Heating a joint quickly

[ToolsRMe]

In another thread ("Bernzomatic JTH7 High Temperature Torch") I posed the folliowing (paraphrased) question:



Is there anything wrong with a joint heating up very quickly with a very hot burner? Does the flux need time to do its work? Since the flame is so hot, perhaps only one side of the joint is getting properly soldered?

I know that copper is a terrific conductor of heat. Yet with a pencil burner I've often seen flux being pulled into one side of a joint and not the other even though I try to heat as much of the joint as possible.


Any comments?

 

[Prashster]

Flux does not need time to work. It works instantly with heat. That being said, it needs to KEEP doing its thing during the soldering process, and if you heat too long, it'll burn off and stop working.

Oxides are formed when metal (copper) reacts with oxygen. These prevent solder flow and prevent bonding btn copper and solder. Mechanical removal via sanding is good for getting most of the oxidation off of the copper prior to heating.

The problem is, heating, itself, causes oxides to quickly form. That's the reason we flux. It prevents oxidation.

The reason you shouldn't overheat a joint is bkz it'll burn all the flux away which will allow the oxides to form unfettered.

So, my guess is that yes, using a super-hot torch means your risk of burning out the flux is higher. That being said, flux doesn't burn off THAT FAST, so you're probably ok, unless you're getting the joint white hot for a long time. If you heat it as quickly as possible, remove, then solder, you're likely fine.

In addition, flux burns at a lower temp than solder, with the El Cheapo tip, it's easier to improperly heat, causing the joint to be just hot enough to run off all the flux, but cold enough to not let the solder melt.

 

[Sponsor]

 

[Lithnights]

When soldering...

1) Is there a standard distance from tip of blue flame to copper pipe that one should use?
2) Is there a standard time in seconds that the copper should be heated?

I'm guessing it depends on what is being joined (copper to brass shutoff or copper to copper coupling etc.) and what diameter pipe it is, but I'm just wondering if there a typical distance and time?

Thanks,

 

[Prashster]

1. Inner blue tip (the hottest part of the flame) should just touch the fitting - not the joint. Want solder to flow TO the heat.
2. No standard time. Happens as quickly as 10 secs and as slow as 45 secs for me depending on ambient temp, size of joint and awkwardness of position.