Fluid Behaviour Of Conformal Coatings

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Information about Fluid Behaviour Of Conformal Coatings
Technology

Published on March 9, 2009

Author: ConformalCoating

Source: slideshare.net

Description

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

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

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

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

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

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

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)

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

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

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

Case for good wetting and adhesion

High Surface Energy = good

Low Surface Tension = good

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

PCB not cleaned = Se low

Low VOC coating = St high

Our job is tough! 

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

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

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