Lots of folks at RCU have had questions about soldering, so we thought we'd give you a "How To" on the basics.  Before we begin, I would like to say that this is not an all-encompassing bible on soldering, but for 95% of what we modelers do, the basics are all that is required.  I'm not even going to start off with the typical "What is Soldering" topic, because most of you don't really care about the technical details anyway, you just need to get your pushrods or landing gear together, or you need to add a new end to your Servo, so I will cover both Electrical, and Mechanical Soldering.

One thing I will point out, since so many have asked, is which type of solder to use.  Regular rosin-core solder should be used for Electrical connections.  It will also work fine for most Mechanical connections.  Silver Solder will work better on Mecanical connections, and I would recommend it for high-stress applications, but for most of what we do, plain 'ol solder will do just fine.

For now, all you need to know are the three basic rules to a good solder connection.  They are:

1) Clean - Your metal surfaces MUST be clean.

2) Flux - Flux allows the solder to flow and adhear better to the metal.

3) Proper Heat - The metal must be hot enough for the solder to adhere to it, but not so hot that it damages the metals properties (for example, if you let landing gear wire get red hot, it will remove the spring-like properties of the metal, and your gear will bend every time you land!)

For Electrical Connections, you'll need some Rosin Core Solder, Flux, and a Soldering Iron (or Gun). You will also need something to insulate the bare wires with afterward.  Electrical Tape can be used for this, but I prefer to use Heat Shrink Tubing, which can be purchased at your Local Hobby Shop, or at a place like Radio Shack.  (Note: If you do some Internet searches, you can find Heat Shrink Tubing in bulk)

The first rule in Soldering is Cleanliness.  This is usually pretty easy to achieve with an electrical Connection, because most of the time, the wires will be protected inside their insulation.  If you are planning to solder a wire that has been previously stripped of it's insulation, I recommend cutting the wire, and stripping the insulation from a fresh section.

You can purchase special pliers to strip insulation, but I find that they are really not needed unless you do it on a daily basis.  For the small wires that we work with, I just carefully nick the insulation about 1/4" from the end with a razor blade, then strip off the insulation with my thumbnail.  

Once the individual wires have been stripped, twist the strands together to keep the ends neat.  Now we're going to impregnate them with solder in a process that is called "Tinning".  To do this, first I dip the bare ends of the wires in Soldering Flux to coat them.  Next, touch the solder to the tip of the iron until it starts to melt.  Allow a small amount of solder to accumulate on the tip.  Then one by one, touch the ends of the wire to the tip of the iron long enough to heat the wire to the point where the solder will "wick" its way into the wire (if you are doing several wires, you may need to add more solder to the tip of the iron after you have tinned a few).

Once the ends are tinned, slip the Heat Shrink Tubing over the wires.  I like to put one large tube over all of the wires, then a smaller one on each.

Now, hold the two wires together as you apply the heat.  In most cases, the solder that is "Tinned" on the wire will be sufficient to join the two wires, and additional solder is not needed.

Once all of the wires are joined, slide the heat shrink tubing over the bare wire, and heat with a heat gun or, carefully (and quickly) run a flame under them (Note: In the picture above, it looks as though the first wire is being cooked!  Actually, in that picture, I am heating the middle (Red) wire, and the heat is nowhere near the first).  After shrinking the tubing, slide the large tubing over the other three, and shrink it to make a nice neat package.

For simple mechanical joints, like soldering a Clevis onto the end of a Push Rod, the rules are the same: Clean, Flux, Proper Heat.  So first, clean the end of the Push Rod with sandpaper, then, dip the end into the can of flux, and insert it into the clevis until you can see the end of the rod is either flush with the other end of the hole, or proturding slightly.  Now, heat the Clevis (not the pushrod). Allow the heat to transfer itself from the clevis to the rod.  (Note:If you are using a Soldering Iron, this may take some time.  A Soldering Gun would be better suited to these larger jobs.)  Then apply the Solder to the joint where the Push Rod meets the Clevis, when the temperature is right, it will melt the Solder, which will "Wick" its way into the joint.

Remove the heat, and allow the joint to cool slowly without being disturbed until the solder has solidified (Usually just a few seconds).  It can now be safely moved, however it should still not be quenched (cooled rapidly as by dipping it in water) for at least a minute.

When it comes to the larger jobs, you will usually find that an iron or a gun will not heat the metal sufficiently.  This is where a propane torch will almost always be a necessity.  The one thing you must be careful of (aside from the obvious hazard of working with an open flame), is that a torch can quickly overheat the metal.

Some metals, like the spring steel used in Landing Gear, are "Tempered" or heat treated during their production.  Without going into a metallurgy lesson, let it surfice to say that you never want to let the metal get RED hot.  This will destroy the Temper of the metal, and remove its "Spring- like" tendencies, leaving it about as soft as a thick wire coat hanger.  But don't let this scare you.  As long as you keep the flame moving, and heat it up a little at a time, you will reach the soldering temperature long before the metal gets too hot.

For this demonstration, I am soldering the Main Landing Gear on a 1/4 scale Piper Cub.  The Plans require you to solder three wires together into a bundle.  The first step is the same as before - Clean all 3 wires with sandpaper.  Once they are clean, I wrapped all three together with soft copper wire, keeping the winds neatly aligned.

Once the wires are wrapped, coat the joint with Flux.  Now it's time to apply the heat.  Moving the torch back and forth, slowly heat the joint.  After a few passes, stop, and touch the solder to the wires to see if it will melt, if not, remove the solder and apply more heat.  Continue these steps until the joint is hot enough to melt the solder.  Once it does, feed enough solder into the joint to sufficiently coat it (applying more heat if necessary).  Once the wires have a nice coat of Solder, remove the heat, and let cool.

That's it!  As I said earlier, there are a lot of technicallities involved, but for basic soldering that's about all you need to know.  In fact, using these same techniques, you could even plumb a new bathroom in your basement (Guess what the Wife had me doing last winter!). - MinnFlyer