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In the days before surface mount printed circuit assemblies, components (such as resistors, diodes, and capacitors) were axial, and printed circuit board components were installed with the “through-hole” method using large conductor spacing. With the advent of integrated circuits, these too were through-hole with good spacing between legs.

If your PCAs were installed into a harsh environment or required high reliability, you would apply a layer of conformal coating. Despite the large conductor spacing and through-hole components, you would still clean the PCB before applying its conformal coating to ensure long-term reliability.

Military contractors—where long-term reliability was essential (roughly 20 years)—would use a two-stage cleaning process: a solvent wash followed by a DI water wash. PCBs for military applications still use a two-step cleaning process to this day.

Things changed with the advent of low solids flux for soldering (sold as “no-clean flux”). Manufacturers saw an opportunity to remove part of the assembly line, process engineers saw one less process to manage, and the era devoid of cleaning before conformal coating came into existence.
In the 1980s and 1990s, automotive manufacturers used process validation to prove to themselves that no-clean worked. This was okay with track width/spacing and packing densities of that period, but an important question remains: will it work with today’s fine line technology and lead-free solder?