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


Joined Mar 13, 2020
Check my point 1. If I add fresh new flux, it doesn't go from dull to shiny. So I don't think the answer is that simple.
Generally it should. If it's not - it may be the wrong type of flux. Without knowing what flux you're talking about it's not possible to give you a definitive answer. [edit] let me ask a stupid question and please don't assume I'm thinking you're not that smart - but after adding flux - you ARE re-heating the joint; right? [end edit]
oxidizing the surface of solder.
This is the #1 reason for using a flux core solder. Adding additional flux won't hurt. Again, as long as it is the right type.
I make 7 joints in 7 seconds.
Even that is slow. But depending on the conditions, materials and consumables - - - at a very least it shouldn't take more than one second for a typical hobby joint.
something is wrong it it takes 7-seconds to melt enough solder to make your joint.
I know you said this was an example of trying to make a joint look dull, but this can also be the result of soldering on a board with large ground and power planes. The board draws so much heat away from the solder joint it's impossible to solder without pre-heating the board. Since you're talking about forcing the issue - I don't think you're working on ground or power planes.
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.
I've seen this time and time again where solder, before it freezes, it's moved, resulting in a grainy and dull appearance.

Another reason why a joint could look poor is if the lead to hole (assuming through hole joinery) is the wrong size hole for the lead. Putting a thin 0.045" lead into a 0.125" hole results in a lot of solder needed for the joint. More solder means more time the process will take. It'll also take longer to cool, allowing time for oxidation.

Solder in its solid form is literally frozen solder. Frozen doesn't mean cold to the touch such as ice or snow. Stones are frozen even in the Sahara. Lava is stone in liquid form. Crust is that point where the surface has cooled sufficiently to begin solidifying. I don't know the temperature range of lava so I can't speak with any authority on the subject. And we're not discussing lava. Nevertheless, solder has three states; solid (frozen), plastic (becoming liquid but not yet flowing) and liquid. Not certain but I think there may be a solder chemistry that literally has no plastic state. But I can't think of any examples of that being a certainty. It's just something I believe I've heard.

After 7 seconds oxidation can be so overwhelming that ordinary fluxes can not restore its shiny appearance. While some may recommend removing and applying new, I've found applying just a very tiny amount of new solder (FLUX CORE) will restore a shiny appearance.

Other things that can bother the appearance of the surface of the solder can be outgassing which is caused by a plated through hole (PTH) who's wall may have glass inclusions, thus allowing gasses from the heated PCB to blow out through the hole. I've seen PTH's with glass inclusions that got copper plated. When the lead is pushed through the PTH the glass breaks - opening a pathway for outgassing. Outgassing can also be caused by other contaminants within the PTH itself or on the lead.

As for what it means chemically - I'm not a chemist and can't answer that part. Summary: Wrong flux, insufficient flux, oxidation, plastic (disturbed) solder, contamination in the hole, lead or even on the solder tip (though the last is rare). One other possibility is that the solder itself may be old. Working in the defense electronics arena solder had an expiration date at which point I'd end up with tons of free solder that otherwise would be tossed into the landfill. Though expired, it's still very useful in hobby electronics. I'd assume an expiration date is the same for medical device manufacturers and other arenas where life/mission critical are serious concern.

Thread Starter


Joined Nov 9, 2015
Okay... this thread is about discussing why dull surface appears. It is not intended to be a guide to solder correctly. Keep this in mind, so now you understand why:

In my example I am talking about solder without flux core. It's crucial to separate as many variables as possible so I can understand what's going on and why.

I am assuming that in lab conditions, I can solder without flux and get a perfectly nice joint, shiny. May be I am wrong and flux is a must always, even in perfect condition, but I was thinking it's not.

As I believe, since we can't work like robots and control perfectly the temperature of the solder, flux helps a lot in this area, as it allows way more "play" to exist instead of very tight time and temperature windows, for a perfect joint.

You know, solder melts at 180ºC but you set your tip at 340ºC for quick heat transfer?
This creates the scenarios for higher temps than necessary, so flux helps there.

I am not talking about solder joints that you move while still cooling, I am talking about well done joints, or simply putting a blob of solder on a trace and play with it until it becomes dull. That's what I don't understand, or didn't, may be this explanation is correct:
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.
Yeah, Peter Pan, hahaha, I am heating and melting the dull solder once flux is reapplied, never goes shiny. I am not dumb hahahaha, but I liked how you were cautious to ask. "Sire, have you plugged in the PC?"

My flux is a mix of polymerized rosin, white oils?, paraffin and wax. That's what I can find in the datasheet.

Anyways, if I came to the conclusion that the dull surface is oxidation, I guess it's oxidation of either Sn or Pb. If adding flux de-oxidizes the solder, that means these rosins are... breaking the SnO2 or PbO2??? Freeing pure Sn and Pb?

I am not sure about that...


Joined Mar 13, 2020
It is not intended to be a guide to solder correctly. Keep this in mind, so now you understand why:

In my example I am talking about solder without flux core. It's crucial to separate as many variables as possible so I can understand what's going on and why.
Oxygen. Or - Oxidation. Without flux solder - ANY solder - will oxidize. The longer it's heated the worse the oxidation will be.