The 3 Essentials of Soldering: Solder, Flux, Heat

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In order to make a successful solder connection with any of these construction options you need solder, flux, and heat. Without just one of them, the soldering process will not work. Let's go more in depth on these three essentials and how to choose the correct soldering trinity for your application. 

1. Solder

What is Solder?

Solder is a metal alloy (mixture of metals) that comes in a variety of forms with the two most common being solder wire and solder paste. Chances are you may have seen or are familiar with solder wire already, it typically comes on a 1 lb spool but can be found on smaller spools or even in tubes as well. In addition, solder is available in many different alloys, each with their own unique melting temperatures and bonding properties.

You may be wondering why we use solder and not something else. Well, the composites of metal that make up solder give it a relatively low melting point. Because of this we can go from a solid wire to a liquid and back to a solid very quickly, using a simple heat source. This heat source could be a soldering iron, a hot air gun or even a butane/propane torch.

When you heat up solder, it flows onto the connection point (joint) of your parts forming a electro-mechanical bond. Since solder is composed of conductive metals, it creates an electronic path between the parts. The combination of mechanical strength and electrical conductivity is what makes soldering such a powerful tool for making solid connections.

2. Flux

What is Flux?

Flux is a chemical agent that de-oxidizes the metal you’re soldering to provide a clean surface for the solder to adhere to. Earlier we described solder as glue but without flux or enough flux, the solder will not be able to flow and bond properly.

Flux is commonly found in flux core solder wire as well as liquid and paste forms. In Latin, the word flux means flow and this makes sense because it helps solder to flow onto the surface of a work piece.

Imagine a thin layer of oxides on the metal surfaces you're soldering, as long as they're there the solder will not be able to flow. The job of the flux is to dissolve that surface oxidation and keep it clean long enough for the solder to do its job. You don't want it to do anymore or any less than that.

Good flux goes in, goes out, and it's done. It shouldn't hang around.

The clean surface that the flux provides improves the wetting (adhesion) between the surface metal and the melted solder because it can transfer directly without any interference. This results in the strongest electrical and mechanical connection at the solder joint.

The component of flux that facilitates the flow of solder and holds everything together is called the vehicle. Most electronic fluxes use isopropyl alcohol as a vehicle because it dissipates easily and doesn't spatter like water when heat is applied to it. If you've ever touched a soldering iron to a hot sponge you know what we mean. The last thing anyone wants is molten solder splattering all over the circuit board potentially causing shorts!

Now before you go and grab whatever flux you've got on your shelf, you need to realize there are different types of flux for different applications. Using the wrong flux like an acid flux can potentially destroy your circuit board and all the connections on it.

For our guide on choosing the right solder and flux click here.

3. Heat

There are three distinct methods of heat transfer when it comes to soldering and each of them use different tools to accomplish the task.  

Conduction - Hand Soldering

This process typically uses a soldering iron as the primary heat source. The mode of heat transfer from the iron to the workpiece is conduction.

Process Order:
Flux comes first, followed by heat and solder simultaneously

Application Range:
Hand Soldering is great for wires and THT but becomes limiting for SMT work.

Convection - Reflow Soldering

This process typically uses a hot air gun, oven, or furnace as the primary heat source. Also called 'sweatsoldering' when a torch is used as in plumbing. 

The mode of heat transfer from the iron to the workpiece is convection.

Process Order:
Flux and solder are applied, followed by heat last

Application Range:
Reflow Soldering is great for SMT work. 

Combination - Wave Soldering

This process typically uses an oven as the primary heat source. The mode of heat transfer from the iron to the workpiece is conduction.

Process Order:
Flux comes first, followed by heat and solder simultaneously

Application Range:
THT (Through-hole Technology)
DIP (dual-in-line packaging) assembly
SMT (Surface Mount Technology)
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