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Fluid Behaviour Of Conformal Coatings
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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

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 Fluid Behaviour Of Conformal Coatings Presentation Transcript

  • 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
  • Summary
    • Viscosity definition
    • Viscosity measurement methods
    • Surface Energy of substrate
    • Surface Tension of liquid
    • Wetting / Levelling / Capillarity
    • Curing profile: IR and UV
  • How conformal coatings behave
    • Spreading the conformal coating over the surfaces as even as possible
    • Spraying adding energy to spread the coating
    • Different viscosities, different application methods…
    • Phase change from wet material to cured especially with a thermal profile
    View slide
  • Viscosity Definition
    • DEFINITION: Measure of the resistance of a fluid which is being deformed by shear stress
    • Describe the ability of a liquid to flow
    • Units: cPs (centipoise) or mPa.s
    • Depends on Temperature
    • Depends on % of Thinner
    View slide
  • 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
  • Viscosity Temperature Dependence
  • Viscosity Thinner Dependence
  • 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
  • Flow cup conversion graph
  • Flow cup Disadvantages
    • A convenient way of checking conformal coating viscosity but not as accurate
    • Generally temperature of conformal coating is not checked (room temp.)
    • Viscosity on Brookfield performed at 25C
    • Conversion graphs available from SCH (as indicative only not part of Humiseal QC test procedure)
  • Surface Energy of a substrate = Se
    • DEFINITION: Quantifies the disruption of intermolecular bonds that occurs when a surface is created
    • Any substrate has a surface energy that can influence the wetting of a liquid
    • High substrate surface energy = good wetting and adhesion
    • Ionic and non-ionic contaminants will lower the surface energy of the substrate
    • Surface treatment (cleaning, primer) will make the substrate wet easier
  • Surface Tension of a liquid = St
    • DEFINITION : A property of the surface of a liquid that causes it to behave as an elastic sheet
    • Governs the degree of contact a liquid can make with another substance
    • Enable to predict liquid behavior
    • Low liquid surface tension = good wettability
    • Surfactants in coating can reduce the surface tension of liquid to help wet better
  • Surface Tension / Surface Energy Theory
    • Case for good wetting and adhesion
    • High Surface Energy = good
    • Low Surface Tension = good
    Substrate = PCB: Se Liquid Coating : St St > Se = dewetting Se > St = wetting
  • 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
  • A modern PCB
    • PCB not cleaned = Se low
    • Low VOC coating = St high
    • Our job is tough! 
    Reminder: Se > St = wetting
  • De-wetting Phenomena
  • The Behaviour of Fluids: Summary
    • KEY NOTIONS:
    • Viscosity, Surface Tension, Surface Energy, Capillarity, etc…
    • Important to understand in order to overcome challenges:
      • Of coating on a difficult PCB substrate
      • Of compatibility with current application technologies available
      • Of curing within cycle time requirements