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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?
 

The Challenges Around Cleaning PCBs

1.    Higher process temperatures
2.    Vitrified flux residues
3.    Unreacted flux residues hidden under the vitrified flux

So what are the problems that today’s surface mount components, increased packing densities and fine line technology bring to long-term reliability?

Let’s start with component shape. Axial components were large and round without sharp edges. As in through-hole designs, flux residues were solely on the underside of the PCB.

Today’s surface mount components—as the name suggests—are mounted on the surface of the PCB. They have metalized ends to allow attachment by solder, where the flux residues remain. The growing trend of miniaturization is causing components to get ever smaller and closer together (see Fig. 4). The fact that surface mount components are predominantly square with sharp edges also leads to problems with coating coverage.

Before we continue, it's important to address that HumiSeal has recently announced a new type of coating designed to adhere better than ever before to the sharp edges found in modern-day PCBs. We highly recommend reading our new blog on the advantages of sharp edge conformal coatings.

Fig4 Components today are so small in some cases they are often referred to as dust.

The combination of reduced conductor spacing and no clean residues causes an increased risk of dendritic growth leading to intermittent faults or total failure. For dendrites to grow you need three things: ionic contamination, voltage bias, and humidity.

Contaminates and Cleaning on PCBs

With electronics invading every part of daily life, it is inevitable that—at some stage in the product’s life span—it will be exposed to high humidity/moisture; hence the need for a conformal coating. Also, for equipment to work, it needs a bias; therefore, the only item that can be easily removed is ionic contamination.

Flux residue is not the only source of Ionic contamination. Handling the PCAs during assembly can lead to finger salts being deposited on the board and components. Any previous process may have left contamination on the board, including bare-board manufacturing.

This is not the only reason to clean before coating; non-ionic contaminants can also cause problems. For a conformal coating to wet the surface and adhere correctly, you need a minimum surface energy of 38 dynes/cm. Anything below this level can cause poor adhesion, de-wetting, and capillary action.

Where do the contaminants come from?

Here are two graphics: one for bare printed circuit boards as received from the PCB manufacturer and one for the final assembled board.

As can be seen from the above graphics, there are a few contaminants that can affect reliability and coating application.
There are two categories of contaminants that need to be removed.

•    Hydrophobic: Water-hating, non-polar, oils and fats, these are the contaminants that cause problems when applying conformal coatings
•    Hydrophilic: Water-loving, polar salts, can cause dendritic growth and therefore PCA failure.

A semi-aqueous (solvent/water) based system is the best method to remove both Hydrophobic and Hydrophilic contaminants.

Resource: https://blog.humiseal.com/cleaning-pcbs-blog