Nowadays, no-clean solder pastes are used to construct many small-component PCBs. By removing the need to clean circuit boards after reflow, no-clean solders help to speed up the production process. You don’t need to remove the solder paste or hold it in place without affecting the circuit board’s function, performance, or expected life expectancy. As a result, no-clean solder pastes are now involved in millions of electronic assemblies.
Mal Fletcher
Experts believe that no-clean solder paste is used in the fabrication of more than half of today’s printed circuit boards. In other cases, manufacturers use no-clean pastes on their whole manufacturing lines and then clean only the boards they want. They may, for example, choose to clean only high-reliability circuit boards, such as those used in security, military, aerospace, and medical systems, where cleanliness is crucial. They may omit other non-critical boards from the cleaning process entirely, thus, saving time and effort for manufacturers.
Rising PCB Complexity:
As printed circuit boards become more complicated and closely packed, some PCB manufacturers have discovered that no-clean soldering isn’t always compatible with today’s power electronics. Micro components like flip-chip, micro BGA, CSP, and QFN packages are squeezed into tighter places on boards as the need for smaller electronics grows.
The manufacturers widely use MOSFETs, which have a low standoff. The number of I/O (input/output) ports is growing, and circuit boards are becoming more multi-functional. If they do not clean the PCBs adequately, this trend towards circuit board reduction, complexity, and high density may increase the chances of complications.
This is especially true for those made with no-clean solder pastes. PCBs that are dirty are susceptible to parasitic spillage, electrochemical migration, shorting, and dendritic formation, among many other issues.
Cleaning no-clean flux residue is no longer a consideration for long-term PCB performance, efficiency, and durability. The reality is that no-clean fluxes were created to minimize the requirement for PCB clean-up. However, they could now provide one of the greatest PCB cleaning challenges.
When the salt enhancers in the no-clean flux come into contact with heat or other chemicals, a white residue is left behind, which can damage delicate circuits and allow dendrite development. No-clean flux, unlike other fluxes, was designed to stay on the board. As a result, it has become some of the hardest contaminants to remove from printed circuit boards.
Better Board Performance:
As current PCBs get more advanced, many manufacturers are preferring to clean all of their PCBs rather than just the high-reliability ones. A major factor is avoiding unreliable performance, expensive board failures, product defects, and product returns. As a result, despite the advent of no-clean soldering, many manufacturers still clean.
The primary reason for cleaning no-clean flux and its residue is to avoid circuit faults and signal transmission disruption. For example, if there is too much no-clean flux on a circuit board, or if a white residue is left behind, the board will produce noise. This is particularly true in the case of high-voltage systems.
Mal Fletcher
In particular, solder balls become trapped between closely spaced components. For optimal circuit board performance, other contaminants such as ink and fingerprints might have to be removed. For other reasons, assemblers may want to clean no-clean fluxes. Among the most common are improved board visual appearance, better inspection and quality management, and improved conformal coating performance.
Improved Aesthetics:
The appearance of printed circuit boards can be affected by flux residue. This is a major concern in the electronic industry. Flux residue might be seen as negligent or sub-par work by some clients. Cleaning flux residue off circuit boards may make them more aesthetically pleasing and appealing to customers.
Better Inspection and Quality Control:
Unclean flux and excessive residue may make inspections difficult. QC (quality control) testing and troubleshooting field repairs become more difficult if flux is left on the circuit boards. It can also cause automatic visual alignment systems to fail. By properly cleaning no-clean fluxes, it may be feasible to make PCB examinations faster and more efficient.
Improved Conformal Coating Results:
Some PCB manufacturers are realizing that no-clean residue can sometimes inhibit conformal coatings from sticking properly. Circuit boards in outdoor or other hostile environments face this issue. Flux residue can absorb moisture.
During curing, this contained moisture may be released, causing the coating to detach off the board. Corrosive materials, dust, or water can penetrate the PCB assembly, resulting in corrosion, signal transmission issues, or even hardware failures. Before conformal coating, thorough removal of clean flux and associated residue may help to avoid this.
How to Clean No-Clean Flux?
Water alone will not remove the flux residue left behind by no-clean solder paste; a saponifier solution in water is required. Assemblers choose the most effective cleaning method that is based on the composition of the flux, whether the residue is hard or soft, and whether the flux contains low or high concentrations of solids.
Fluxes containing halides produce more residue, but assemblers can quickly clean them using mild cleaning chemicals and short wash durations. Halide-free no-clean fluxes, on the other hand, leave less residue but are more difficult to remove.
When no-clean solder pastes are subjected to high temperatures, they oxidize, leaving hard residues and poor solder connections. For the greatest soldering results, Rush PCB suggests following the paste manufacturer’s guidelines.
Cleaning No-Clean Flux (Lead-Free):
The residue left behind by lead-free no-clean pastes is hard and difficult to clean since reflow temperatures for melting lead-free solder are typically 240-250°C. As a result, before the assembler can add other materials like coatings, process engineers must handle the cleaning concerns. Selecting pastes that are easy to clean increases the PCB’s quality, according to Rush PCB.
Conclusion:
No clean fluxes, in general, can be incredibly difficult to clean, especially after any reflow processes, because they can be coated on and hard to remove.
It’s critical to address any difficulties with no-clean technologies and the cleaning challenges that come with them early in the device design phase, especially for high-reliability products like avionics or medical equipment. Cleaning professionals should be consulted for the most effective, board-safe cleaning technique in situations where cleanliness is important.