CONFORMAL COATINGS A newly manufactured printed circuit board will generally perform well but performance will quickly deteriorate. Common causes of deterioration and PCB failure include: o Atmospheric moisture o Contamination of the surface with airborne ionic material o Electrostatic attraction of dust to the surface o Corrosive environments, such as salt fog Conformal coatings can be applied to a PCB giving protection against the environment, extending the life of the board and therefore can be used in a vast array of electronic applications. Domestic Industrial Automotive Aerospace Military
CONFORMAL COATINGS Conformal coatings are thin coatings, typically 25-75 microns, which ‘conform’ to the contours of a PCB. The ideal conformal coating will have performance requirements that include: o Good electrical properties o Low moisture permeability o Good mechanical durability o Excellent adhesion properties o Good thermal resistance Additional properties such as flame retardance or chemical resistance may also be required.
Different chemistries utilised in conformal coating technology require different types of cure. It is important to understand the terms used in conformal coating in under to understand cure schedules: ‘Tack Free’ – this is the time at which the coating is no longer tacky but slight indentations may be left in the coating when pressed firmly ‘Touch Dry’ – this is the time at which the coating is dry enough to touch without leaving any fingerprints on the surface of the coating It is important to note that some coatings are very soft and can feel tacky even when fully cured. The touch dry time in these cases is more a reference to when you can move the board without affecting the coating ‘Cure Time’ – this is the time at which the coating is fully cured and will offer protection It is also important to note that most coatings actually reach their optimum cure level after around 7 days but they will provide protection prior to this CONFORMAL COATING TERMINOLOGY
CONFORMAL COATING APPLICATION Conformal coatings can be brushed, dipped or sprayed depending on the requirements. Customers often specify their desired application method. Brushing: AdvantagesDisadvantages Cost effectiveUneven coating Ideal for small scale production Difficult to get under components Selective coatingOnly one side at a time can be coated
CONFORMAL COATING APPLICATION Dip Coating: AdvantagesDisadvantages Ideal for medium scale production Initial capital outlay Uniform coating thicknessMasking essential for connectors Easy to operate machinerySpecial extraction may be required Both sides can be coated at the same time Suitable for higher viscosity materials Coating can penetrate under components Operator training required but not difficult
CONFORMAL COATING APPLICATION Spraying – aerosol or manual spray gun: AdvantagesDisadvantages Cost effectiveOverspray can occur Ideal for bench top work and repair Difficult to get under components Ideal for small to medium scale production Special extraction may be required ConvenientOnly one side at a time can be coated
CONFORMAL COATING APPLICATION Selective Spraying: AdvantagesDisadvantages No masking requiredInitial capital outlay Uniform thicknessLow under component penetration – requires good flow properties with the coating Automated process – good for inline procedures Special extraction may be required – but can be linked to the machine easily Ideal for all scales of production Requires a trained operator to set the software
Application Conditions Temperature – Affects viscosity and evaporation rate Humidity – Contaminates dip coat tanks, blooming in hand spray and affects curing / pot life. Ventilation – Affects operators
Considerations for Choice As there are many different applications for conformal coatings, there are also a number of elements that must be considered as part of the selection process: Coating properties Adhesion Flexibility Temperature range Mechanical durability Electrical performance Environmental performance Flame retardance Mould resistance Application Viscosity/application Touch dry time Full cure time Curing conditions Is re-work required? Cost Quality Approvals required?
CONFORMAL COATING TYPES Conformal coatings are available in a number of different chemistry types and also cure via different mechanisms. The different types of conformal coating include: o Solvent-based – Evaporation Cure o Solvent-based – Heat Cure o Low VOC/Water-based – Evaporation/Heat Cure o Non-VOC – Moisture Cure o UV Cure – Dual Cure: Moisture/Heat Common conformal coating chemistries include: o Acrylic o Polyurethane o Silicone o Modified-silicone
DCA (SCC3) MODIFIED SILICONE COATING DCA is a flexible, transparent & unique modified silicone conformal coating specifically designed for the protection of electronic circuitry. Key Features: o Resistant to most solvents, lubricants and chemicals associated with most aerospace and automotive applications. o Excellent adhesion under various climatic conditions to a variety of substrates, including high altitude/decompression o Fluoresces under UV light for ease of inspection. o Extreme operating temperature range (-70˚C to +200˚C) o Excellent electrical properties
DCA (SCC3) MODIFIED SILICONE COATING DCA is a solvent based coating and can be sprayed, dipped or brushed. For optimum properties, DCA should be heat cured (2hrs at RT, 2hrs @ 90˚C advised). If RT cured then the solvent resistance will be significantly reduced. Should be used with DCT thinners and removed with CCRG – Electrolube Conformal Coating Remover Gel Approvals o Def-Stan 59/47 – Standard now obsolete o UL746 – File Number E138403
