Coating over a dirty board does not protect it—it traps the contamination under a film that then drives adhesion failure, ionic creep, and hidden defects, so cleaning to a verified cleanliness level is the prerequisite step that makes conformal coating a protection rather than a liability. The first rule of any coating line is that the coating seals whatever is already there, good or bad.
What Goes Wrong If You Coat Over Contamination?
Flux residue, ionic salts, particulates, and handling oils left under the coating become active problems instead of visible ones. Residue breaks the coating-to-substrate bond, so the film lifts or blisters; ionic material wicks under the coating and drives electrochemical migration and corrosion; particulates become nucleation points for voids. I tell buyers the coating is the last inspection barrier, not the first cleaning opportunity—once it is on, the defects underneath are locked in and often invisible to visual inspection.
Which Contaminants Matter Most Before Coating?
The contaminants that hurt coating most are the same ones that hurt reliability: flux residues (especially activated rosin and some no-clean formulations that are not truly inert), ionic salts from handling and process water, particulates from drilling and handling, and silicone or oils that poison coating adhesion. Of these, ionic and adhesive-incompatible films are the silent killers—ionic because it drives creep corrosion under the film, adhesive-incompatible because it guarantees delamination. I prioritize ionic and surface-energy testing before coating far above a simple “looks clean” check.
How Clean Must the Board Be Before Coating?
Clean enough to pass the same cleanliness discipline you would apply to a bare reliability build: visual acceptance per IPC-A-610, ionic control by a method such as resistivity of solvent extract (ROSE, IPC-TM-650 2.3.25) or ion chromatography (IPC-TM-650 2.6.3.3), and a surface-energy or adhesion check appropriate to the coating chemistry. There is no single universal numeric cutoff—the targeted level is set by your product class, the coating chemistry, and the customer or IPC-J-STD-001 requirement—so I qualify the pre-coat clean to the actual spec rather than quoting a generic number. The point is that “clean” is a verified state, not an assumption.
What Cleaning Method Fits a Pre-Coat Step?
Aqueous cleaning is the usual fit because it removes flux and ionic residue thoroughly and leaves a well-controlled, low-residue surface that coatings bond to, validated against IPC-CH-65. Solvent cleaning fits boards or coatings that cannot tolerate a wet-dry cycle, and it must be matched so no solvent film remains to poison adhesion. Whichever method, the pre-coat step must end with a verified dry, ionic-controlled surface—an incomplete rinse or a trapped-water dry-out defeats the whole purpose. I treat the pre-coat clean as a qualified process with recorded parameters, not a rinse-and-hope.
How Do You Verify Cleanliness Before Coating?
I verify in three layers: visual and magnification per IPC-A-610 to catch residue and particulates; ionic testing (ROSE or ion chromatography per IPC-TM-650) to confirm the ionic load is within the qualified limit; and an adhesion or surface-energy check (often a simple dyne or tape test qualified to the coating) to confirm the surface will actually bond. The verification is what converts “we cleaned it” into “it is clean enough to coat,” and it is the step I never skip on a reliability or automotive-class build.