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[GG]

Unless you limit your modelling activities to sailing RTR (Ready To Run) models and buy the same when in need of replacement, it is almost inevitable that the skill of soldering will be required (or wished for?) at some time in this hobby.  Many modellers have learnt and successfully practice the rituals needed to solder but perhaps without a true understanding of what the process involves.  These are sometimes the ones who try to make soldering more complicated than it really is, maybe in an attempt to preserve their illusion of superiority?  This might be fine until something goes wrong, there is only "magic" in the process of joining two or more pieces of metal together with solder if you don't understand what is happening.  Perhaps the worst crime is to suggest that soldering is like using glue in that the solder "sticks" to the surface!


Welding is simple to comprehend, supply enough heat to locally melt the metals you are trying to join, probably introducing more metal into the molten mixture, then allow to cool.  The result is a solid joint between the metals and, if done correctly, it is very strong and tough.  But, soldering is clearly different since we operate at temperatures much lower than the melting points of the metals we are trying to join.  I have yet to melt any copper wires let alone steel, with my soldering iron.


If you take two metals with different melting points and heat them up together then you might expect the one with the lowest melting point to melt first.  Then, as the temperature rises, the second metal eventually liquefies on reaching its melting point.  However, it is common for metals to form alloys, which is in the liquid state they mix together and do not remain physically separate. This results in the metal with the lowest melting point liquefying first and in effect "dissolving" the second metal at well below its melting point.


Usually these alloys do not have a single meting point but a range over which they solidify.  Solidification starting with an alloy rich in the highest meting point metal.  Melting such an alloy being the reverse process in that an alloy rich in the lowest melting point metal will start to melt first.  Some people might remember the technique of repairing old domestic lead water pipes by using a solder that had such a temperature range over which it solidified thus allowing the semisolid solder to be "wiped" into a neat smooth surface with a rag.


By adding different metals together to make an alloy, it is possible to achieve surprisingly low melting points.  You may have seen the joke of giving someone a spoon made from "Woods Metal" to stir a hot cup of tea in which the spoon promptly melts!  It is worth adding that this alloy being made from Bismuth, Lead, Tin and Cadmium, none of which are wise to ingest, means that after laughing at the joke the tea must definitely not be drunk.


Thus, the process of soldering is simply applying a suitable metal which melts a conveniently low temperature onto the base metal surface which it can start to dissolve into and form an alloy with.  Once this occurs, the heat is removed and the molten metal at the surface is allowed to solidify.  The result is a joint which has no definite boundaries as the chemical composition changes from the base metal to solder at the outside, Fig 1.


So, with this understanding of how soldering works, it is not hard to appreciate what is needed for success.  If it still sounds a little too "magical", just ask yourself how we get rid of ice frozen onto footpaths in winter.  Throwing another equally cold solid onto the ice doesn't sound very promising yet, that just what we do with salt!  The ice quickly melts and shows that things can behave in ways that might seem odd until you understand what is happening (hopefully readers who enjoy warm winters can still appreciate this example?).
Glynn Guest   

[GG]

HEAT
Enough heat has to be supplied to allow the solder to be in a molten state on the surface of the metals to be joined.  Without this the solder has no chance of dissolving into the metal to create a strong joint.  If using an electric soldering iron it has to be powerful to do the job quickly, ideally the solder should form a bright molten surface within a second or two of it being applied.  Having to keep the soldering iron in place for several seconds suggests that it is not powerful enough for the job.


Dirty Surfaces
Molten solder needs to be in direct contact with the metal it is to bond with.  Unfortunately most metal surfaces will tend to corrode given half a chance, the most common being an oxide layer formed by a reaction with atmospheric oxygen.  Sadly, molten solder will be stopped dead by such layers, things might be getting hot but no proper joint will be made.


The obvious thing to do is make sure the surfaces to be joined are clean.  This usually means rubbing with a file or abrasive paper until things look shiny.  This is sometimes confused with abrading a surface to allow a glue to have a good "grip", hopefully the previous explanation of how solder works has dispelled this myth.  Also, for the same reason any grease or oil must be removed from surfaces before soldering is attempted.


Flux
The actual process of heating up the metal at the start of soldering can create the oxide layer we want to avoid.  This can be prevented by using a "Flux" which forms a protective barrier on the metal whilst soldering.  Some solders already have a core of flux and can be used directly. If needed, I apply extra flux to the surfaces to be soldered with a small paintbrush.  A small tin of flux was bought many years ago and looks like it will outlast be!


Not to be forgotten is the tip of Soldering Iron also needs to clean as any buildup of oxides will reduce the rate at which heat can flow into the joint.  If during a soldering job, my iron hasn't been used for a few minutes, it gets wiped across a damp cloth or sponge to remove any rubbish off the surface of the tip.


Gaps
When making joints that will be subject to stresses, the gaps between base metals must be as small as possible.  Solder by itself is mechanically weak and a large gap will mean that there is a band of weak solder that if subjected to a load, it may not be able to resist.  So, solder is not for filling gaps in load bearing structures.


Smooth Joints
Soldered joints should have a smooth and shiny appearance.  This shows that the solder has flowed into the joint and melted into the metal for maximum strength.  The solder is said to have "Wet" the metal surface as opposed to forming a "blob" and just sat on the metal, Fig2.  A dirty surface and/or insufficient heat are likely causes of such blobs.


[size=78%]Even if the solder has made a joint but it has a dull and granular appearance, it may lack strength.  This can appear when soldering stranded electrical wires as the solder may be "sucked" away from joint by capillary action along the wire strands.  This can be avoided by using a soldering iron powerful enough to do the job in a couple of seconds and keep the solder where it ought to be.[/size]


A blob can occur at a joint by adding too much solder. It is unlikely to add any strength to the joint and can look unsightly on exposed surfaces.  Sometime the excess can be removed by touching the joint with the tip of the soldering iron, quickly removing the iron before all the solder is remelted and hoping some of the excess will follow the iron.  There are "desoldering tools" and "wicks" that can be to remove excess solder but there is always the risk of a soldered joint coming apart.  On some joints it can be safer to file the excess away.


Hopefully this has given some readers a better of the processes involved in soldering.  Yes, you can make good soldered joints without this knowledge, maybe even with completely the wrong ideas!  It is what happens when things go wrong and you can find yourself repeating the same actions over and over again whilst hoping for a different outcome.
Glynn Guest





 

[SteamboatPhil]

Very well put together information Glynn, I am sure it has cleared a few questions up.

[GG]

Hope you are right Phil.
It was an item that has been sitting in the files, not going anywhere, for too long.  To be honest, it was written out of the sense of frustration at seeing people blindly stabbing at a mass of solder with their soldering irons.  Plus, a little annoyance at some who offered poor advice often based on limited "knowledge" and sometimes a worrying ignorance of what was going on.
Must have another look at what is in the files....
Glynn Guest

[wynthorpe]

Im scared to death with the amount of solder im going to have to do on my Tamar, but im always willing to learn and this is great information, thanks.

[GG]

Wynthorpe,
               The whole idea of this note was to help people avoid getting stuck in a repeating a ritual that isn't working. It's disheartening when you see people stuck in a cycle of failure but possibly amusing when they'd like to be thought of as experts!


If you practice and understand what is happening, or maybe not happening? then success will surely come.
Good luck with your model,


To quote Goethe, "  What a man doesn't understand, he doesn't have"


Glynn Guest