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Is No-Clean Flux Actually Clean?

No-clean solder paste

No-clean solder paste is being dispensed from an automated machine.

PCB manufacturing involves many procedures that are sometimes ignored or overlooked. These include things like testing, inspection, cleaning, and quality control procedures that ensure your board is produced to industry standards and your operating requirements. Within cleaning processes, there is one unfortunately-named product that implies cleaning is not needed: no-clean flux.

All PCBAs must undergo soldering processes, and within reflow soldering, there is a type of solder flux known as no-clean flux. The very name of this product implies that it will never require cleaning, but studies have shown that this is not necessarily the case. In some designs, the leftover residue will probably not interfere with your board’s functionality. In other cases, the design will require thorough cleaning in order to remove leftover chemical residue, including no-clean flux residues.

What is No-Clean Flux?

In general there are three types of solder fluxes: water soluble flux, rosin-based flux, and no-clean flux (which may contain rosin). Solder pastes used for manual soldering, selective soldering, or reflow soldering will include some type of flux, and this is commonly a no-clean flux formulation. Typically, the off-the-shelf solder pastes that are available to consumers include a no-clean flux, and could best be called no-clean solder pastes. These solder pastes also come in lead-free varieties that intend to comply with Pb-free/RoHS directives.

No-clean solder paste

Example no-clean solder paste that can be purchased from Digi-Key.

All solder flux leaves behind residues, including no-clean flux. The residue that is leftover by flux is claimed to not require any cleaning, meaning you can leave these residues on the PCB and you should not expect any interference with product performance. If you look at a hand-assembled PCB under light at an angle, you can most likely see the leftover no-clean flux residue covering the surface of the PCB a/s a thin transparent film.

What’s Wrong With No-Clean Flux Residues?

The problem with these residues is that they can be conductive after they are deposited on the PCB, or they can slowly become conductive over time. There are several reasons to clean this leftover material:

  • Prevent leakage currents between different circuits
  • Prevent contamination of circuits by residues
  • Prevent outgassing from the flux residue
  • Prevent moisture absorption into the flux residue
  • Ensure better adherence of conformal coatings
  • Ensure more accurate inspection and quality control

In low-voltage, low-density boards, the leftover flux residue probably won’t matter. In high-density boards, it’s possible for solder balls to become trapped in leftover no-clean flux residue between components.

In some cases, leftover no-clean flux residues will be benign and will not create any problems in the PCB layout with respect. As long as the flux has been fully temperature-cycled based on the paste manufacturer's solder profile, then the residue should be safe. Even if the flux residue is fully activated, it will still be hygroscopic, meaning it can still absorb moisture and it can still prevent some conformal coatings from adhering to the board. It is best to simply specify a cleaning procedure to remove the residues.

No-Clean Flux Removers

As a designer, it is generally not your job to select a no-clean flux remover, but rather to ensure the requirement for such a remover is specified if required. That being said, there are many no-clean flux removers that can be purchased from electronics distributors. These cleaning fluids are available in spray cans or in bulk so that they can be implemented in automated equipment. The cleaner and the solvent should generally be strong enough to remove any flux residue, but not so strong that it damages the solder mask, silkscreen, components, or any exposed conductors. No-flux remover vendors can provide guidance on pairing their products with certain fluxes.

No-clean solder flux cleaner

Flux-Off is one popular flux removal spray. [Image credit: Chemtronics]

Should Water-Soluble Flux Be Used Instead?

Someone might ask this in an effort to try and avoid the need for cleaning. The reality is that water soluble flux residues should always be cleaned from a PCBA. Water-soluble fluxes are much more reactive and will leave behind corrosive residues. Therefore, these will need to be cleaned by thorough rinsing with distilled water. In modern boards that use moisture-sensitive components, it may be best to simply avoid the use of water-soluble flux altogether.

Specify Your Cleaning Needs

It’s easy to assume your assembly house will go through best practices and protocols when handling chemical residues on your PCBA, including removal of no-clean flux residues. Unfortunately, budget assembly houses can still cut corners with your product, either by using a substandard or overly rough cleaning method, or by omitting this step completely.

If you are planning to put a new prototype into production, these points surrounding flux residue removal should be determined during the prototyping phase. To prevent problems, make sure you think through the best flux cleaning procedure for your board. If you’re unsure of the right procedure, consult your volume manufacturer; they should be able to provide a suitable procedure that you can specify for your prototype. Make sure to communicate these needs to your prototyping house before beginning production.

When you’re ready to define your assembly requirements in your manufacturing deliverables, you can take greater control of your board with the complete set of design tools in OrCAD from Cadence. OrCAD includes the industry’s best PCB design and analysis software, complete with a set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.

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