Rosin vs. Non-Rosin Wave Flux? - More Reliable Electronic Assemblies

Adam Murling and Ron Lasky, Ph.D.
Indium Corporation
Clinton, NY, US

Is there a benefit to using wave soldering fluxes that contain rosin versus ones that do not? Rosin-based fluxes are some of the original types of fluxes used in the early years of the electronics industry. They are based on material obtained from pine trees and other plants, primarily conifers. Rosin-based fluxes are non-corrosive at room temperature, hygroscopic, and normally cure at room temperature to entrap potentially corrosive activators.

In comparison, non-rosin fluxes - especially water-wash - contain aggressive acids that need to be cleaned off after the wave soldering process. If the assemblies are not cleaned, the residue can cause corrosion and dendritic growth.
A manufacturer can choose either a rosin-containing or a non-rosin-containing flux based on the solvent used, the ratio of flux to solvent, and the current cleaning process, to name a few. The choice is made as a result of multiple factors that come into play when wave soldering, such as thermal profile, type of flux, flux application, solder alloy type, preheat time, temperature, and wave contact time.

In this series of experiments, the independent variable was the flux type. The previously mentioned variables were varied in a designed experiment format. To evaluate the quality of the resulting assemblies, the current IPC surface insulation resistance (SIR) test was employed. Data were collected to determine the reliability of three rosin-containing and three non-rosin-containing wave fluxes. In addition, microphotographs of typical resulting solder joints were taken to observe workmanship and esthetic properties of the solder joints.

Both the organic fluxes and the rosin-based, no-clean fluxes passed the SIR test, which made it difficult to ascertain a trend. This is why the data was broken down further and represented in Figure 16. Having the data presented this way makes it easier to see a clear disparity between the rosin containing fluxes and their organic counterparts. The log10 of the overall average for the rosin-based fluxes was 11.76 ohms, while the organic fluxes were stable at 9.33 ohms. The rosin-based fluxes are more than 2 orders of magnitude greater in SIR than the organic fluxes. Having a higher resistivity implies that the material will be more resistant to dendritic growth and other defects. However, it is important to remember that the organic fluxes were above the minimum of log10 R = 8 ohms and passed per J-STD-004B.

Five out of the 6 fluxes' pattern down means were lower than the pattern up means, which is to be expected. It is intriguing that Flux A's SIR was better than the other 5, requiring future research and experimentation to arrive at a conclusion.