' date=' 63 percent tin, 37 percent lead, melts at 361 degrees Fahrenheit, which is 182 decrees Centigrade. That mixture melts completely at its melting point instead of having a phase where it is pasty, like 60/40 and 50/50 solder do.

 

The Wikipedia article describes the broader class of alloys that are called solders as having melting points up to about 450 C.

 

If it is true that solder joints are weaker with lead-free solder, if I had the choice I would balk at demands to use lead-free solder. Too many safety-critical systems depend on electronics. Anything that increases their failure rate can cause more danger to the public safety than the lead in the circuit boards. Solder joints fail all the time. They will fail more often if they are mechanically weaker.[/quote']

 

 

not to be boasting, but ive done alot of soldering (not electronics, but mecanical, such as wierd things and metal art) and ive found that lead solder is crap. Its weak, poisonous, soft, and has a higher surface tension than all the other lead free things ive used. The ultimate solder ive found is silver solder (tin 96%; silver 3%; copper 1%)

[gnpatterson]

I believe that idea of disrupting the crystal lattice is in fact more subtle in that it is really only properly explained if a discussion of the entropic principle is included

 

in the eutectic alloy a highly ordered structure is obtained as the solid phase and the contribution of this order/disorder to the transistion in the mixture is the important factor.

 

the question is if you are a liquid mixture of 90%tin and 10%lead and your are being cooled what do to accomodate this. the answer is that you separate into some impure tin crytals and a liquid mixture of tin and lead that maximises the entropy of the whole system.

 

the concept of bond energies arent as important as the amount of entropy given to the liquid by being more disorganised.

[mmalluck]

not to be boasting, but ive done alot of soldering (not electronics, but mecanical, such as wierd things and metal art) and ive found that lead solder is crap. Its weak, poisonous, soft, and has a higher surface tension than all the other lead free things ive used. The ultimate solder ive found is silver solder (tin 96%; silver 3%; copper 1%)

 

Silver solder is the king of pipe joints, but the higher melting tempreature (430 degrees to over 1300 degrees F) make it rather prohibitive to use with electronics.

 

All those little junctions in semiconductor chips don't like heat too much. The less heat you use, the less chance you'll cook the chip.

[BigMoosie]

Are there any liquid metals that are safe to handle without gloves?

[Primarygun]

Are there any liquid metals that are safe to handle without gloves?

To melt a metal, lots of heat energy is required to overcome the strong metallic bonds.

Hence, a high temperature needed to be achieved. Though mercury has a low melting point, it is very toxic.

I don't think we can handle any liquid metals in that mean.

[YT2095]

Galium will melt at hand temperature, and is also non toxic.

[BigMoosie]

Galium will melt at hand temperature, and is also non toxic.

 

I so gotta ge me somma that!

[YT2095]

I`m sure one or 2 of the lads on here have a few samples that they would probably swap with you for something of interest, I`ve only got a few grams left myself, I keep using it all

[BigMoosie]

Wow, people actually trade chemicals here? I dont posess any chemicals though, none that aren't readily availible. Where would I find some gallium, other than by breaking apart several hundred LEDs?

[YT2095]

Ebay or maybe United nuclear, Or someone from here, I think quite have a little bit of it and would probably break some off for ya

and yes many of us swap chems and electronic components etc... mostly in our IRC chatroom though.

 

tell ya what, I`ll ask a mate, and come back and re-edit this with an answer....

 

edit: even better he has an alloy that`s liquid at Room temp! send Lance a PM and ask him about it, he`ll sell ya some

[BigMoosie]

OMFG, I must persue this. Thanks for that YT!

[DQW]

Why does Solder have a lower melting point than the parent metals ?

 

Is there a reason as to why an Alloy can have such a different melting point to its composing metals?

I'll go through something of an introduction before I attempt to answer the question itself.

