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The Need for Conformal Coating. |
The most important mechanisms that degrade PCB assemblies in harsh environments are corrosion, electrochemical migration, and crack formation in solder joints.The high requirements put on the reliability of electronic products make it necessary to protect the printed circuit board assemblies from degradation by contaminants and humidity. In most cases the only feasible way to protect the PCB assemblies is to conformally coat them.
Conformal coatings qualified to MIL-I-46058C or IPCCC-830 have been shown to meet requirements such as: electrical properties, appearance, fungus resistance, resistance against thermal shock, moisture resistance, flexibility and flammability. Only one coating material, Parylene, can provide total encapsulation of the PCB and therefore exceptional protection from moisture and contaminant attack.
Section of copper lead on PCB coated with Parylene
Stringent Independent Testing.Independent tests to evaluate a number of proprietary conformal coatings have been carried out by The Swedish Institute of Production Engineering Research organization. (IF Research Publication 96807, "Evaluation of the Capability of Conformal Coatings to Prevent Degradation of Printed Circuit Assemblies in Harsh Environment, Part2").
As part of the test, FR-4 surface mount populated test assemblies were humidity tested according to IPCTM-650 test method 2.6.3.1 class 3 (the temperature is cycled between 25 and 65°C at 90-98% Relative Humidity for 160 hours, three cycles per day, with a 5V DC polarization voltage). Parylene coated assemblies showed no sign of corrosion. Test boards were subsequently subjected to a Flowing Mixed Gas test, both acid and alkaline, exposure to salt mist and thermal cycling.
The Parylene coated test boards gave excellent protection against crack formation of the solder joints, exhibiting only minor crack commencement in corners of the LCCC6B component. Virtually all other coatings tested showed evidence of severe cracking of the solder joints.
It was concluded that "the ability of Parylene to completely encapsulate the assembly and therefore prevent moisture reaching the solder joints hindered corrosion and therefore corrosion fatigue and subsequent cracking of the joint."Para Tech Coating Scandinavia AB In summary it was stated "One coating, Parylene, has been outstanding in the test -it gave excellent protection against corrosion and crack formation in solder joints."
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