Liquid conformal polymers – resins of acrylic (AR), epoxy (ER), silicone (SR) and urethane (UR) – use wet application processes to attach to substrates. Most prominent of these are brushing the wet coating onto an assembly, dipping (immersing) the assembly in a bath of liquid coating, or spraying the conformal film onto the designated surface. The coating materials are wet when they are applied. If
- application processes are inadequate or
- targeted substrates are inadequately cleaned
- conformal adhesion diminishes and can lead to delamination.
One of the failure mechanisms that can emerge under these circumstances is de-wetting.
De-Wetting of Liquid Coatings
De-wetting is the tendency of the coating material to refuse to wet the surface of assemblies to which it has been applied. De-wetting deteriorates the conformal coating. Thin polymer films can fracture into small, non-conformal droplets through de-wetting, which has several distinct phases:
- Hole-formation occurs either spontaneously (spinodal de-wetting), or because of film contact with surface contaminants.
- Reduced surface area of the polymer/air interface stimulates further hole growth.
- Other holes may emerge in consequence, further decreasing the diameter of the coating’s polymer fibers.
- Hole collision creates thinning polymeric lines throughout the film, which continues to diminish in thickness as the film material drains to the apexes of its polymeric rings.
- Rayleigh instability – increased fluctuations on the film surface -- develops.
- Holes eventually dissolve into droplets, disrupting the uniformity of the liquid coating material, and jeopardizing conformal protection.
De-wetting and hole-growth in wet polymer conformal films is a major failure mechanism, diminishing their protective qualities. Liquid films polymeric nature adds to the non-linear viscoelastic effect of their shear thinning.
Surface contamination prevents coating solutions from evenly sticking to and ‘wetting’ the substrate. Lack of proper adhesions leaves assembly areas uncoated, exposing the substrate to additional contamination and subsequent coating failure. Cleanliness is the key to preventing de-wetting; causes of surface contamination include:
- flux-residue when no-clean flux is used,
- soldering processes,
- hot air solder leveling (HASL) rinse-operations stimulating corrosion,
- component mold release agents,
- silicone oil left from production adhesives,
- cleaning bath contamination and
- operator handling.
When de-wetting occurs, solder fails to adhere to components. In addition to contamination and corrosion, extremely high temperatures above a film material’s glass transition temperature can stimulate de-wetting; by increasing the mobility of the polymer-chain molecules, a tendency toward separation from each other and the substrate surface develops, stimulating de-wetting.
The only viable solution is stripping the damaged coating from the affected area, re-coating it with a rigorous, manual re-work process.
Parylene and De-wetting
Providing an entirely conformal, durable, pinhole-free coating for PCBs and similar electronics, parylene (XY) offers a protective, insulative coating for a wide range of products and materials. Applied by chemical vapor deposition (CVD) rather than the liquid methods used by AR. ER, SR and UR, XY is converted from a solid to a gas, with no wet stage. Thus, unlike liquid coatings, parylene is not pre-synthesized and dispensed during application in a wet format.
Parylene’s CVD free radical polymerization technique creates XY coating, synthesizing the coating during application, using a reaction mechanism that forms resonance-stabilized XY diradicals, which eventually adsorb on and into a substrate near room temperature. The result is generation of a much better. conformally-thin polymer film on virtually all substrate surfaces than those supplied by conventional wet-solution methods
However, XY’s specific material conditions and the CVD application method also quash chemically-based film adhesion for parylene; only mechanical adhesion is possible. Penetrating substrate surfaces gives a parylene a more dependable conformal coat than those provided by liquid polymers, as mechanical adhesion enters and fills pores/voids along covered surfaces, holding together by interlocking film elements.
No wet processes/liquid materials are used, The absence of solvent in XY CVD avoids de-wetting and pinhole-related defects, by enabling growth of high-purity, ultrathin (<10 nm) layers of conformal coating. Precisely-controlled parylene CVD enables direct chemical synthesis of thin-ﬁlm conformal coating formation in one-step processing. Unlike liquid materials, monomeric reactants in the CVD sequencing process require no solubility, bypassing de-wetting potential and other detrimental impacts accompanying solvent-use.
To learn more about what to look for when inspecting parylene, download our whitepaper now: