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Issues of Burnt Flux on Class 3 PCB Assemblies

Question: I have always had in the back of my head that burnt flux is a defect for all class 3 assemblies. Upon further review I have not been able to identify the specific section of the IPC-A-610F standard which identifies burnt flux as an acceptable or defect condition. I understand that if the laminate is burnt it is a defect condition, but I am not clear on burnt flux for a no-clean process. Any clarification would be appreciated.

Answer: Flux residues are flux residues, regardless of whether it is burnt or not. Reference section 10.6.1 Cleanliness – Flux Residues in the 610.

The low solid content fluxes are designed to have the activators in the flux volatilize off during the soldering operation and the residues left behind are benign, which is they won’t hurt or degrade the product if left on the boards after soldering.

Fully active fluxes, if left behind, are susceptible to the absorption of moisture which is a problem when the product is in the field as the flux residues are still active and when moisture is absorbed, will activate the flux and corrosion can result.

Another reason to remove the flux is for testing purposes. Rosin type of fluxes, once polymerized, are very difficult for pin type of test stations (fixtures) to pierce through the rosin, creating lots of open connections.

Burnt flux implies the carbonization of the rosin material or any other material and this material is very susceptible to the absorption of moisture and the issues we just mentioned. Secondly, if the residues are burnt it will impact the ability to visually inspect the solder joints, so it has to come off.

You mention burnt flux for a no-clean process, again, this is an issue with the chemical material phase changes which happens with the residues once they exceed their boiling temperatures. These residues are no longer impervious to the environment and pose a reliability problem with the product. Secondly, I’m not sure I fully understand how you are burning the flux as one would have to have an extra long dwell time when making a solder joint to burn the flux, which will definitely impact the laminate in the area where the burn flux is visible.

Ultimately, from my perspective “burnt flux”, regardless of clean or no-clean chemistries, is a defect under all classes.

This is an excerpt from IPC-A-610 Revision “F” on page 10-40 for further clarification if this will help:

Courtesy of IPC from IPC-A-610
Courtesy of IPC from IPC-A-610