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NordsonASYMTEK.com Conformal Coating Proc ess Characteriz ation Considerati ons Page 4 Higher Viscosities Higher visco sity coatings require the use of atomi zing coating te chnologies th at are slower in coa ting rate a…
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Conformal Coating Process Characterization Considerations Page 3
mask process. The labor savings with selective coating is typically significant as is the
volume of products produced in a shift. In addition, the manual masking process
increases the opportunities for failure because of the additional board handling during
the assembly phase. Selective coating equipment only has the initial capital
equipment investment that is depreciated over the ten to fifteen year lifetime of the
24/7 capable equipment; the only significant reoccurring cost is that of the coating
itself. Using automated equipment eliminates operator-to-operator variation, further
increasing yield over manual processes.
Optimized Coating Processes
For optimized coating processes, the conformal coating material and the equipment
should be thought about collectively. Manufacturers commonly select a specific
conformal coating based on the cured properties of the material. This is a logical first
step as the conformal coating is in a cured state when the electronics are in their final
use model; however, the material is applied in a liquid state that is subsequently cured
by thermal, ultraviolet, moisture, and other solidifying processes. For a high yield
process a manufacturer also needs to strongly consider the material’s liquid properties
and to characterize the coating process in conjunction with the design phase to
minimize the PCA layout while respecting process tolerances. An understanding of the
specific material’s rheology allows for automated coating equipment selection and
defining process capabilities.
When considering design for manufacturing, some of the specific properties to
consider when selecting a conformal coating are the material’s viscosity, chemical
composition and associated work time, and environmental considerations. The
viscosity is important because it determines how the material will flow once applied to
the board as well as the type of applicators used to dispense the material. Low
viscosity coatings (typically less than 100 centipoise) can be applied with high speed,
highly selective non-atomized film coating technologies that can offer keep-out zones
less than 2mm in production.
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Conformal Coating Process Characterization Considerations Page 4
Higher Viscosities
Higher viscosity coatings require the use of atomizing coating technologies that are
slower in coating rate and have keep-out zones typically greater than 3mm. Jetting or
needle dispensing technologies can be used to decrease the keep-out zones to the
1mm neighborhood and are generally used to precisely coat specific parts rather than
for coating an entire board. The addition of a second coating valve to increase
selectivity can decrease process throughput. Considering the need for conformal
coating, applying the required process tolerances for the material, and evaluating
conformal coating equipment and characterization at the design phase greatly aid in
manufacturability. It makes
the manufacturing process
less difficult, time-
consuming, and costly, and
avoids having to use
specialized equipment,
applying additional
conformal coating, or using
masks after the fact due to
unplanned for parts or
areas that have zero
tolerances.
Chemical Composition
The chemical composition is also very important as often materials have finite work
times, commonly referenced as pot life, before the equipment must be cleaned to avoid
curing inside the equipment. The presence of hazardous solvents is common, so
consideration needs to be given regarding process controls and the environmental
impact of these materials. This specifically applies to the health of the employees,
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Conformal Coating Process Characterization Considerations Page 5
potential for cross contamination to other processes, and ventilation of the vapors.
Finally, the cure mechanism is determined by the material as well, be it acrylics,
urethanes, silicones, or epoxies. The time to cure is related to the material itself, and
automated equipment selection depends on the cure mechanisms of the specific
material. To minimize the time to cure and the associated work in process as well as
line length, designers need to look at the specific
coatings and not the equipment.
A high reliability coating process occurs most
efficiently when there is characterization of the
equipment prior to introducing the final product. The
equipment used to apply the conformal coating has
numerous input parameters to optimize the process;
some of these are the applicator selection, nozzle
orifice size, pressure causing the fluid to flow, the
transversing speed of the applicator, height of the
applicator from the substrate, and the overlap of
each pass, to name a few of the most critical. The
output of these variable settings provides the
coating thickness, repeatability, throughput, and
defects, such as bubbles and voids.
Curing the Coating
The equipment used to cure the coating material also has input variables depending on
the cure mechanism. For thermal cure materials these are typically the temperature of
the oven, belt speed, temperature ramps, and for some materials, the humidity inside
the oven. For UV cure materials they are the height of the UV lamp and the speed the
board passes through the oven. The output of these variable settings, in conjunction
with the material thickness, provides the throughput of the cured product as well as
defects, such as incomplete cure or over cure defects.
Input parameters to
optimize the process
include the applicator
selection, nozzle
orifice size, pressure
causing the fluid to
flow, the transversing
speed of the
applicator, height of
the applicator from
the substrate, and
the overlap of each
pass.