No clean flux vs washing

I have a PCB with barbed port pressure sensors. In the past I’ve used these and don’t believe no clean flux was used. Now I am told these parts can’t be washed. Seems there are three options: Hand solder the pressure sensors after the rest of the board is manufactured (+$20 per board), use no clean flux, or plug the senor ports then wash.

What are the down sides of using no-clean flux? Any reason not to?

Thanks,
G.

1 Like

Hi Greg, use the no clean solder paste to install the pressure sensors. Only you will see the flux residue on the solder joints.

1 Like

Like so much about soldering, this question appears simple enough to deserve an unqualified answer but is more complicated than it seems. After all, lessons about flux take up several hours in my Science of Soldering classes.

Of the three options you list, only the no-clean flux route makes sense. In fact, no-clean flux is the best choice for almost all modern assemblies. (Even the most aggressive post-solder washing using the best solvents can’t remove all flux trapped under low-lying components and test solutions don’t reach areas that the cleaning solvents couldn’t penetrate.)

But here’s where things get complicated. Flux manufacturers slap “no clean” designations on a wide variety of chemical formulations, some of which pose serious reliability risks. In other words, be wary. Although most no–clean fluxes leave little visual residue, what can be seen is less serious than what can’t. Flux-related failures mostly occur because the invisible material (mostly ionic residues of acids) can be destructively conductive or corrosive. How much conductivity can be tolerated depends on the assembly involved.

A low solids (small concentration of rosin) ROL0 or ROL1 formulation tends to be the safest no–clean flux. I’ve never encountered a failure using such flux despite the countless millions of assemblies used in the harshest environments like automotive. Rosin repels moisture, so encapsulates the acids and acts like a conformal coating. (But be careful if you apply real conformal coating. Some conformal coatings react badly when applied over even thin layers of rosin. That’s not a reason to reject rosin-based no–clean flux; change to a conformal coating that tolerates some flux residues.)

Finally, here are my rules when changing process elements with which you don’t have experience. First, recognize that no one has more at stake than you. Outsiders who give advice can go on to other clients if your change goes badly, but your entire company could be at risk. Second, solicit but don’t blindly follow a supplier’s recommendations. The better-known suppliers have a lot of experience and don’t (usually) lie, but they also can’t know your product as well as you. Third, remember that consultants are not necessarily as informed as they would have you believe. Fourth, put in the effort to understand why you are considering the change(s). Above all, qualify the change through environmental stress testing.

If you would like to know more, Sierra Circuits is sponsoring one of my online Science of Soldering classes in September. Please consider attending.

Jim Smith
President
Electronics Manufacturing Sciences, Inc.

2 Likes

You can find more info about Jim’s soldering class here: Seminar: Science of Soldering© by Jim Smith | Sierra Circuits

No-clean flux can be a good solution, but it has some important considerations to keep in mind:

  1. Residue Removal: No-clean solder paste residues are generally harder to remove compared to other types of flux. While the name suggests no cleaning is required, in certain cases, cleaning may still be necessary.
  2. Cleaning Process: No-clean solder pastes cannot be cleaned with water alone. You’ll need aqueous or solvent-based cleaning agents, and in some cases, automated cleaning systems like conveyorized inline cleaners or batch cleaners may be required. These systems also often operate at higher temperatures and spray pressures than those used for water-soluble residues.
  3. Residue Types: No-clean fluxes that contain halides leave more residues but are relatively easier to clean. On the other hand, halide-free no-clean fluxes produce less residue but are more challenging to clean.
  4. Heat Exposure: Be cautious with temperature exposure. If no-clean solder is subjected to excessive heat, the residue can harden, making it significantly harder to remove later on.
    In short, no-clean solder paste is a practical option, but if cleaning becomes necessary, it can introduce complexity and require specialized equipment.
1 Like

There’s no such thing as a truly “No Clean” solder paste. What exists are low-residue pastes, which might not require cleaning for low-complexity, low-value boards. However, for high-reliability boards, particularly those dealing with relatively high voltages, skipping the cleaning step is not advisable.

When discussing flux, it’s important to note that the corrosion of lead-free solders (such as SAC types) is primarily galvanic. This occurs due to the dissimilar metal corrosion caused by the different anodic potentials of the metals involved. Additionally, lead-free solders are generally more difficult to work with than traditional SnPb solders. They require longer reflow times, and in hand soldering, the process can take around 5 seconds compared to just a second for SnPb. This increased difficulty can lead to a higher risk of solder splashes and flux outgassing, which needs careful attention during the reflow process.

If your boards intended for high-humidity environments, conformal coating is essential to prevent or minimize corrosion. In humid conditions, airborne salts combine with moisture, potentially creating unwanted current paths and accelerating solder joint degradation. Improperly cleaned boards (where residues, especially salts, were left behind) can lead to poor adhesion of conformal coatings. While conformal coatings offer some moisture protection, they are not perfect barriers. In the event of residue contamination, the coating may not adhere properly, allowing moisture to seep beneath it. Residues left on a PCB can crack under environmental stress (such as temperature changes or vibration), which exposes them to moisture absorption. This can result in electrochemical migration or leakage paths, both of which can cause major reliability issues. Additionally, flux residues are essentially salts, which, when combined with moisture, can lead to dendrite growth and localized short circuits.

