PCBs
Cleaning no-clean flux residue: when, why and how?
By Elizabeth Norwood, senior chemist at MicroCare I
n the ever-changing world of electronics manufacturing, the use of solder fluxes has been crucial to the efficiency and reliability of printed circuit board (PCB) assembly. One significant innovation in this area is the introduction of no-clean solder fluxes, designed to end the need for traditional cleaning processes after soldering. But why, then, do we find ourselves discussing the cleaning of something explicitly labeled “no-clean”? To understand this paradox, let’s delve into the history, development and solutions surrounding no-clean solder flux.
The genesis of no-clean flux To understand the need for cleaning no-clean flux, we must first rewind to the late 1980s. Prior to this era, electronics manufacturers relied heavily on cleaning flux and solder paste residues with aerosol spray solvents containing chlorofluorocarbons (CFCs). However, the detrimental environmental effects of CFCs, notably their contribution to ozone layer depletion, prompted global action. In 1987, the Montreal Protocol banned the use of CFCs, causing a shift in PCB manufacturing practices. Rather than seeking alternative cleaning solvents, the electronics industry took a different approach. They developed solder fluxes that didn’t require any cleaning. Thus, no-clean solder flux was born.
The catch with no-clean flux The catch to the success of these new no-clean fluxes was that they need to be properly heated. No-clean flux relies on specific heating processes to oxidize activators into benign, non-ionic, or low-ionic residues, encapsulating them to prevent them corroding electronic components or causing connectivity issues. It is important to follow the solder paste manufacturer’s recommended heating profile. Not enough heat can leave harmful residues behind, but exposing no-clean solder pastes to extreme heat may cause the flux residue to darken and solidify, making the baked-on residue harder to clean.
In automated surface mount technology (SMT) environments, where specific heating profiles can be meticulously controlled to the
22 March 2024 Components in Electronics
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Chip, FPGA and QFNs, these densely packed boards offered little room for the flux to fully heat-activate due to the limited space and standoff. This caused the electronics industry to reevaluate the need for cleaning their no- clean flux residues.
The presence of visible residues can detract from the overall quality of the assembly
precise required temperature, the no-clean fluxes worked well, and benign flux residues were safely left on the PCB without cleaning. However, challenges arose in hand soldering or touch-up operations, where
Vapour degreasing is ideal for batch cleaning larger quantities of PCBs
achieving the precise heating required for residue encapsulation proved more difficult. In addition, as electronics evolved towards high density surface mount boards featuring components like BGAs, Micro-BGAs, Flip
Four reasons to clean no-clean 1. Preservation of signal integrity: No-clean flux residues have the potential to disrupt signal transmission, resulting in diminished performance, especially in the case of high-frequency or high-voltage assemblies. 2. Conformal coating adhesion: Even the smallest residue of no-clean flux can impede adhesion in conformal coating applications, emphasising the need for thorough cleaning to achieve the best coating performance. 3. Improved aesthetics: The presence of visible residues can detract from the overall quality of the assembly, highlighting the
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