Ask the Experts
November 9, 2023 - Updated
January 8, 2013 - Originally Posted

Reduced pH and Specific Gravity of Flux

If both the pH and specific gravity of the flux used in our wave soldering operation are reduced, what effects will we see in the wave soldering process?


Expert Panel Responses

Let's take specific gravity first... if it is low, that means that there is more solvent and less solids present (I'm assuming a solvent-based flux, not water-based). If that's the case, you may see reduced fluxing action, depending on how great a reduction in solids.

If the reduction is moderate, you may in fact see no detectable effect. How much dilution of the flux is required to show a process effect is dependent on a lot of variables,including the properties of the flux, the solderability of the boards, the process settings in the wave, among other things.

Now let's take the pH part of the question. When you say pH is reduced,I interpret this as there being a lower pH number, which actually means more acidity. That normally does not occur if the solids content is on the low side.I'd expect a slightly less acid pH (higher number).

pH is not a great indicator of flux performance in any case, because the flux is relatively weakly buffered and the pH can be affected by other things. A better way to measure the acid content is by a simple titration to determine the acid number. Your flux manufacturer should be able to guide you as to how to implement such a test.

Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

By lowering the pH of the flux, it becomes more acidic and the same applies to changing the specific gravity. As the alcohol in the flux evaporates the flux gets more concentrated or stronger and more aggressive.

Both of these issues will create performance changes on the product if there are not cleaned of regardless of whether or not they were low solid content flux.

The effects seen on the wave solder process may be minimal but the flux residues remaining on the board will be more aggressive and this will need to be reviewed and analyzed. From an equipment perspective the stronger the flux or the lower the pH of the flux, may cause increased maintenance problems on the conveyor system, fixtures and the venting system.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

If the same flux is used, when both pH value and the specific gravity are reduced, it usually means the activator content is below the normal. You might see a degradation on the activity of the flux, which may cause bridging, icicling, skipping, and poor hole fill etc.

David Bao
Director New Product Development
Metallic Resources, Inc
David Bao has more than fifteen years of experience in developing new solder paste, wave soldering fluxes and other SMT consumables. He currently serves as the Director of New Product Development at Metallic Resources Inc. He received a Ph.D. in Chemistry at Oklahoma State University.
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