Understanding Soldering – Part 2: The Difference Between Flux and Solder

I am fascinated by a good magic show, but I’ve read that in a performance it is actually the magician’s assistant that does a lot of the work. Although the magician appears to be the source of the illusion, they are more often the showman drawing the attention of the audience. This allows the assistant to make the mechanics of the illusion work by flipping a switch, folding themselves up in a box, or opening a trap door, all while the attention of the audience is focused on the magician. Magician assistants usually don’t get any billing but are in fact highly skilled actors, dancers, and on-stage performance facilitators. Without them, the show couldn’t go on, yet they rarely seem to get the recognition they deserve.

When it comes to soldering a printed circuit board, there is a very similar relationship between solder and flux as there is to a magician and their assistant. We talk about soldering our circuit boards using wave soldering, solder reflow, selective soldering, and even good old manual hand soldering. But in all of those phrases you never once hear the word “flux,” even though it is as equally important as solder in attaching a component onto a printed circuit board. Just as with a magic show though, without flux, the solder couldn’t go on. Let’s take a closer look at exactly what these two materials are and the difference between flux and solder.

What is Flux and What is the Difference Between Flux and Solder?

While solder is used to assemble components onto your circuit board, the flux is used prior to assembly to prepare and help your board through the soldering process. The unprotected metal on your printed circuit board can become contaminated with impurities from handling, as well as from oxides that form due to the metal’s exposure to air. This contamination can prevent the soldering process from creating good solid joints between the circuit board metal and leads of the components that are being attached to it. In order to remove this contamination, it is vital that the board be cleaned with flux prior to soldering. Solder flux not only removes these different contaminations, but it also helps promote the wetting of the molten solder between the metals and protects the surfaces from re-oxidation during soldering.

There are three different categories of fluxes used to solder electronics according to IPC J-STD-004B, Rosin and Rosin Substitutes, Water Soluble, and No-Clean. After the soldering process, the rosin and water-soluble fluxes need to be chemically cleaned to remove any excess flux residue that remains. These residues if not removed would continue their activity and cause corrosion of the board while it was in operation. The no-clean fluxes do not need any special cleaning for performance, and any visible residue can be cleaned as desired.

Flux is available in liquid, paste, and solid forms, and there are different factors that go into deciding which type of flux should be used. These factors include:

• The type of printed circuit board that is being assembled.
• The density of the component placement on the PCB.
• Which solder process is going to be used for the PCB assembly.
• The solderability of the metals that are going to be joined.

Now that we’ve introduced you to flux and how important it is to the process, let’s look at the PCB solder itself.

PCB Solder and How it Works Together with Flux

Solder is the adhesive material used when two metals need to be joined together, such as component leads and their corresponding pads or holes on a printed circuit board. The solder is used as a metal filler between the metals to be joined and is heated until it melts. Since the molten solder has a lower melting point the metals it is joining, it provides a strong metal-to-metal joint without causing any harm to the board or the components after cooling. The solder that is used for circuit board assembly has to conform to the standards set in IPC J-STD-006 and is available in different forms depending on how it will be used. Solid bars are used for applications that require a large reservoir of molten solder, while solder powder and paste are used for reflowing under surface mount leads. Manual hand soldering technicians usually use a wire or ribbon of solder to work with.

As we discussed earlier, the complete soldering process involves both the solder and the flux to create good solid solder joints. The job of the flux is to prepare the surfaces for soldering, and then protect those surfaces during soldering as well. As such, flux is an integrated part of soldering, and its application is considered to be part of the entire process. Here is how flux is applied for the different soldering processes:

Soldering Process		Flux Application
Wave Soldering	The circuit board is passed over a molten wave of solder which wicks up into the holes soldering the component leads in those holes.	Sprayed on prior to being soldered.
Solder Reflow	Solder paste is applied to the pads on a circuit board using either a jet application or a stencil. This paste holds the component leads in place and then reflows when passed through an infrared oven that heats it creating a solid joint.	A thick flux chemical combined with solder powder to form the solder paste.
Selective Solder	This is an automated system that applies solder to individual holes or pads on the board. The system will either deposit molten solder through a nozzle or use a laser to melt solid solder that is fed to the joint.	Sprayed on, or dispensed as single drops on the surfaces to be soldered.
Manual Solder	A technician using a soldering iron to melt solid solder that is fed to the joint.	A solid core within the solid solder wire used by the technicians.

How to Get the Best Quality Solder Joints on Your PCB Assembly

The next step is to make sure that you get the most robust solder joints possible on the printed circuit board that you are designing. The best way to do this is to build your PCB component footprint land patterns to the correct size, and follow good design for manufacturability (DFM) rules specified by your manufacturer. As you proceed with your design, don’t forget to continually check your design as well to make sure that there aren’t any DFM violations that have crept up on you.

At Tempo Automation, we are ready to work with you on how best to design your boards for the assembly and soldering processes that you will need. We have vast amounts of experience working with engineers like you who need the best quality manufacturing possible for their designs.