Understanding Fluid Behavior in Coating Applications

1. 8-The Behaviour of Fluids

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

3. 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

4. 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

5. Viscosity of Liquids Examples

6. 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

7. Viscosity Temperature Dependence

8. 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

9. Viscosity Thinner Dependence

10. 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

11. Viscosity Measurement Methods

12. Flow cup conversion graph

13. 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)

14. 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

15. 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

16. Surface Tension / Surface Energy Theory Liquid Coating : St Substrate = PCB: Se St > Se = dewetting Se > St = wetting • Optimum case for good wetting and adhesion • High Surface Energy = good • Low Surface Tension = good

17. 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?

18. What are the additives? • 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

19. Example of Surface Tension / Surface Energy St = Low value = Better Se = High value = Better Reminder: Se > St = wetting

20. 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

21. Example of dewetting phenomena

22. Example of dewetting phenomena

23. 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)

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

25. 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

26. Example of a typical coating line 4 m IR oven 1m2 1m2 SOLVENT BASED Flash Conveyor Selective Coating Machine Curing Oven 1m2 1m2 1m UV oven UV CURABLE

27. 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.

28. Typical Recommended Thermal Profile for Solvent Based material

29. Examples of bubbles

30. UV Curing • 3 keys parameters: • Spectral distribution • UV irradiance • UV dose

31. 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

32. Lamps Spectral Distribution: H vs D bulb • 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

33. 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

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

35. 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

36. UV40 Curing Window Profile

37. 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

38. The way Humiseal work: Customer   Process Solution Equipment Manufacturer Coating Manufacturer 

39. Thank you for your attention! • www.humiseal.com • europetechsupport@humiseal.com