 

If two substances melt at temperatures [imath]T_A[/imath] and [imath]T_B[/imath] one can only expect their mixture to melt at the exact same above temperatures (notice that I'm saying that a thing melts at two temperatures) if the the two components do not interact with each other in any way. For instance' date=' consider a misture of snow and gallium pellets. If you slowly heat this mixture up, the snow (ice) will melt at about 0C, and from then on, up to about 30C you will have a mixture of solid gallium in water, at which point the gallium will melt, giving rise to a liquid-liquid mixture. No matter whether this mixture is 90% snow or 90% gallium, you will see essentially this same behavior. This mixture clearly has two melting points - one where the first component melts (often called a solidus temperature or curve) and another where the second does (called the liquidus temperature or curve). Between these temperatures, there's a solid-liquid mixture.

 

An alloy is nothing but an intimate mixture between metals (as primary components). In this context, there are two (pertinent) different kinds of alloys : (i) isomorphous alloys or solutions, and (ii) eutectic alloys.

 

An isomorphous solution is a substitutional alloy formed between metals with similar crystal structures(lattice parameter and symmetry) (eg : Cu-Ni or Ag-Au alloys). In an alloy of say, 70%Cu-30%Ni, the Ni atoms occupy exactly the lattice positions of the Cu atoms - hence the term 'substitutional'. In such alloys the liquidus temperature and solidus temperatures often lie between the two melting points and are a function of the composition because the components are intimately interacting with each other. An isomorphous alloy is what one may refer to as a "homogeneous mixture" or a solid "solution".

[img']http://www.ce.berkeley.edu/~paulmont/CE60/alloys/img003.gif[/img]

 

 

 

Solder is a eutectic alloy, not an isomorphous solution. A eutectic is formed between metals of very different latice parameters and crystal structures (eg : Sn-Pb or Ag-Cu). A eutectic alloy is called a two-phase alloy because if you look at it under a microscope, you can distinguish two different types of regions each with its own composition and crystal structure. Each of these phases is in fact, just an isomorphous alloy. So here's what happens when I try to alloy Pb with Sn : I start with pure Sn and slowly add Pb to it. At very low concentrations of Pb, the Pb atoms actually occupy the positions of the Sn atoms in the Sn lattice. But Pb atoms being larger than Sn atoms, this will strain the lattice locally. Beyond a certain concentration, it becomes infeasible to substitute the bulky Pb atoms in the spaces meant for Sn. Instead, it becomes easier to make a new phase which is predominantly Pb, with a small portion of the Sn atoms substituting for the Pb. So, a eutectic alloy consists of two distincet phases - each phase rich in one of the components. But each of these phases is now strained because of the distortion resulting from different radii. This strain causes a lowering of the melting temperatures. In addition to the strain, there is also a lowering that results from the poor bonding between the two phases. If the alloy is predominantly one of the two phases (say 85%A-15%B or 15%a-85%B) there will not be much lowering due to poor interphase bonding because there is so little of one phase (ie: the alooy is mostly single-phase). While the first factor plays a dominant role near the extreme compositions, the second factor is what is important at intermediate compositions. Among the various compositions that can be made between Sn and Pb, the one with the lowest melting point is called the eutectic composition (62%Sn-38%Pb).

[DQW]

Where would I find some gallium, other than by breaking apart several hundred LEDs?

You will not find any gallium in an LED - only gallium nitride, which is a hard ceramic that melts at 2500C or about 4500F.

 

For gallium, see this thread (particularly posts #26, #34 and definitely, #63) : http://www.scienceforums.net/forums/showthread.php?t=9680

[H2SO4]

and thats only in certain LED's, right?

[Lance]

and thats only in certain LED's, right?

 

Blue. I thought it was gallium arsenide though.

[DQW]

Blue. I thought it was gallium arsenide though.

GaN has a blue photoemission peak, so yes, it is used to make blue LEDs.

 

GaAs is used in high frequency devices like CD/DVD players (due to much higher mobilities than Si) and in heterostructures, for all kinds of fundamental and applied research.

[d22k]

Gallium Here: https://secure.avdns7.com/~scitoys/cgi-bin/shop.exe?page=metal_desc.html&QX=0a&SID=3802959333&something_to_remember_me_by_an_bye_and_bye&

 

may i also suggest that you browse the whole site, its fascinating if you are new to magnets and electrikery =)