Why high temp and quick is best for soldering

[R.G.]

I thought it might help those who don't have an ME or EE background to understand my comments about soldering if I explained a bit.

It does seem a little counter-intuitive that to avoid damaging components when heating their leads you should use a high temperature, not the lowest one you can get away with. But it is quite true, and it has to do with the way that heat flow works.

Heat only flows between two places when there is a temperature difference. The temperature difference is the "pressure" that makes heat move, just like pressure differences are what make water move in a hose. How much heat energy flows is analogous to how much water flows.

Every material has a thermal resistance, just like it has an electrical resistance. It turns out that you can describe heat flow just like current flow, by a variant of Ohm's law if you get the variables correct, with V(that is, temperature difference) = I (that is, heat flow) times R (or the thermal resistance).

Each object has a thermal capacitance - its mass. If you think about it, it takes ten times as much heat energy to heat up a 10 pound block of metal by five degrees as it does one pound of metal by the same temperature. 

Where this gets different from electrical flow as a model is that (a) there are no thermal inductors - they just don't exist - and (b) thermal resistance and thermal capacitance are much more closely bound than electrical resistance and capacitance. To get low thermal resistance you generally have to have big masses, and that inherently brings in lots of thermal capacitance, (c) all thermal capacitances are to "ground"; there are no series thermal capacitances.

The real model for all heat flow is as follows:

Heat source -> thermal resistance 1 -> thermal capacitance 1 to ground ->thermal resistance 2 -> thermal capacitance 2 to ground - thermal resistance 3 -> thermal capacitance 3 to ground ->...

Each new object that carries heat has its own thermal resistance and at the same time its own mass that must be heated before it can carry the heat. And that R-C sets up a time delay. If we go back to the garden hose analogy, it's like a 25 foot hose is made up of 25 one-foot sections, and between each two sections there is a one-gallon tank that must fill before the water can go to the next section. So the water (heat) applied to the source end fills the first gallon through the first one foot of hose, then it flows into the second foot of hose to fill the second gallon, and so on. There is a time delay associated with each section of hose you fill.

And now we're down to soldering. What if all you want to do is fill the first one gallon container in the 25 feet of hose, but not get any water into container #25? What's the correct pressure (that is, temperature) to apply? It's not just barely enough, because some of the water dribbles along the rest of the hose as you go and before you get the first one full, there's already water in most of the 25 containers. It's a high pressure, applied suddenly. When you apply a sudden high pressure, the time it takes to fill each container actually slows down the flow reaching each successive container. So you fill the first one almost totally before the water has time to get into the third one.

That's another way of saying that by applying a soldering iron that's quite a bit hotter than you need to just melt solder, you make the solder you first touch molten before the heat has time to conduct through the leads into the device being soldered. So you get the part you want to get hot (the lead) hot enough before the time delay of heat moving into the part can overheat the part. Get the surface of the leads up over soldering temperature quickly, let the solder wet them, and get the iron off the joint quickly. The heat remains local to the joint because it does not have time to conduct.

This is one reason laser machining is precise. The heat carried in a laser pulse is big - tens of kilowatts in industrial setups - but it's very quick, nanoseconds per pulse. The application of so much power so quickly vaporizes the material it hits before the heat has time to conduct to the surrounding material. And the vapor carries off the heat energy so it does not remain to conduct into the surrounding material.

[petemoore]

  Steam Layer...OT
  If you lick or wet your finger and touch a super heated metal surface, you will hear the tsizzss, as a steam layer is created, the heat doesn't time to get through the layer, to the wet on your finger and burn it.
  If tried on a bit cooler surface, instead of a steam layer 'insulating' the liquid on the fingertip, your finger and the liquid on it will be heated, to a much greater degree, transferring the heat to your fingertip, burning it.
 

[R.G.]

Yep - sessile effect, ablative cooling. Also used in some re-entry vehicles. The heat gets eaten up boiling off a layer of the surface, and the boiled-off gas carries away the heat before it can conduct through the rest of the coating.

[dmk]

interesting stuff there guys.
learning is fun

[Thomas P.]

Thermodynamics are fun - for sure!
And to enter another odd observation to this thread:

If you know those self cooling beer cakes...what happens there is the water boils till it freezes

hehe - physicians sure are geeks...

[Transmogrifox]

Ummm...in your response to your signature, tomboy:

[Meanderthal]

 
I Absolutely agree on this one! I use a 60 watt(used to use 30 watt), and get in and out quick. Way fewer problems that way... no cold solder joints, no screwing around waiting for the solder to melt, neater joints with much less solder globbed up, everything sticks the first time, and I have yet to burn up anything with the hotter iron. Not even my fingers.
  I'm also using thinner solder lately, but that's a whole different subject...

[brett]

Good analgy.
The weakest part of my soldering is surface preparation.  I don't use Vero much, so when I do, I should make sure I run over it with 800 grit sandpaper, because otherwise, the copper oxide coating on the tracks (i) provides an insulator, and (ii) makes the alloying process slow and difficult (chemically).  Meanwhile, lots of heat is flowing down the lead into the component. 
Note to self: take this advice and stop making a mess with old vero. 
cheers

[bwanasonic]

Thanks for the lesson RG! I had already learned the lesson that a proper soldering *station* cranked up a bit, vastly improved my soldering chops, but I appreciate the effort to explain the fundamental concepts behind that.

Kerry M