Make Sure to Clean Your No-Clean Flux

Alexsander Tamari
|  Created: April 25, 2024  |  Updated: May 22, 2024
no clean flux

No-clean flux has become popular due to its convenience. However, despite its name, the question of whether no-clean flux should indeed be left on the PCB after assembly is a topic of ongoing debate. This applies in hand soldering of through-holes, where a wire might be used with flux, or when solder pastes are used with SMD parts which may have a no-clean flux formulation.

No-clean flux is intended to leave behind a harmless residue after soldering as the residue is supposed to be non-conductive and non-corrosive. This would eliminate the need for post-soldering cleaning, which can save on time, materials, and ultimately costs. Yet, the decision to clean or not to clean these residues is not as straightforward as it might seem. Various factors, including the operating environment of the PCB, the complexity of the assembly, and long-term reliability concerns, can influence this decision.

Why No-Clean Flux is Used

All fluxes used in PCB soldering are intended to aid wetting by removing oxidation from the metal surfaces being soldered. It's designed to break down during the soldering process, and ideally it would leave behind minimal residue. There are three types of flux used in solder formulations:

  1. Rosin core flux
  2. No-clean rosin-based flux
  3. Water soluble flux

No-clean flux is formulated to leave behind very little residue on the PCB after the soldering process has completed. The remaining residue should be non-conductive and non-corrosive, which allows elimination of a cleaning step at the end of PCB assembly. Learn more about various solder fluxes in this article.

If you have ever looked at a picture of an older PCB which did not receive a thorough cleaning, one can see what happens when flux residues start to degrade over time. The image below shows one example of remnant flux residues contributing to oxidation over long time periods. Flux residues are hygroscopic and will form conductive electrolytes when they absorb water from humid air, which then leads to accelerated corrosion.

The image below shows how flux residues spread across an assembly during reflow. By spreading across multiple land pads, even with conservative pitch, it becomes a concern as to whether these residues create reduced surface insulation resistance (SIR), or contribute to electrochemical reactions that would create an ECM short. Read the source article in the image caption below to learn more about this important study.