Brass and other metals Soldering Tips

 

Fluxes: Unlike electronic component soldering, you cannot rely on the flux in the core of the solder to clean a brass joint for you. You must use a separate liquid Flux of some sort. A good one if you can find it is CARR's Red Flux, which is designed to both clean the joint and then neutralize with the heat of the solder. This stops it corroding the joint later. Other commonly available flux {e.g. Duzall, or Bakers) don't do this, however if you can't get anything else they will work, as long as you make sure that you thoroughly wash and dry your work after completing each session to get rid of any excess flux. If you don't do this you will get a lovely verdegries effect which might look nice on church steeples but wont help your attempts to paint later.

If you use a solder with a flux in the center put a little on the Soldering iron tip to let the flux in the core boil off (i.e. wait for the fumes to stop) before you apply it to the work.

Soldering

To solder metal joints on a model, use the following technique:

·          Test fit your parts and make sure there is plenty of metal making contact (solder has very little strength)

·          Sand or burnish the mating surfaces clean

·          Using a small paint brush, paint the flux on each part where the joint is to be made

·          'Tin' each part separately by melting a little solder on the tip of your iron (lets the flux burn off) and "painting" it onto the fluxed surfaces.

·          You should aim for a thin even coat rather than a thick or lumpy buildup. A thicker solder joint will be weaker than a thin one, as the real solder join is only made in the top layer of molecules an each part of the two pieces and a thick solder joint just leaves extra soft solder between the parts.

·          Fit the tinned pieces together, with another coat of flux painted onto the now tinned surfaces.

·          Clamp or hold them steady with weights

·          Heat the joint with the iron or a torch until the solder 'flashes' {you will see it change to a brighter liquid state).

·          You may find you have to use a wooden or steel pointer to hold  small parts down if you are using an iron, otherwise the iron tends to pick the piece up as you take it away.

 

 

Sub-assemblies

Once you start putting together sub-assemblies you have to be careful that you don't put too much heat into the built-up unit otherwise other joints will fall apart while you are putting new ones together! There are a couple of things you can do to get around this problem:

·          Put a wet piece of cloth on the body between the joint you are working on and the other joints. The water should take up the heat if you work quickly.

·          If you use the resistive soldering technique this keeps the heat within the joint you are working at.

·          Use a high wattage iron. This way you can get a lot of heat into your joint quickly and if it is tinned and fluxed correctly it will take a very little time to flash giving the rest of the work no time to heat up.

·          With a small iron you tend to find that it takes a longer time to heat up the joint to the solder flow temperature and because brass is such a good conductor the small irons heat gets rapidly lowered as it heats up the whole piece  (which not only loosens the other joints but also hurts like hell if you happen to be holding it).

I favour a third method.

·          Make the main major joints (side-frames, boilers and heavier pieces) with the high melt solders

·          Use Tin/Copper solder for the lesser joints (that still need a bit of strength)

·          Then use low melt solder for the final joints. This special solder melts at a much lower temperature than normal solder and hence you can work with it and there is less risk of loosening other joints.

 

 

Commercial Solders:-

Melting point depends on the metal content

5/93/1 Tin/Lead/Silver (high melt point) at 300 C

40/60 Tin/Lead alloy, melts at 234 C, tip temp approx. 354 C

99/1 Tin/copper (lead free) melts at 220 C

'Savbit' alloy (copper in), melts at 215 C, tip approx. 335 C

60/40 Tin/Lead alloy, melts at 188 C, tip temp approx. 308 C

62/32/2 Tin/Lead/Silver (low melt point) at 179 C

Low melt solders are mixtures of Tin, Antimony and Bismuth and melt at temperatures between 170 C and 70 C

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