Features

Dipping

A good low-cost method for low volume coating operations starts with a dipping process. Here the assembly is immersed in a bath of coating so exposure to the chemistry is complete. Dipping does offer benefits in covering complex topography since the entire board is immersed. Thickness is controlled by the withdrawal rate of the assembly from the bath. Dipping is a process conducted vertically so coating can be uneven as the material flows to the bottom of the board during the withdrawal process.

Since the chemistry sits in an open vat, any chemistry that is reactive to the ambient environment must be closely monitored as the viscosity of the coating can change with exposure to elevated temperatures or humidity. Also, since the entire board is immersed, any keep-out areas must be meticulously masked to prevent impingement into these zones.

Manual Spray

Figure 3. Non-Atomized Film Coat valve utilizing the patent pending continuous film calibration process.

A more common application approach is manual spray via an aerosol can or handheld nozzle. This process is also very easy to implement with minimal equipment costs. There is certainly a level of variability in this process as thickness is difficult to control and operator dependent. As the operator moves the nozzle over the board surface, shadowing can occur as coating is applied in each direction. It is suggested that after each cycle that the board is rotated 90 degrees to assure an even film build and counteract shadowing in any one direction.

Manual spray is a fast process, though the nature of mass coverage does require masking of keep out areas which can be a laborious process. You will also see a significant loss of coating in the process as the vast majority of the chemistry is wasted. This puts a strong emphasis on operator safety and ventilation of vapors or solvents lost in the spray process.

Selective Coating

Selective coating provides an automated approach to eliminate process variability, increase throughput in high volume applications, and greatly reduce or eliminate masking due to its inherent robotic control. These programmable units greatly reduce material waste in the application with transfer efficiencies of 99% (vs. 25%-40% for dipping or manual spray) so coating cost per board is much less.

The selective process can entail numerous application methods depending on the desired application and conformal coating type. Solvent-based coatings contain a low percentage of resin and subsequently are very low in viscosity (sub 100 cps). As the solvent evaporation for these coatings can be as high as 80%-90%, the coating is put on the assembly much thicker wet, to obtain your target dry film thickness. For instance, it would not be unusual to have to apply a 200 μm wet film to achieve a 25 μm dry film after solvent evaporation. As a result, solvent-based coatings are often applied via an airless film coating process at robot speeds up to 500 mm/sec. Film coating provides superb edge definition over larger coated areas.

Thinner films of coating can be achieved with the introduction of air to create an atomized process. Low volume (0.5 psi – 5 psi), pattern shaping air can be used to break down a coating into small particles or droplets to result in a much thinner wet film. This process can be used for solvent-based, low viscosity fluids when a user is looking for a very low dry film thickness.

Atomization is also the preferred method for higher viscosity coatings (often silicones) or UV chemistries. There are very few limits on what you can atomize. Very thick materials can be sprayed by altering the atomizing air pressure, though selectivity can be compromised in high pressure applications. Conformal coating films are often created with pressures below 5 psi. A series of spray caps can result in pattern shapes varying from conical to fans in pattern widths down to 3 mm. For added control, most selective coating applications integrate a detailing tool to jet, or needle dispense in and around small keep-out areas.

Selective coating has a higher initial cost than dipping or manual spray operations, but can often be justified by the higher throughput, material savings, and reduction in masking labor. All of these factors must be taken into consideration when evaluating the best process for your production environment.

Conclusion

Implementing a conformal coating process is a delicate balance of material selection, understanding the engineering requirements for coverage, keep-out area, and thickness, and choosing an appropriate application method. Every variable in the process has its own pros and cons that you must weigh against your internal resources and production volume. Changing one variable may also impact other decisions you make in the process so it is always important to qualify your process prior to implementation.

This article was written by Frank Hart, Global Sales and Marketing Manager, PVA (Cohoes, NY). For more information, Click Here .