Electronics

If your soldering iron was perfect in terms of giving off its heat you could perfectly solder regular snpb 63/37 at 184 degrees c ( 63/37 is the eutiectic alloy, meaning it melts exactly at a given temperature, in this case 183. 60/40 is a cheaper non eutectic alloy that is 'syrup' in a narrow band. 185 to 190 degrees. Eutectic materials transition from solid to liquid at 1 point, non eutectic allows transition slower , which can lead to brittle solder joints... )

Now, in real life you have to deal with a number of factors:

• The thermal resistance between the tip and the , to be soldered elements.

• The thermal inductance. It takes a while to get the heat in....

• The thermal capacitance of your iron. If you start pulling heat out of the tip it takes a while to replenish that.

The thermal capacitance is determined by two things : thermal mass and electrical power.

If your soldering iron has a lot of metal and a big fat tip it has a big 'buffer'. If the iron itself has a lot of electrical power it can replenish the missing heat very quickly. A good soldering iron for electronics should have 80 to 100 watts.

The thermal inductance you have no real control over. It is mainly determined by the parts that need soldering. If you have to solder a small sot23 there is virtually none... If you need to solder the pins of a through hole heatsink ... Yeah there's plenty there...

The thermal resistance is a matter of correct soldering technique. Never,ever solder 'dry'.

The correct method is to apply a bit of solder to the tip first , put the little blob of tin in contact with the pin and pad to be soldered , wait until it flows and then add a bit more solder if needed.

This lottle blob of pre-melted tin on the tip ensures good thermal transfer. If you tip is 'dry' there is very little contact surface. That little blob of tin mitigates this.

So , how Do we traditionally compensate for the shortcomings of our soldering station? By jacking up the temperature ... Which is totally WRONG !

You need to use an appropriately sized tip for what you do and you need an iron with enough thermal mass. That is why , on a real assembly floor, the technicians have 2 ,3 or more different irons with different tips and they cycle according to what they are soldering.

On the subject of flux.

Flux activates at a very low temperature. Most fluxes kick in at about 90 to 120 degrees c.

Flux is an element that improves the wettability of the , to be soldered, elements. Wettability is a measure for how good solder flows. For example : sprinkle some water on a glass plate and it will form beads. This is poor wettability. Add some soap and it flows nicely.

Flux is just that : it is 'soap' for solder.

Flux does two things: it lowers the surface tension of the solder which allows solder to flow better, instead of forming beads. But it is also ahighly reactive compound. Flux has the capability of ripping oxygen atoms out of binding with other elements. If your copper pad has a thin film of copper oxide on it the flux will rip the oxygen out of binding with the copper and transform copperoxide back into copper. At the same time the flux covers the now clean copper , preventing re-oxidation.

Flux needs some time to react... The fluxes in solder wire are much more reactive than fluxes in solder paste. Paste is intended for slow heating .. Over a time of minutes. Where as solderwire has to melt in seconds. So the flux needs to be more agressive. It has a shorter time to work.

Here is a problem though : flux evaporates when heated. The mass of the solder is large enough to push it out of the way, but, if the temperature is too high , the flux evaporates too fast and it will 'spatter' you will get little beads of flux all over the place. Its like a mini eruption... Worst case this mini eruption flings pieces of liquid solder around as well...

If you run it so hot the flux also falls chemically apart and can no longer do its work.

So, what is the correct answer for the question: what is the correct temperature for the soldering iron?

The answer is: the lowest common denominator of all these factors.... In this case .... The flux !

Followed by what alloy of solder you are using.

You go look up the datasheet of the solder you buy, and you really need to go to the specific manufacturer and model , because every manufacturer has different fluxes !

In that datasheet they will give you the 'working band' and optimum temperature.

And that's it. Those numbers are tuned for that particular solder. Now, in practice they will be pretty close between manufacturers, although there can be some differences. Few tens of degrees c.

If you solder joints are still no good : you need more thermal mass and power , or your soldering technique is simply no good.