2 Likes

No-clean flux has been widely adopted, with approximately 70-75% of global electronics using it without any flux residue cleaning—saving significant costs in the process. However, while it has a good track record, there are important considerations if you choose to clean the residue.

Pros of No-Clean Flux :

  • No-clean flux is reliable for a majority of assemblies, particularly in high-volume manufacturing. It minimizes the need for post-solder cleaning, leading to substantial cost savings.
  • The rosin in the no-clean flux forms a protective, inert layer over the residue, preventing moisture from causing ion migration, which could lead to corrosion or electrical failures.

Cons of Cleaning No-Clean Flux Residue :

  • If cleaning is necessary, deionized (DI) water alone cannot be used. The rosin will react with the water, forming white powdery deposits. Instead, specialized solvent-based cleaners or saponifiers are required, which can be costly and may require additional capital investment in cleaning systems.
  • For components with low standoff heights, it becomes increasingly difficult to remove the flux residue effectively, making the cleaning process even more challenging.
  • If the no-clean flux residue is not cleaned thoroughly, it can pose the same risk as leaving water-washable flux residue behind. Any ions remaining on the board can eventually lead to corrosion and long-term reliability issues.

Ultimately, no-clean flux is a great solution in most cases, but cleaning its residue introduces challenges that may outweigh its initial benefits. Proper process control is essential if cleaning is required, as improper cleaning can cause more harm than good.

1 Like

No-clean fluxes offer significant cost, but they also come with some disadvantages that should be carefully considered:

  • No-clean residues can sometimes slip past masking and interfere with testing, especially on bed-of-nails test points. This can lead to false test results or the need for rework, which can offset the initial cost savings.

  • The residues from no-clean flux are often brittle and transparent, which can make solder joints appear cracked or improperly formed during visual inspections. This can complicate quality control, especially in high-reliability applications.

  • Residues left behind by no-clean flux can inhibit the adhesion of conformal coatings. This becomes especially problematic in high-humidity environments or when additional protection against corrosion is needed.

  • If the flux is not properly activated by heat, it can remain corrosive, leading to long-term reliability issues. Ensuring that the soldering process reaches the correct temperature is crucial to avoid this risk.

1 Like

No-clean flux is a type of mild rosin core flux with reduced flux activity. It’s less aggressive in nature, designed so that the residue left after soldering contains substances like binders and pH stabilizers to minimize reactivity. The ‘activity’ of solder flux refers to its ability to remove oxidation from pads and pins during the heating process, which is crucial for proper solder wetting. Ideally, flux acts as a heat-activated acid that evaporates after the solder bonds. However, some residue always remains, which can affect the board.

No-clean fluxes, particularly the less active ones, are meant for use with fresh boards and components that have little to no oxidation. These fluxes struggle with older parts where oxidation is heavier, leading to poor wetting. On the other hand, highly active fluxes work wonders for cleaning heavily oxidized surfaces and preventing oxidation for longer periods. However, their downside is that the leftover residue remains reactive and, over time, can attract moisture, leading to corrosion. In one case, a board exposed to ~85°F and 75% humidity over a period of 2-2.5 years lose entire footprints due to highly active flux residues left behind.

Interestingly, cleaning flux residue can sometimes accelerate corrosion, as removing the protective binders can expose active components to the environment. If you want to clean this residue, a mixture of 70%-90% isopropyl alcohol and water works well. You can gently scrub the areas with a toothbrush to remove any thick layers.

The reason cleaning can sometimes accelerate corrosion is due to the nature of flux. Flux residue is a combination of inert substances and more reactive components (like metal halides). When you clean, the easier-to-remove inert substances get scrubbed away, exposing more of the reactive elements. These reactive residues, even in tiny amounts, are more susceptible to humidity, which can lead to corrosion over time if left unprotected.

That said, if you’re using a “no-clean” flux without halides or other highly active components, the risk is lower. However, if you see blackened or “burnt” flux, it suggests improper reflow, meaning the flux wasn’t fully activated. In that case, thoroughly cleaning is important to avoid long-term damage, especially with water-soluble fluxes, which must be completely removed with warm water to prevent corrosion. So, while cleaning isn’t always necessary with “no-clean” flux, if you prefer a clean board, use the right method and tools to ensure all active residues are effectively removed.

1 Like

If your assembly won’t be exposed to extreme environments and doesn’t require a conformal coating, using a no-clean solder process without washing should be acceptable for most applications. The no-clean flux residue generally isn’t harmful in standard operating conditions.

However, if the board requires conformal coating or will be used in high-reliability applications, high-impedance circuits, or harsh environments, it’s better to clean the board regardless of the soldering process. Residual flux can impact the performance and longevity of the board in those scenarios, potentially leading to issues like corrosion or contamination over time.

1 Like

Using no-clean flux does come with some risks, such as the potential for electrochemical migration and dendrite growth, especially in environments with moisture. However, if the flux reaches a high enough temperature during soldering, it typically deactivates, reducing these risks.

For assemblies used in harsh or outdoor environments, it’s generally a good idea to clean the boards regardless of the soldering process. Water-soluble fluxes, like OA fluxes, can be easily cleaned with deionized water but can cause corrosion if not fully removed. While RMA and no-clean fluxes can also be cleaned, they may require specialized cleaning agents to ensure all residue is eliminated.

Ultimately, the choice of flux and cleaning method depends on the specific reliability requirements of your assembly. I’d recommend consulting with your flux or paste supplier to make sure you’re using the right process for your application.

1 Like