Effective soldering relies on PCB cleaning both before and after soldering

Understanding Soldering Part 8: PCB Cleaning Before & After Soldering

Cleaning is undoubtedly important and not just because our mothers place so much emphasis on it. But is a deep level of cleaning necessary for everything, such as printed circuit boards before or after soldering? There is no question that a circuit board can be contaminated by ordinary grease and grime, oxidation of its exposed metal surfaces, and the effects of PCB manufacturing. But while PCB cleaning is necessary for some manufacturing processes, it is not necessarily required for all. Here is a deeper look into PCB cleaning before soldering and PCB cleaning after soldering to help you understand when cleaning is necessary and when it is not.

PCB Cleaning Before Soldering

Whether or not circuit boards should be cleaned before soldering depends on the unique scenario. Those circuit boards coming straight from PCB fabrication should be clean and ready for automated assembly due to the quality control techniques practiced throughout fabrication. Likewise, a fully assembled circuit board delivered to the end-user should not require any additional cleaning if it has been packaged and shipped carefully. However, older boards may have oxidation or contaminants that accumulated over time and may require cleaning before soldering is performed. When reworking such a board, you may first use isopropyl alcohol to remove the contaminants. And be sure to avoid making the mistake that the flux used during soldering will provide enough cleaning of the board—before and after cleaning should always be performed.

Flux is a cleaning agent that prepares metal surfaces for soldering by removing oxides and impurities. Flux is either combined with the solder being used or applied separately depending on which soldering process is employed. In addition to its cleaning capabilities, flux also helps PCB assembly by altering the surface tension of the molten solder.

The question then becomes, does the circuit board require additional cleaning before soldering, or is the flux adequate in itself? Ideally, circuit boards should not be handled prior to assembly. Preventing the board’s contact with human hands can help reduce or eliminate potential contamination. Human contact can leave grease, grime, and salts on the board and lead to the corrosion of exposed metal areas that can affect its solderability. Additionally, dust, inks, tape, and condensation contamination can also add to these difficulties. However, cleaning circuit boards prior to assembly in large-scale production processes involves a cost that may not be warranted for the level of the circuit board being assembled. Therefore, the best practice is usually to exercise caution in handling the circuit board prior to assembly and omit any pre-soldering cleaning unless absolutely necessary

Will You Need PCB Cleaning After Soldering?

Cleaning after PCB assembly is a different story. Depending on the type of flux, application, and the desired aesthetics of the circuit board, cleaning may be required:

Fluxes

There are different types of fluxes used in circuit board assembly, and the type used will determine if it needs to be cleaned or not:

  • No-clean flux: Many circuit boards used in non-critical applications can be built safely with no-clean flux. As long as the flux is fully activated by bringing it up to the full soldering temperature, it will not require any special cleaning for regular performance.
  • Water-soluble: While these fluxes should be cleaned off after soldering, cleaning agents such as deionized water and detergents will do the job.
  • Rosin-based: These fluxes need to be cleaned with specific chemical solvents that often include fluorocarbons.

No-clean fluxes, as their name suggests, do not need to be cleaned off after soldering in all cases. The other fluxes do. Water-soluble, rosin, and rosin substitute fluxes leave an ionic residue that can form dendrites or conductive tendrils when in contact with moisture in the air and current. Not only can the dendrites create a short between different circuits, but the other flux residues could continue their chemical activity and cause more corrosion while the board is operating.

Board applications

Even though no-clean solder does not require clean-up, certain applications of circuit boards still require cleaning. Mission-critical circuit boards used in medical devices or aircraft often have design standards that specify cleaning the board after soldering. The flux residues on circuit boards with a long operating life can absorb moisture from the air leading to oxidation on exposed metal surfaces, which over time can create corrosion. Also, it is important to remove all flux residue for circuit boards that will be conformally coated to provide the coating with a clean surface for adherence.

Aesthetics

The board’s aesthetics may dictate cleaning in some cases, even if a no-clean flux is used. For automated circuit board assembly, several different cleaning processes are available:

  • Ultrasonic: Sound waves are used to break up and lift flux residues from the PCB. Care must be exercised, however, as ultrasonic cleaning can cause damage to on-board clocks and crystals.
  • Vapor degreaser: PCBs are submerged in boiling solvent and then rinsed in ultrasonics and solvent vapors.
  • Batch flux removal: Circuit boards are held in a rack and water-based flux remover is sprayed over the rack.
  • Inline flux removal: Circuit boards are subjected to water-based flux remover spray again, but this time they travel through the process on a conveyor system.

Circuit boards can also be manually cleaned of residual flux, although this is a labor-intensive operation that will vary between operators:

  • Aerosol: Chemical solvent sprayed onto the board to remove the flux.
  • Trigger spray: Water-based cleaners or isopropyl alcohol (IPA) are often sprayed from standard bottles.
  • Immersion: Circuit boards will also be bathed in a tray of solvent for cleaning.
  • Swab: This is the most common way to clean flux from an area of a circuit board being repaired. The swab is treated with IPA, then rubbed around the repair area.

Remember, though, some components simply cannot be cleaned due to the risk of internal damage to the part. The use of those types of parts needs to be carefully evaluated in the project’s design phase. Additional protection for those parts should be provided, or a substitute part should be used.

As you can see, many questions surround PCB cleaning before and after soldering. Fortunately, your PCB contract manufacturer can help.

Understanding PCBA Complexity: Designing for Trouble Free Manufacturing

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Where You Can Find More Information About PCB Soldering and Cleaning

In PCB manufacturing, the goal is to build an error-free circuit board that will deliver its expected performance throughout its projected life-cycle. To do this requires all aspects of PCB assembly, including soldering and PCB cleaning, to be performed according to industry standards and customer expectations. To understand which assembly procedures will be the best choice for your circuit board, talk with the engineering and manufacturing staff where your board will be built.

Tempo's Custom PCB Manufacturing Service
  • ISO-9001, IPC-600, and IPC-610 commitment to quality certifications.
  • Execute your full development cycle from proto to validation, NPI, and low volume production.
  • Accurate quote in less than a day.
  • Performs entire turnkey process in as fast as 4 days.
  • DFX support, including DFM, DFA, and DFT from Day 1 of design.
  • Sources components from the most reputable suppliers in the industry. to reduce procurement time.
  • Software-driven smart factory with monitoring and control throughout the manufacturing process.
  • Performs multiple automated inspections during PCB assembly to ensure PCB quality for prototyping.
  • Smooth transition from prototyping to production.

At Tempo Automation, our staff is ready to answer your questions about soldering and cleaning and help you determine which assembly process will be the best choice for your project. We’ve been working with various customer requirements for a long time and have the soldering and PCB cleaning expertise you need to build your circuit board to the highest level of quality.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on why PCB cleaning before and after soldering is important, contact us.

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