Whether you are working on a copper pipe in your bathroom or trying to repair a speaker in your portable radio you are likely to find situations where you need to solder metal parts together for structural or electrical reasons.
The first thing we should cover is very important. When you are working with home electrical systems you never solder the joints / connections. Wiring that is used to deliver electricity from your circuit breaker box to an outlet, switch or device should always be mechanically joined and never soldered or welded.
However there are many times that you do need to solder metal in your home and like anything preparation is the key to getting the job done quick and easy.
Many times when you see a technician soldering a pipe or working on electronics you may wonder how can they do that so cleanly and fast. The main reason that they are able to complete joints so fast and clean is proper use of flux and metal preparation. This in addition to understanding how to heat the metals correctly to accept the solder is what you need to learn.
The next thing you should understand is Soldering is not Welding. Welding is where two pieces of metal are joined by melting and the pool of metal actually mixes together to become one.
Soldering is where you join two metals the same way you would use duct tape.. a metal (the solder) is melted and it adheres to the parts you are trying to join. None of the metals actually melt and mix together.
This is why surface preparation is very important when soldering. If you were to place a piece of duct tape on a oily or dusty surface you would never expect it to stick. The same is true when you are soldering wires or pipes.
Proper Selection of Flux
There are two basic types of solders that you can choose.
Corrosive Flux is normally used for soldering where the primary concern is etching the metal to make it clean. A small amount of acid is added to the mixture and must be wiped away and cleaned after the soldering is completed or it could cause corrosion of the parts you are joining.
Non-Corrosive Flux is normally used for electronics because it can be very difficult to clean the parts after joining them and when you are using stranded wire the flux will flow behind or into the shielding plastic that covers the wire resulting in corrosion inside the insulating jacket in weeks or months to come.
Preparation of metal parts for soldering
When pipes are made and delivered to your supply center they have a light coating of oil that was used in the extrusion process that keeps the pipe from corroding. Additionally everyone has seen a bright penny and a brown one well the brown corrosion which will eventually turn green is oxidation of the copper. For the best results your pipe must be bright and shiny for the solder to adhere.
Most plumbers will use a special wire brush specifically designed to clean pipe, the use of sandpaper is also good for removing most of the oxidation.
Applying the Flux
Flux in soldering is used to protect the metals while they are heated so they won’t oxidize and to allow the solder to flow on to the metal. The flow part is what is most important for most people to understand.
When you apply flux you will use a small brush that is included or you may need to buy a flux brush. You should not use something like a qtip because the cotton fibers will get into the flux.
Apply a very thin coating only where you want the solder to adhere.
If you are soldering pipes you want to apply it on the female and male sides and you only want to apply it where solder will be used. If in doubt dry fit the pipe but make sure the connections are fully inserted. On the male end of the pipe you can add about 1/8th of an inch however once inserted the flux will probably seep out a bit.
If you mess up wipe it off before you start.
Applying the solder
When you solder any plumbing joint or electronic part it is important to understand that you should heat the part and not the solder.
If you heat the solder and drip it on to the part it will not adhere.
Heat the part and then touch the solder to the part. You have got the part hot enough when the solder flows.
The exception is electronic parts. The only trick here is that when using a soldering iron you want to melt a small drop of solder on the tip of the soldering gun that way heat will be transferred to the part you are heating. You do not want to load up the soldering iron tip with so much solder that you can complete the connection.. just enough to allow good contact between the gun and the wire.. then once the wire is hot enough touch the solder to the wire and it will flow over the area you prepared with flux.
Final Note
I hope this helps you understand a little more about the use of Flux while soldering.
The best bet is to use less than you think is needed however you want to coat any area that you expect to accept solder. Solder may flow in areas not prepared with flux but the use of flux makes sure solder will get into pipes and connections.
A trick for soldering electronics parts and wires is to Tin the wires before you solder them. This is where you apply flux and a very small amount of solder to the wire end prior to beginning your soldering. It makes things go much faster because less heating is required when actually joining the parts. Some electronics parts will come with tinned wire ends from the supplier .. if not you can do this yourself and your job will go much faster.