Why solder becomes dull? What does it really mean internally, chemically?

Thread Starter

rambomhtri

Joined Nov 9, 2015
520
No, I am not talking about Lead free vs Leaded solder. I am talking about the very same solder behaving different.

I got my solder wire (Sn38Pb60Cu2 + rosin core) fresh new, put the tip of my iron at 340ºC and I heat the solder (and the parts where it's going to go) to melt it nicely and quickly. If I do the job in less than 7s I would say, the solder ends up shiny and nice. All fantastic.

However, if I keep "playing" with it, melt it down, let it cool, melt it again because I did something wrong or need to adjust one leg or pin or the cable is not fully inserted or whatever, there's a certain point at which the finish starts to go dull. Then, no matter what I do, it says dull forever unless I replace that solder by new one.

My question is...
1. Why does this happen? The Sn + Pb + Cu don't evaporate, and the only thing smoking (evaporating) is the rosin I guess.
I've thought several times may be it's the rosin, but then I add to the dull solder a lot of rosin (flux) and no matter what, it stays dull.
2. Why does it become dull? What it means, what's really going on chemically or physically to the solder?
Since the composition is the same alloy except for the rosin part, but adding it does not change anything.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
520

SamR

Joined Mar 19, 2019
4,548
Metals also oxidize faster when hot and repeated heating speeds up the process even more so by prolonging the exposure. Flux acts as a shield to oxidation so burning it off also exacerbates the situation.
 

MisterBill2

Joined Jan 23, 2018
14,244
Chemists know that for every ten degrees "C" the temperature is raised, the chemical reactivity doubles. That applies also to oxidizing the surface of solder. So that is part of your answer. The other part is that no matter what the label states, some solder brands are not as good as others. I am describing the alloy, not the flux or the spool or the package it all comes in.
 

dl324

Joined Mar 30, 2015
15,511
If I add fresh new flux, it doesn't go from dull to shiny. So I don't think the answer is that simple.
Dull solder doesn't necessarily mean the joint is bad. If, upon close inspection, you find that the joint is bad, the only way to get a shiny joint is to remove the solder and apply more.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
520
Dull solder doesn't necessarily mean the joint is bad. If, upon close inspection, you find that the joint is bad, the only way to get a shiny joint is to remove the solder and apply more.
Yeah, never claimed dull equals bad, I was just shocked about why it happened.

So it's basically oxidation?
That's why if you heat and play with shiny solder, after a few seconds it becomes dull and has no solution?

That would make sense, since adding flux won't remove the oxidation. So the next question then is... why, instead of flux that only prevents oxidation if correctly applied and used, we don't pour some other gel or liquid that not only prevents oxidation but de-oxidizes the solder?

May be something like that doesn't exist?
 

MrSalts

Joined Apr 2, 2020
2,767
Chemists know that for every ten degrees "C" the temperature is raised, the chemical reactivity doubles. That applies also to oxidizing the surface of solder. So that is part of your answer. The other part is that no matter what the label states, some solder brands are not as good as others. I am describing the alloy, not the flux or the spool or the package it all comes in.
Chemists also know the Q10 Law that you reference is generally true (but not always true) for organic chemical reactions (carbon-containing molecules in solution - like biological systems). A 10-degree change will get much less predictable results for a metal-gas reaction like the oxygen oxidation of a metal surface. In general, a reaction happens more quickly but, some reactions happen slower with higher temps. It is a dangerous game to apply "rules of thumb" from biology class to inorganic chemistry and metallurgy.
 

MisterBill2

Joined Jan 23, 2018
14,244
Nothing is done to prevent the surface oxidizing because it does no harm. After the oxide layer forms it usually stops oxidizing. And so the reaction is faster but not always doubled.
I have repaired equipment where most of the solder on the PCB had oxidized and many of the the connections had failed. A clean iron and good solder and in very few minutes all new connections were made, and the equipment suddenly worked well again.
And OK on the Q10 rule applying reliably to only organic reactions. Organic was not required for EEs.
 

MrSalts

Joined Apr 2, 2020
2,767
No, I am not talking about Lead free vs Leaded solder. I am talking about the very same solder behaving different.

I got my solder wire (Sn38Pb60Cu2 + rosin core) fresh new, put the tip of my iron at 340ºC and I heat the solder (and the parts where it's going to go) to melt it nicely and quickly. If I do the job in less than 7s I would say, the solder ends up shiny and nice. All fantastic.

However, if I keep "playing" with it, melt it down, let it cool, melt it again because I did something wrong or need to adjust one leg or pin or the cable is not fully inserted or whatever, there's a certain point at which the finish starts to go dull. Then, no matter what I do, it says dull forever unless I replace that solder by new one.

My question is...
1. Why does this happen? The Sn + Pb + Cu don't evaporate, and the only thing smoking (evaporating) is the rosin I guess.
I've thought several times may be it's the rosin, but then I add to the dull solder a lot of rosin (flux) and no matter what, it stays dull.
2. Why does it become dull? What it means, what's really going on chemically or physically to the solder?
Since the composition is the same alloy except for the rosin part, but adding it does not change anything.
As said above, something is wrong it it takes 7-seconds to melt enough solder to make your joint. Either you are using some very fat solder vs the size of your solder tip or your iron isn't really really appropriately powered for the job.
 

