Title Conformal coating defects and solutions
Bob Willis
The following covers examples of good and defect coating processes and some of the problems
experienced in industry.
Conformal coating defects
and solutions
The use of conformal coating has provided coating coverage on the board and cleaning fluid recommended by the coating
benefits to industry for many years, either component surfaces, coating has capillaried supplier. The cleaning material should
in the high reliability market sector or under the PLCC. This is indicated by the not damage the board assembly or the
where products have to deal with extreme tide line around the component. Based on connector body. As gold contacts are inert,
environmental conditions. In recent inspection criteria, this still meets common there should not be an issue with contact
years, due to changes in technology and standards, but the thickness of the material resistance, but other plated connector
miniaturization, coatings are being used will vary on the surface of the board. termination surfaces may need to be
in different industries, like telecommunica- Changing the viscosity of the material, tested. It’s important to run test boards or
tions, automotive and consumer products. reducing the travel speed of the spray or cards to check for correct set up and spray
Mobile phones and other professional increasing the amount of material or solids application, then inspect the sample prior
handheld products have benefited from the applied during spray application should to running the process. This is not time
use of selective coating for environmental improve the results. consuming and can be conducted in less
and robustness. The following are examples Coating on sharp corners can be than five minutes.
of good and defect coating processes and difficult. The flat surfaces on the PLCC Coating has separated from the
some of the problems experienced in device in Figure 5 shows satisfactory surface of the plastic QFN/LGA body in
industry. coating, and provided the surface of the Figure 7. This is not uncommon on many
Figure 1 shows satisfactory conformal board and all terminations and separation plastic parts. Devices often have release
coating on plastic leaded chip carrier gaps between adjacent exposed pads have agents on the surface which make coating
(PLCC) terminations under UV light. been covered, the reliability of the product adhesion difficult to maintain. Minor
The image shows that the coating has should be satisfactory. However, based loss or separation may not normally be
covered all the joints surfaces and the areas on many standards the no coverage of reworked, but in this case, the root cause
between the terminations on the surface of edges of plastic parts would be considered should be established as all the coating has
the printed board. unacceptable. It is felt that inspection completely lifted.
Coating on the SOIC in Figure standards and criteria do need to be Cleaning the board assembly after
2, shown under a UV light source, is regularly reviewed and updated. soldering may overcome the problem in
satisfactory. There is good coverage on To achieve a better coverage on devices future production runs, but equally it may
the component leads, component body like this, it would be necessary to coat the not. The fault can be seen on most coating
and the surface of the board between the board assembly again but also look at the systems, more likely on thinner coating,
0.050” pitch terminations. viscosity of the material being used. If the although the poor adhesion will still be
Satisfactory dip coating of the solids content was low, it may have reduced apparent on thicker coating that may not
next SOIC (Figure 3) is shown during the coatings coverage. be easily displaced.
inspection under UV light. The coating Coating contamination, shown in Coating failure has occurred, allowing
has completely covered the surface of Figure 6, was found under UV light the copper dendrites to form across
the board, terminations and body of the inspection. The coating process has caused the surface of the solder mask between
component. If a spray coating process were the material to contaminate the surface of two terminations. This was found after
used, all the surfaces would be coated to the connector pins. This board assembly removing the coating during failure
an equal standard, but there would not was coated with an automatic spray system analysis of the circuit board. Either the
be coating filling the gap between the which was probably incorrectly set up soldering materials used or the cleanliness
component termination and body. Its for production. The overspray is visible of the basic PCB were inadequate for the
always important comparing like for like!! under UV light on the surface of the gold product environment. Comparing the
Coating inspection under UV light connector pins and the body of the part. soldering materials, process and coating
in Figure 4 shows the impact of capillary Rework of the connector pins in a combination can be conducted using a
action. Although there is evidence of UV inspection can be conducted using a standard test board and measuring surface
4 – Global SMT & Packaging – September 2009 www.globalsmt.net
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