Understanding Soldering Part 7: Rosin Flux

Many popular foods and restaurant chains raise interest in their products by advertising a “secret” ingredient. They aren’t the only ones to rely on a secret, though; soldering electronic components to a circuit board relies on a hidden ingredient, as well. Working together with the solder is a substance that many aren’t aware of called flux, but without it, we wouldn’t be able to reliably solder a single part.

Solder is a combination of metal alloys that, after melting, fuses the pin of a component to a metal pad or hole on a circuit board to create an electrically conductive connection. Flux facilitates this soldering process by cleaning the metal surfaces before they are fused. Different types of flux are distinguished by the base materials used to create it, with rosin being one of the more common variants used in circuit board assembly. There’s more to tell concerning this secret ingredient; let’s explore the use of rosin flux in PCB assembly.

What Is Rosin Flux?

When metal is exposed to air, oxides can form on unprotected surfaces. In the case of electronics, these oxides can impede the formation of a good solder joint during circuit board assembly. To help with the soldering process, flux is used to chemically clean the metal surfaces by removing the oxides as well as any other impurities. The flux also helps promote the wetting of the solder, which establishes how well the molten solder will flow onto the surfaces being joined.

Fluxes for soldering electronics are separated into three categories: Rosin, Water Soluble (or Organic Acid), and No-Clean. The water-soluble fluxes made from organic materials other than rosin are extremely active during soldering. This is very effective for preparing the surfaces for soldering but requires thorough cleaning and inspection soon afterward to guard against flux contamination or board damage if not removed in a timely fashion. No-clean flux doesn’t require cleaning due to its low level of activity, but as a result, it is also not as effective in preparing the surfaces for soldering.

Rosin flux’s base material is composed primarily of rosin extracted from the sap of pine trees. It also contains different kinds of acids as activators, which promote the wetting of the molten solder by removing oxides from the metal. The level of activity of these acids is further separated into three categories of rosin flux:

• R (rosin): This is the mildest form of rosin flux, which contains the least amount of activators and is only intended for clean metal surfaces. Due to its low level of activity, this rosin flux will not produce any harmful residue during soldering.
• RMA (rosin mildly activated): This category contains a higher level of activators sufficient for cleaning PCB pads and holes, as well as the component pins, during soldering. This flux will leave some residue that is normally not an issue.
• RA (rosin activated): This rosin flux contains the highest levels of activators for cleaning, and it will also leave the most residue after soldering.

In addition to the rosin and the activators, the flux will also contain other solvents and additives to assist with the soldering and protect the metal against corrosion. Now let’s examine how rosin flux is used during the soldering process.

How Flux Is Used During Soldering

Rosin flux can be used for automated PCB assembly depending on the needs of the circuit board. For wave soldering, the flux is typically sprayed onto the circuit board before it passes over the molten wave of solder. Circuit boards that are processed in a solder reflow oven use a solder paste composed of tiny particles of solder and sticky flux that hold the components in place before reflowing.

Rosin flux is used frequently in hand soldering and can be found in the core of spooled soldering wire. Rosin core solder helps the user by ensuring that the flux is distributed in consistent amounts for the solder application. If more flux is needed for larger areas, however, a liquid or paste can be applied with a brush, cotton swab, or a specific flux applicator. Rosin flux can also be found in commercially available dispensing pens for added convenience.

Once the soldering is complete, you may find residual amounts of rosin flux on the circuit board—especially if you’ve used a flux with higher levels of activators. Lighter amounts of rosin flux residue are often left on the board after soldering unless its removal is required for aesthetic reasons. The residue left by more active rosin or organic, water-soluble fluxes may be more corrosive and should be cleaned off. Usually, deionized water (DI) is used to accomplish this. Remember not to overuse flux during soldering, or you will have more residue to clean up than expected. Next, we’ll look at a few soldering best practices to provide the results that you seek.

Tips for the Best Results While Soldering

The above discussion about rosin flux is meant to help you create the best solder joints possible when you are soldering wires or components on your circuit board. Here are some additional basic soldering tips to remember:

• Make sure that the surfaces to be soldered are as clean as possible before you start so that the flux isn’t overwhelmed.
• Set your soldering iron high enough to thoroughly heat the type of solder that you are using.
• Add additional flux to wires that need to be tinned, or other larger areas on the board to be soldered.
• With the tip of your soldering iron bring the two surfaces to be joined, usually a component pin and the circuit board pad or hole, up to temperature at the same time.
• Add solder to the heated joint making sure that it flows and covers the surfaces being joined.
• Once you are finished, clean off the excess flux.

Making changes to your circuit board is a normal part of bringing up a new design, but be careful. Hand soldering involves risks that could damage the board with a simple slip of the soldering iron. Perhaps the best soldering tip of all is to use the professional soldering services of your PCB contract manufacturer to make any necessary changes. This will ensure that the parts are soldered with a robust connection and that there isn’t any damage to the surrounding areas of the board.