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How to Solder

How to Solder

by Andy Slater

This document deals primarily with the technique of soldering as it related to electronic components. The majority of this also applies to the soldering of brass, copper, tin and other metal parts which is generally somewhat easier as the parts being joined are less likely to suffer heat damage. There are some notes at the end relating specifically to the soldering of metal parts.

Equipment

As in all fields, it's worth investing in decent equipment however, do consider how much you intend to use these tools. Articles about soldering often go on about ensuring that your soldering iron has a changeable tip and if you intend to be soldering for 4 hours a day for the next couple of years this is very sound advice. The author has such a soldering iron but has not yet needed to replace the tip in the ten years that he has owned it. I'm not saying avoid them, just don't get hung up about it.

How to Solder

Soldering is not difficult. Doing it properly however requires some attention to detail although this should not be a problem for model makers who can usually list an attention to detail as on of their defining characteristics. Some practice is still probably appropriate and considering the relatively low cost of electronics components there is little excuse for not having a practice run first.

The main thing at stake is the long term viability of a soldered joint and this is largely dependant on the way the joint is made but may not be apparent at the time. For example, overheating of an electronics component may destroy it on the spot but may also cause damage such that although it works for a while, its life has been reduced. The same applies to the solder joint itself. A bad joint will look bad but even one that looks and works okay may fail at a future time if it hasn't been properly made.

A Little Theory

Cleanliness is important for soldering. The iron should be clean and the parts to be soldered should also be clean as any crud that gets into the joint will affect its long term, if not its immediate, viability. It is usual to use solder with a core of flux. Flux is a corrosive material that helps to clean the parts being joined as they are made. This however is no excuse not to ensure that the parts are as clean as possible in the first place. If the metal is not clean and bright, use fine sandpaper or wire wool to clean it. It is also essential that the soldering iron itself is kept clean by wiping it frequently on a damp sponge or pad of kitchen paper to remove, amongst other things, burnt flux.Your may well need to use your soldering techniques on such things as pewter gifts .

In order for a good joint to be made it is essential that the parts are held still while the solder cools and solidifies. It is usual to use some kind of clamp to hold the pieces being soldered. You're going to need both hands for holding the soldering iron and solder in any case.

The biggest issue with soldering is to apply enough heat without overheating and there are two contradictory factors. The first is that in order to make a good joint the parts being soldered should be hot enough that the solder melts on them. Flowing molten solder onto parts that aren't hot enough will result in a bad joint that is likely to fail. The second issue (try not to laugh) is that most electronic components will be damaged if they get to hot.

There are two things you can do to improve your odds:

  1. Firstly, be quick. Follow the method below and you should be okay.
  2. Apply heat shunts. A heat shunt is simply a device that is employed to move heat away from the delicate component. The job can be simply done using a crocodile clip. Simply attach it to the part being soldered at some point between the joint and the body of the component. Heat from the joint will then be diverted into the crocodile clip reducing the amount that gets to the component. Remember that failing to do so may well reduce the life of the component and it's such a simple precaution that there really is not excuse for not doing it.

How to Do It

Assuming that your iron is up to temperature (test it by touching the end with the solder - it should melt immediately) we can set about making some joints:

  1. Check that the parts to be soldered are clean and bright. If not, clean them.
  2. Hold the parts together using some form of clamp so that they are in the position in which they are to be soldered and will not move.
  3. Attach crocodile clips as required to act as heat shunts.
  4. Wipe the tip of the iron on the damp pad to remove dirt and old solder.
  5. Touch the end of the soldering iron with the solder so that a tiny amount of solder melts onto it.
  6. Put the iron in contact with your work so that is touching both of the parts to be joined at the same time. The tiny amount of solder on the tip of the iron will help the heat flow into the parts.
  7. Touch the solder against the hot parts at a point where they both meet. The solder should melt and flow around them. Do not use too much solder. The ideal join will be completely covered in solder however the shape of it will be concave such that you can see the shape of the wires. A convex blob of solder is NOT required.
  8. Remove the solder and the iron.
  9. Allow the joint to cool for several seconds before removing it from the clamps and removing the heat shunts.

Steps 4 to 8 should take about 2 seconds from start to finish and the soldering iron should only be in contact with the work for about half a second.

Soldering Metal Parts

Most of what has already been said also relates to the soldering of metal parts such as those in brass kits of railway vehicles, etc. There are other things to bear in mind however.

The areas to be joined are usually larger than those of electronic components and will require more heat. The good news is that we no longer need to worry quite so much about overheating as the metal parts will not be damaged by heat. Care should still be taken however as there is still the possibility that a joint that has already been made will come loose if it receives too much heat while you are making a joint on another part of the model.

The use of jigs to hold the parts while they are soldered is pretty much mandatory and these must obviously be able to take the heat. Balsa wood is probably the ideal material because as well as being able to handle the heat it is easy to work with.

Cleanliness is still of paramount importance and it is usual to smear flux onto the edges to be joined after they have been cleaned with sandpaper or wire wool. Note that this should be cleaned off again afterwards as it is corrosive and any surplus will continue to affect the metal after the joint has been made.

Whenever possible, the parts are 'tinned' before the actual join is made. This involves heating each edge that is to be joined and melting a small amount of solder onto it. The result is that all edges to be joined are given a thin coating of solder before they are brought into contact with each other. The solder will of course have solidified before they are brought into contact however the iron can then be run along the length of the joint re-melting the thin layers of solder such that they flow together. More fresh solder can be applied if required.

Sometimes, particularly with small parts, it may not be possible to tin the parts beforehand and the method described for electronic components should be employed or the part can be attached with cyanoacrylate glue.

As suggested in the section about electronic components, before embarking upon your first real project, some practice with scraps of brass sheet and pieces cut from tin cans will probably pay dividends.

This article is copyright (C) Andy Slater and is used here with permission
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