This is a difficult question to answer simply. However, the best advice is if the product is safety critical and you have not tested the product for long term reliability it may be worth cleaning!

The problem lies in not knowing what contaminants are on the surface of the board before coating. These contaminants could be from a variety of sources including the bare board manufacture, the solder resist used and whether it is compatible with the coating, the assembly processes including fluxing and the handling process.

Determining if the board contaminants are relatively benign is possible using techniques like Surface Insulation Resistance (SIR) testing? However, it can be complex and could be quite costly depending on the level of investigation. This cost however needs to weighed against the potential costs of returns, reputation and consequential losses if it suddenly goes wrong.

An alternative is to ionic test the PCB. This is where the PCB is washed and measured by an ionic contamination system such as an Iconography which SCH use. The system washes a test board completely, measuring the residue removed off the board. This residue is then expressed as an absolute value which can be checked against an industry standard on cleanliness. The test is low cost, takes approximately 30 min and can be run after cleaning to check the process has removed all of the residues needed.

That said, many companies conformal coat over a no clean process and approximately 50% of SCH coating service work is no clean. Problems with coating that can occur with “dirty” no clean PCBs include excessive de-wetting and a lack of adhesion of the coating to the board. This in turn leads to excessive re-work / finishing after coating which can cost more than the original cleaning process!

What problem could I see if I do not use cleaning for the conformal coating process?

From the benefit of our experience, as long as the bare boards are of good quality (ionically clean, free of surfactants) and the process is well under control (no handling of bare boards without gloves etc), then the vast majority of no-clean pastes are compatible with coatings.  If you have any specific combinations of flux and coating, we can check our database for any compatibility issues. 

• Rosin from paste melting at T>80°C and phase change (solid/liquid) resulting in increased volume, which stresses coating, causes coating to crack. This is a point of ingression for water and a possible failure site.
• Solvent in coating leaches activators from the rosin, free acids in coating or surface of board which can cause failures.
• Residues cause coating to de-wet, resulting in no coating present in certain areas

What conformal coating material works best for protecting against salt spray test and salt in general?

Salt is pretty inert to the resins but really is aggressive to the metals. So, the coating has to be homogeneous and a repeatable minimum thickness all over the board. Unfortunately, this is difficult to achieve. When the coating is applied to the components it naturally slumps (relaxes). This is where the coating pulls back from the pointed parts of the components leaving them exposed to the salt. Hence, the salt then attacks these points and corrodes the PCB.

To ensure that the coverage on the sharp edges of the components is excellent you have to change the way you process. In general a single pass of coating is not enough to improve the coating coverage and multiple layers is required. This then builds up the layers on the sharp tips and provides less access for the salt.