The reason behind no-clean solder pastes was to get rid of the need to clean printed circuit boards. However, most assemblers still clean PCB assemblies. Some PCB manufacturers have designed and fine-tuned the PCB assembly processes to have no-clean paste. Therefore, they prefer to run all production using one process and clean those that need it.
No-clean fluxes are specifically designed to get rid of any need to clean. Any flux residues left by no-clean pastes are harder to remove than other fluxes. Therefore, these residues can’t easily wash off which causes a problem when you need to clean them. A No-clean flux is a “low-residue” solder paste.
PCB assemblers started cleaning no-clean fluxes as a result of issues found at in-circuit testing. Also, residues from no-clean pastes were hard to remove. Test pins would also not penetrate through to achieve good electrical contact with test pads. Also, this residue would end up building up on pins impacting accuracy. However, this is no longer a problem as recent formulations have made things better.
Why Clean a No-Clean Flux
PCB assemblers started cleaning PCBs soldered with no-clean paste when problems occurred during in-circuit testing. No-clean pastes once earlier produced tacky residues after every reflow. Also, there was build up of residue on pins. This affected the required maintenance and accuracy of pins.
These days, it has become essential to clean a no-clean flux residue. Also, cleaning is crucial for long-term circuit board functionality and performance. No-clean fluxes were initially designed to eliminate the need for circuit board cleaning. Rather, they now result in more difficulties in PCB cleaning.
No-clean fluxes contain salt activators. When these salt activators come in contact with chemicals or heat, they form a white residue. This residue may enable dendrite growth as they corrode fragile circuits. Most manufacturers now choose to clean all their circuit boards due to its importance.
Easier inspection and quality control
The residue left behind by no-clean pastes can make inspections difficult to perform. Also, if flux remains on the PCB, it can make troubleshooting field repairs more challenging. Furthermore, it can cause failure in automated visual alignment systems. Thorough cleaning of no-clean pastes will make inspections much easier and more accurate.
Improved board performance
Cleaning no-clean pastes helps to prevent any malfunctions in printed circuit boards. Also, it helps to prevent interference with signal transmission. Build up of no-clean pastes on a PCB can create noise on the PCB. Also, solder balls can trap between densely-packed components. PCB assemblers may need to clean contaminants for optimal PCB performance. Also, better conformal coating performance is another benefit of cleaning no-clean fluxes
Early specification
As circuit boards become more complicated, PCB designers nee to determine cleaning options earlier in the design process. Also, they need to determine cleaning requirements before deciding about coatings and other materials. Early fluid specification before production helps to resolve any cleaning problems.
Improved Aesthetics
Flux residue can reduce the cosmetic appearance of a circuit board. Aesthetics is a major concern for consumer electronics. Also, some customers believe that flux residue indicates sub-par work. Therefore, cleaning the flux residue from PCBs can give them better aesthetics.
Testing
Testing also occurs earlier when you clean no-clean solder pastes. The majority of manufacturers now test-clean in small batches before full-scale production. Also, it is beneficial to manufacture and clean a small batch first. Therefore, this ensures cleanliness before starting full production mode. If the cleaning fluid works on a smaller scale, higher-volume will be manufactured.
How to Clean a No-clean Flux
To thoroughly clean the flux residue, simple water won’t do the job. Therefore, you need a solution of saponifier in water to clean these residues. The chemistry of the flux used determines the most effective cleaning process to use. Also, fluxes having halides leave more residues. However, PCB assemblers can clean these residues using mild cleaning agents and incorporating short wash times.
Halide-free no clean pastes leave fewer residues behind. However, they are more difficult to clean. Furthermore, a no-clean solder paste exposed to high heat can oxidize and leave residues. This will form dull solder joints. Dull solder joints aren’t good for PCBs. It is important to follow the instructions of the solder paste manufacturer to achieve the best soldering experience.
Some no-clean fluxes are lead free. Lead-free no-clean fluxes leave hard to clean flux residues. This is because the reflow temperature used for melting lead-free solder is within 240-250oC. Therefore, process engineers need to find a solution to this cleaning problem before the PCB assembler adds coatings.
Therefore, it is advisable to choose no clean pastes easier to clean. This will help to enhance the PCB quality. The cleaning process for lead-free no-clean solder pastes is different. You can run no clean paste with water soluble flux.
How to Choose No-Clean Flux Removers
There are several no-clean paste removers in the market. However, the strength of these removers varies. Therefore, choosing an ideal no-clean paste remover needs matching the fluid to the contaminant. Also, the cleaning fluid needs to be compatible with the PCB substrate and other materials.
Furthermore, the cleaner should be strong enough to get rid of the flux residues effectively. Workplace safety requires using a cleaning-fluid dispensing tool. Also, this helps to remove the contaminants from the workers’ hands. Therefore, this regulates workers’ exposure to fluids and fumes.
No-clean flux removers have various strengths. PCB assemblers need to select a cleaning fluid by matching the fluid to the contaminants. Also, they should select a nonflammable no-clean flux remover to ensure safety at the workplace.
If a PCB assembler has trouble in removing a no-clean residue, a different cleaning fluid can be of help. Also, they could use a paste with less resins or solids in the flux. Paste with a low amount of solid will have less residue left on the PCB. Therefore, this makes it easier to clean the residue. Furthermore, no-clean fluxes having fewer halides are much easier to remove.
Conclusion
No clean fluxes leave residues behind. This is a major reason most manufacturers clean no-clean fluxes. Also, cleaning no-clean paste has transformed PCB manufacturing. Following the recommendations of the paste manufacturer will help to achieve a great result.
