Choosing the Correct Flux—Advantages/Disadvantages

Estimated reading time: 2 minutes

While often overlooked, the flux chosen for the selective soldering process has a great impact on solder joint quality, long term reliability and overall selective soldering performance. This article outlines the critical factors of commonly available selective soldering fluxes and how they impact the soldering quality, reliability and equipment performance.

Fluxes essentially fall into three basic categories or flux types:

• Low-solids/no-clean fluxes
• Rosin fluxes (full/high-solids rosins)
• Water soluble fluxes

When discussing fluxes for the selective soldering process, we are generally referring to low-solids/no-clean fluxes, and it is the most commonly used flux type in selective soldering.

If a company is using a full rosin or water soluble flux in their selective soldering process, they are usually mandated to use them by their customer, or industry, and are usually producing a legacy product with a legacy reliability standard. From a flux performance standpoint, both of these flux types solder very well, and there is little to evaluate. But most companies using selective soldering avoid them because of the need to install expensive cleaning processes as well. After all, one of the benefits of selective soldering is the ability to selectively flux so that cleaning can be eliminated.

NOTE: If you are required to use a rosin or water soluble flux in your selective soldering process, then you should consult with your equipment manufacturer to make certain you have the appropriate options or materials for handling these types of fluxes.

Low-solids/no-clean fluxes breakdown into a few other categories:

• Alcohol-based, rosin or resin containing
• Alcohol-based, rosin or resin free
• Water-based, rosin or resin free (VOC-free); rare occasions will contain rosin or resin

In this category, there are a variety of manufacturers and many more flux choices. So how do you decide? Even if your customers, corporate management, or your available manufacturing processes dictate the flux you use, it is important to understand if the flux you are using is a help or hindrance to your selective soldering process.

Low-solids/no-clean fluxes in general, have less active chemistry and are more challenging to solder with than rosin or water soluble. Some fluxes are made to overcome particular issues—issues you may not have; while others may be weak in an area that is an issue for you. Frequently the type of products you manufacture can impact the flux that is best for your process. Or, simply the variety of products you manufacture can influence your choice of flux—and it may even require using different fluxes for different products.

One issue is that many of the available fluxes being used in selective soldering were originally intended for wave soldering. Regardless of the product or application, the wave soldering process was relatively the same across the industry and easier to adapt for these different fluxes. Only recently have flux manufacturers started producing fluxes specifically for selective soldering, recognizing that it is a distinctly different process than wave soldering.

However, among these various flux options, the rosin/resin containing, alcohol-based, low-solids/no-clean fluxes are usually the best option for the selective soldering process. They work well across various surface finishes, have a relatively wide process window, handle a wider range of time at high temperature, work with leaded and lead-free solders, and burn-off well, generally leaving safer residues.

To read this entire article, which appeared in the January 2017 issue of SMT Magazine, click here.