Fluid Behaviour Of Conformal Coatings

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A technical analysis of conformal coatings looking at viscosity, the behaviour of surface tensions and energies and problems such as wetting, levelling and capillary

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Fluid Behaviour Of Conformal Coatings

  1. 1. Fluid Behaviour of Conformal Coatings A technical analysis of conformal coatings looking at viscosity, the behaviour of surface tensions and energies and problems such as wetting, levelling and capillary
  2. 2. Summary <ul><li>Viscosity definition </li></ul><ul><li>Viscosity measurement methods </li></ul><ul><li>Surface Energy of substrate </li></ul><ul><li>Surface Tension of liquid </li></ul><ul><li>Wetting / Levelling / Capillarity </li></ul><ul><li>Curing profile: IR and UV </li></ul>
  3. 3. How conformal coatings behave <ul><li>Spreading the conformal coating over the surfaces as even as possible </li></ul><ul><li>Spraying adding energy to spread the coating </li></ul><ul><li>Different viscosities, different application methods… </li></ul><ul><li>Phase change from wet material to cured especially with a thermal profile </li></ul>
  4. 4. Viscosity Definition <ul><li>DEFINITION: Measure of the resistance of a fluid which is being deformed by shear stress </li></ul><ul><li>Describe the ability of a liquid to flow </li></ul><ul><li>Units: cPs (centipoise) or mPa.s </li></ul><ul><li>Depends on Temperature </li></ul><ul><li>Depends on % of Thinner </li></ul>
  5. 5. Viscosity Examples Liquids Type Viscosity in centiPoise Water 1-10 Solvent 5-10 Humiseal 1R32A-2 PB 65 65 Olive oil 80-100 Humiseal 1A33, 1B31 180-220 Humiseal UV40 250-500 Castor Oil 1000 Honey 2000-10 000
  6. 6. Viscosity Temperature Dependence
  7. 7. Viscosity Thinner Dependence
  8. 8. Viscosity Measurement Methods Test Method BROOKFIELD FLOW CUPS (Zahn, DIN, etc) Type Static Kinematic Principle Measure the resistance of a spindle rotating at a constant speed in the liquid Measure a flow time of a liquid through a cup with a known volume Measurement Units Centipoise Or mPa.s Seconds Purpose of Use Quality Control Laboratory Testing Indicative only On field Testing Accuracy Excellent Fair Instrument Example
  9. 9. Flow cup conversion graph
  10. 10. Flow cup Disadvantages <ul><li>A convenient way of checking conformal coating viscosity but not as accurate </li></ul><ul><li>Generally temperature of conformal coating is not checked (room temp.) </li></ul><ul><li>Viscosity on Brookfield performed at 25C </li></ul><ul><li>Conversion graphs available from SCH (as indicative only not part of Humiseal QC test procedure) </li></ul>
  11. 11. Surface Energy of a substrate = Se <ul><li>DEFINITION: Quantifies the disruption of intermolecular bonds that occurs when a surface is created </li></ul><ul><li>Any substrate has a surface energy that can influence the wetting of a liquid </li></ul><ul><li>High substrate surface energy = good wetting and adhesion </li></ul><ul><li>Ionic and non-ionic contaminants will lower the surface energy of the substrate </li></ul><ul><li>Surface treatment (cleaning, primer) will make the substrate wet easier </li></ul>
  12. 12. Surface Tension of a liquid = St <ul><li>DEFINITION : A property of the surface of a liquid that causes it to behave as an elastic sheet </li></ul><ul><li>Governs the degree of contact a liquid can make with another substance </li></ul><ul><li>Enable to predict liquid behavior </li></ul><ul><li>Low liquid surface tension = good wettability </li></ul><ul><li>Surfactants in coating can reduce the surface tension of liquid to help wet better </li></ul>
  13. 13. Surface Tension / Surface Energy Theory <ul><li>Case for good wetting and adhesion </li></ul><ul><li>High Surface Energy = good </li></ul><ul><li>Low Surface Tension = good </li></ul>Substrate = PCB: Se Liquid Coating : St St > Se = dewetting Se > St = wetting
  14. 14. Example of Surface Tension / Surface Energy St = Low value = Better Se = High value = Better Reminder: Se > St = wetting Liquids Type Surface Tension in dynes/cm Solvent 22-26 Coatings 32-38 Water 72 Mercury 480 Substrate Type Surface Energy in dynes/cm Cleaned PCBs 40-42 Average PCBs 36-38 Not cleaned PCBs 32 Teflon, Silicone, Plastics <<<32
  15. 15. A modern PCB <ul><li>PCB not cleaned = Se low </li></ul><ul><li>Low VOC coating = St high </li></ul><ul><li>Our job is tough!  </li></ul>Reminder: Se > St = wetting
  16. 16. De-wetting Phenomena
  17. 17. The Behaviour of Fluids: Summary <ul><li>KEY NOTIONS: </li></ul><ul><li>Viscosity, Surface Tension, Surface Energy, Capillarity, etc… </li></ul><ul><li>Important to understand in order to overcome challenges: </li></ul><ul><ul><li>Of coating on a difficult PCB substrate </li></ul></ul><ul><ul><li>Of compatibility with current application technologies available </li></ul></ul><ul><ul><li>Of curing within cycle time requirements </li></ul></ul>

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