DCA (SCC3) MODIFIED SILICONE COATING The DCA (SCC3) Range includes: o DCA-FD – Fast-dry version o DCA-HT – With improved high temperature performance o DCR – Opaque red version o DCB – Opaque black version o DCRT – Opaque red, high viscosity version o DCE – Thixotropic gel version (dam and fill applications) Unique range of products based on a silicone modified alkyd Avoids issues associated with silicone migration Easier processing than standard silicones Performance compares with many silicone conformal coatings
TFA – TOLUENE FREE ACRYLIC COATING TFA is a flexible and transparent acrylic conformal coating for the protection of electronic circuitry. It has been formulated to meet many of today’s commercial applications, meeting both Military and IPC standards. Key Features: o Toluene free product. o Fluoresces under UV light for ease of inspection. o Wide operating temperature range (-65˚C to +125˚C) o Excellent Dielectric properties o Clear coating with excellent resistance to yellowing when exposed to UV light
TFA – TOLUENE FREE ACRYLIC COATING TFA is a solvent based coating and can be sprayed, dipped or brushed. Should be used with DCT thinners and removed with CCRG – Electrolube Conformal Coating Remover Gel Approvals o UL746 Approved o IPC-CC-830 approved o Automatically meets MIL-46058-C Also available: AFA – Aromatic Free Acrylic
VOCs or Volatile Organic Compounds are carbon based compounds which vaporise easily under ambient conditions VOCs contribute towards the formation of ground level ozone Such pollution can have many detrimental effects on the environment, damaging forests and vegetation In addition, some materials classed as VOCs can act as irritants and over exposure can lead to a variety of health problems
VOC DEFINITION EU Solvents Emissions Directive “Any organic compound having at 20˚C a vapour pressure of 0.01kPa or more, or having corresponding volatility under the particular conditions of use”
UVCL - UV Cure Extremely rapid initial cure schedule Ideal for high volume production In-line process, curing equipment required Very high level of protection; highly cross-linked coating Excellent chemical and solvent resistance Using a dual cure system can ensure that in shadowed areas under components the coating still cures and protects the PCB Moisture Heat UV cure coatings are available as 100% solids materials Non-VOC options
WBPS – WATER-BASED COATING WBPs is a water-based conformal coating, based on polymeric materials which has been specifically formulated for the protection of electronic circuitry. It offers an excellent blend of both physical and electrical properties in a more environmentally and user friendly package when compared to conventional solvent based conformal coatings. Non-flammable and contains very low levels of VOC’s. WBP is for dipping/brushing applications whilst WBPS (Sprayable) has been specifically formulated for use in selective film coater equipment.
WBPS – WATER-BASED COATING Key Features: o Environmentally friendly and eliminates hazards associated with solvent based conformal coatings o Excellent solvent resistance. o Very low VOC content o NMP (N-Methyl pyrrolidone) free. o Fluoresces under UV light for ease of inspection. o Excellent adhesion on a wide variety of substrates. o Excellent Dielectric properties and resistant to mould growth.
PUC – POLYURETHANE COATING PUC is a tough, flexible, modified polyurethane conformal coating and is specifically designed for the protection of electronic circuitry, particularly in harsh conditions. Key Features o Excellent adhesion to a wide variety of substrates. o Fluoresces under UV light for ease of inspection. o Good resistance to a wide variety of chemicals including acids and alkalis. o Wide operating temperature range. o Excellent mechanical properties including abrasion resistance o Solvent-based, can be sprayed, dipped or brushed o Should be used with PTH thinners and removed with CCRG
Modified Silicone – LFCC, FSC (Modified with acrylic; up to 125˚C) Acrylic – HPA (MIL Approved), APL, AFA, APLSP (Sprayable version), TRV Water-Based – WBP (Dipping Version) Polyester – CPL Surface Modifier – TFCF (Fluorinated) OTHER COATINGS
High Temperature – DCA (SCC3) Range Excellent Clarity – TFA or an Acrylic Coating Chemical Resistance – PUC or DCA (SCC3) Range Cosmetic Appearance – CPL (no UV trace) Low Solvent Content – WBP or WBPs Reworkable – Acrylic Coatings or FSC UL Approved – DCA and TFA UV Cure - UVCL CHOOSING THE RIGHT PRODUCT
Electrolube Conformal Coating Selector Chart Is optical clarity required? YES DCA Is a fast touch dry time required? Is low/non VOC required? UVCL WBP(s) YES Is easy removal for re-work required? Is an approval required? YES Is chemical resistance required? DCA/WBP(s) YES Is the operating temperature above 130°C? YES Is the operating temperature below -55°C? WBP(s) PUC PUCAF YES NO APL HPA (MIL-I-46058C) TFA (IPC-CC-830/UL746) AFA (UL746) FSC (IEC61086) DCA (UL746) UVCL (IPC-CC-830) Is an approval required? YES NO
Adhesion Loss Incorrect curing could cause adhesion loss. This could be due to insufficient curing, i.e. the coating has not finished cross-linking. It could also be a problem with removing masking material – ideally this should be done when the coating is still soft.
Coating Removal Products For acrylics, a solvent based product like Electrolube ULS is ideal for quick and clean removal of the coating. For coatings with a higher chemical resistance, a product such as Electrolube RRS will be required. For localised rework, a remover gel like Electrolube CCRG is ideal.
In order for conformal coatings to be applied correctly some complimentary products are required: Thinners – where applicable conformal coatings are diluted to the required viscosity, Electrolube have a range of thinners suitable for this process Masking – PCM and PCS are coating masks suitable for covering connectors during the coating process OTHER PRODUCTS
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