MisterBill2

Joined Jan 23, 2018
14,244
Either the solder is not making adequate contact with the hot part of the iron, or the solder alloy is not uniform. Or rgere may be particles of contaminant on the iron tip.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
520
No, it normally doesn't take me to solder something 7 seconds at all, I was simply setting a number at which point a perfectly shiny joint goes dull. If you read, I said "if I play with the solder", meaning I am purposely trying to make it dull to check why and how it happens.

However, there are some soldering jobs that are harder than others, not every soldering job is an open clean PCB with one medium nice leg sticking out. Sometimes you solder a tiny cable and are not happy with the angle or position of the cable, or it moved while cooling, or it's in a crowded area where it's really hard to work and you have to re do it, and if you sum all the time it gets to those 7s and the joint becomes dull.

Anyways, answer is, correct me if I'm wrong:

Let's use a Pb60Sn40 solder, no flux core.

1. If you just melt it very quickly, with the lowest temperature possible, that doesn't give you a cold joint, in a very short period of time and do the joint nicely, you will get a shiny surface because all went great and fast without oxidation.

2. However, flux is added to help with a less efficient job, like heating a bit too much and taking a bit more time. Flux is added so people not working perfectly get basically the same result as point 1.

3. If you take way too long (7s or more), or heat it too much, even with flux, the solder will oxidize and then you have no more Pb60Sn40, but a mix of that plus SnO² or PbO or PbO². I don't know the exact chemistry but I guess at least one of those oxides will form and is the responsible of the dull surface.

Then it makes sense why I am not able to make it shiny again even adding flux: those oxides are there forever and the only way to make the joint shiny again is removing those oxides or put some new solder in the joint to "camouflage" the oxides (like adding clear water to some red tinted water to make it transparent again)

I guess that's the official explanation.
 

Ya’akov

Joined Jan 27, 2019
7,027
Flux is not a substitute for good soldering technique or conditions, it is good soldering technique and conditions.

Flux dissolves the oxide layer on the surface of the pieces to be joined by the solder joint. It also creates an oxygen free environment to make the joint which is needed because the oxides will immediately form again if you remove then in air.

As far as a joint becoming ”dull”, that’s not descriptive enough. Most of the time a ”dull” joint is a cold joint—at least if you are not using lead free solder which never gets the nice shiny appearance. Despite its name, a cold joint isn‘t about the temperature used to make it. Instead it is a joint that was mechanically disturbed while the solder was in its plastic range.

The solder is plastic when it can be moved but doesn‘t behave as a liquid. Sn60/Pb40 solder has a plastic range of about 5℃ after melting at 188℃. From ~187℃ down to ~183℃ Sn60/Pb40 solder will be neither fully liquid nor fully solid. If the joint is disturbed during this time it will be made “cold”, and is mechanically and electrically unreliable.

However, if the joint is not disturbed, but spends a long time in the plastic range as it is cooling—a highly heated or even overheated joint, one that is made into a large thermal mass, or the like—it will get a dull surface appearance. This is not significant chemically, mechanically, or electrically. It is purely aesthetic to prefer the shiny result to the dull one.

That is, in those case where you are convinced of the integrity of the soldering process and know the joint was not disturbed Because a dull join may not signal trouble but a shiny joint is a guarantee that the trouble hasn’t occurred, when hand soldering reflowing dull joints is a good practice.

Aside from the eutectic alloy’s lowest possible melting point, Sn63/Pb37 solder has the advantage of no plastic range. It goes from liquid to solid directly. This makes cold joints impossible.

So, don’t worry about the dull appearance of an otherwise good solder joint, always use flux (make it a no clean variety), and use Sn63/Pb37 for hand soldering. Sn60/Pb40 does have a better wetting action, that is, it forms the alloy between the solder and the workpiece that makes soldering what it is more readily. So, for tinning leads, traces, etc. Sn60/Pb40 is actually preferable. But that’s a special application, the eutectic alloy wins the hand soldering game.
 

dl324

Joined Mar 30, 2015
15,511
Let's use a Pb60Sn40 solder, no flux core.
60/40 solder is more likely yield joints that aren't shiny, even without your repeated working of the joint. Eutectic solder melts and solidifies at a single temperature.

Don't recall ever seeing solder that didn't have a flux core...
 

MisterBill2

Joined Jan 23, 2018
14,244
I have seen and used non-flux-core solder.It is sort of OK with tinned PCBs and parts. Also, not all consumer electronics is built with good solder. As an example I was given a Kenwood brand stereo amplifier that "just quit" working. No service information was available, in fact Kenwood did not even acknowledge the model number. I removed the bottom cover and saw that most of the solder joints were a dark gray and not at all reflective. A complete resolder job on every connection and it plays perfectly. It took much longer to gain complete access than it took to resolder. And many of the solder joints had mechanically faild completely. So certainly there is such a thing as poor quality solder.
 

Lo_volt

Joined Apr 3, 2014
286
I'll note that if the solder joint is not stationary, it won't turn out shiny. In other words, if I'm soldering two wires together and I don't hold them completely still, I'll get a dull solder joint. This is especially true if I've burned off all of the flux. The same is true when I solder wires to a PC board. Generally, if I reheat, with flux, and let it cool without wiggling anything, the joint stays shiny.
 
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