8-The Behaviour of Fluids

Summary Viscosity definition Viscosity measurement methods Surface Energy of substrate Surface Tension of liquid Wetting / Levelling / Capillarity Curing profile: IR and UV

Why fluids are behaving? Spreading a liquid over a surface as even as possible Spraying = adding energy to spread the liquid Different viscosities, different application method… Phase change from wet material to cured = 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

Viscosity of Liquids Examples Liquids TypeViscosity in centiPoise Water1-10 Solvent5-10 Humiseal 1R32A-2 PB 6565 Olive oil Humiseal 1A33, 1B Humiseal UV Castor Oil1000 Honey

Viscosity Temperature Dependence Important to control the viscosity of the coating (constant temp. = repeatable process) Determine accurate coating application parameters Heating system on coating equipment helps to overcome temp. variation in coating room

Viscosity Temperature Dependence

Viscosity Thinner Dependence Important to control the % of thinner in the coating (solid content and viscosity) Important key criteria for spray coating and dip coating Humiseal can provide preblend material to overcome operator mixing variability Preblend viscosity measured with Brookfield instrument (high accuracy 65 ± 3cPs) at 25C

Viscosity Thinner Dependence

Viscosity Measurement Methods Various test method depending on instrument and accuracy Laboratory test equipment or production tool Brookfield test method is Humiseal QC test procedure (at 25C) Only test method recognized by Humiseal for any quality issues

Viscosity Measurement Methods Test MethodBROOKFIELD FLOW CUPS (Zahn, DIN, etc) TypeStaticKinematic 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 AccuracyExcellentFair Instrument Example

Flow cup conversion graph

Flow cup measurement Disadvantages A convenient way of checking viscosity but not as accurate Generally temperature of coating is not checked (room temp.) Viscosity on Brookfield performed at 25C Conversion graphs available (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 behaviour 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 Substrate = PCB: Se Liquid Coating : St St > Se = dewettingSe > St = wetting Optimum case for good wetting and adhesion High Surface Energy = good Low Surface Tension = good

Compatibility between solder resist and conformal coating Solder resist formulation include some additives (as well as some coating formulation) Additives used to improve substrate quality (e.g esthetic finish, abrasion resistance, wetting, etc…) What are the effect of those additives responsible for dewetting of the coating?

Silice based : used to modify the rheology of a liquid (example a thixotropic gel) Silicone based: used to improve wetting, levelling, glossy/matt finish Organopolymers based (organopolysiloxanes) : used to trigger the crosslinking mecanism (UV curable for example) Fluoropolymers based: used to improve scratch / abrasion resistance and create an anti stick surface What are the additives?

Example of Surface Tension / Surface Energy Liquids Type Surface Tension in dynes/cm Solvent Coatings Water 72 Mercury 480 St = Low value = Better Se = High value = Better Substrate Type Surface Energy in dynes/cm Cleaned PCBs Average PCBs Not cleaned PCBs 32 Teflon, Silicone, Plastics <<<32 Reminder: Se > St = wetting

The real life on a PCB nowadays! PCB not cleaned = Se low Low VOC coating = St high Application method is playing as well: theory not always true Our job is tough! Reminder: Se > St = wetting

Example of dewetting phenomena

Capillarity Capillarity = phenomenon of fluids to fill voids Capillary action will suck material through vias and underneath components Can be very helpful (under filling flat components) but also very annoying (coating the inside of connectors)

Capillarity Example Influenced by: - low viscosity of material - high amount of material - low surface energy of substrate - high surface tension of liquid

Solvent evaporation / Curing Process Need to understand: –Whole process of a coating line –Coating chemistry to apply –Coating curing mechanism of the coating –In order to troubleshoot coating defects problems

Example of a typical coating line Selective Coating Machine SOLVENT BASED 1m 2 1m UV oven 4 m IR oven Flash Conveyor Curing Oven 1m 2 UV CURABLE

Solvent evaporation / IR-Curing Solvent evaporate faster at higher temperature: Can create bubbles if: – Too much solvent left in coating –Oven temperature too hot –Flash off zone too high –Flash off zone not sufficient –Important to set up the thermal profile of a curing line depending on coating chemistry and board design.

Typical Recommended Thermal Profile for Solvent Based material

Examples of bubbles

UV Curing 3 keys parameters : –Spectral distribution –UV irradiance –UV dose

Spectral Distribution Wavelength under which the lamp is emitting Lamp type can be single output (365nm only) or spectral output H bulb and D bulb have different spectral distribution Will determine the curing profile of the UV coating

H BULB : - Hg doped - Focused on short wavelength - Design for thin film D BULB: - Fe doped - Focused on medium long wavelength - Design for thick film and pigmented system Lamps Spectral Distribution: H vs D bulb

UV Dose / UV Irradiance DEFINITION UV irradiance (mW/cm2); –Intensity delivered to a surface –Characteristic of lamp and geometry of reflector –Independent of conveyor speed UV dose (mJ/cm2): –Total energy delivered to a surface –Proportional with number of passes under UV light –Inversely proportional to speed

UV Dose / UV Irradiance Definition Time Intensity UV Dose (mJ/cm2) = Energy Output UV Irradiance (mW/cm2) Peak irradiance

UV Curing Process Control Radiometer instrument to monitor the amount of UV dose delivered by a lamp Controlling a curing process and efficiency of a lamp overtime Ensure coating is tack free after UV exposure And cured under the recommended curing profile to obtain optimum physical properties

UV40 Curing Window Profile

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

The way Humiseal work: Process Solution Customer Coating Manufacturer Equipment Manufacturer

Thank you for your attention!