Sections in this article:
-Soldering AC and DC Wires
-General Technique
-Soldering Heavy/Thick Wires
-Useful Fun Fact about Soldering
The Importance of Good Soldering to AC and DC Current
Whether there is AC or DC electricity running through your copper, proper soldering increases reliability in general. A good soldered joint should look clean, somewhat shiny, and be stronger than the wires it connects (a good way to test your skills is to try and pull the joint apart. If the wires breaks before the solder joint, you have a strong joint). You can only tell with a microscope, but the most perfect joints have very little bubbling or none at all. The melted and boiling solder will create pockets that make solder less conductive. With this in mind, lets examine the pros of a good joint to AC and DC electricity.
AC current travels through its path. So a thick, gap (air) free cable is best depending on the size of current. So here, a solder joint that sticks to every surface and leaves no gaps aids in conductivity. It is because AC current travels through things that it makes it most dangerous to the human body.
DC current travels along the surface of its path. Ultimately, breaks in a wire amended with solder do harm and act as a resistor. However, this still makes 100% adherence to every surface important, as well as cleanliness of the cooled solder. Blackened solder is no good for anything with DC current especially. DC current is not as harmful to the body because it will tend to travel over-top your skin, leaving vital organs alone. Don't be stupid, however, as any electricity is harmful given enough power.
General Techniques:
So lets get into some actual soldering technique. The goal here is to both clean the surface to be soldered and have it reach the same temperature as the melted solder. Cleaning can be achieved with flux or rosin. These agents can be bought in bottles or found in what is called flux or rosin-cored solder. I buy bottles even with cored solder because I find it makes things easier. As the flux or rosin burns off, it will clean the metal and distribute heat along the wire.
As for the heat source, add some solder to the tip of the iron (if that is what you use, it increases conductivity) and position it near the bottom of your joint. Heat travels up and will evenly heat the total surface to be soldered. You know the wire is ready when the solder you apply melts not on the heat source, but the wire itself. The solder will bond with the metal and spread on its own as the wire heats. If you did it right, you will get something like in the checked picture above. The solder will appear to suck into the metal, especially if you are bonding a wire to a terminal.
Some cautions:
1) Unless you are a beginner, use high heat whenever possible. Heat will spread everywhere and can melt plastic insulation and circuit boards. Using high heat ensures quickly heating the place of interest, while not giving the heat time to spread.
2) Ultimately, a soldering iron is best. You have controlled temperature for a soldering environment and no flame to quickly leave black filth. If using a flame, be direct and keep away from plastics. I recommend a jet-flame from a small butane lighter.
Soldering Thick/Heavy Joints:
This becomes a huge problem when you realize your soldering gun can't keep up enough heat. You will be melting everything else around you except for your solder. There are a couple techniques that you can use to avoid the obvious and expensive solution of buying a more powerful iron
1) Small Butane Torch: I use my pipe lighter. The flame is intense, concentrated, and hot. This is a very controllable heat source that you can easily keep away from plastic parts. I can quickly solder 4 gauge wire to big ring terminals in seconds with this. I find that the butane flame tends not to leave black marks. This is recommended.
2) Aluminum Foil Method: Another solution, especially if you are bundling many small wires (why..., but I've been there) is to wrap enough solder around the bundle, seal around it cleanly with aluminum foil, and then hold a lighter to it for a while. You need to absolutely know what you are doing to get this right, since you can't see what is going on under the foil. Effective, but hard to achieve a good joint (at all?) with.
3) Stove: I did this out of desperation for some 0 gauge wire. I have no heavy heat source (yet), so I pressed the wire to a red hot element to get it heated. It was fast, scary, and I managed to burn back a little too much insulation. I won't judge you for doing it, I will do it again until I have a good heater for this situation. Maybe this method can be perfected...?
There you have it. This should leave you with enough information to judge what heat source you need for what situation. The key is always to get in and get out fast with concentrated heat. Experiment, have fun, but try not to burn those tiny components on your circuit board.
Fun Facts About Soldering:
1) Traditionally, there is lead and silver solder. Lead is toxic and cheap, while silver is much safer and more costly. Especially regarding silver, the more purity (silver to additive ratio) there is, the easier it is to solder and gain a conductive joint.
2) Rosin is sap obtained from Pine trees and some other plants. Flux can be many things, but self-cleaning flux such as petroleum-based flux makes jobs easier. There is simply no flux to brush off later. Find what works for you best.
3) Although solder can melt with irons at about 400F, heat as high as 650F to 800F can be used.
4) More solder does not equal better. Use only what is necessary to ensure there is maximum adherence to all surfaces.
5) It doesn't take many watts. As little as 15W can do, though some irons reach 120W. Cordless irons are to be treated with caution, as they don't sustain heat well. I prefer a butane torch when an outlet isn't near.
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