- Conformal Coating Introduction
- Application of Conformal Coating
- Coating Material and Cured Thickness Check
- Coating Process Flow
- Machine Capability And Settings
- Coating Inspection Guideline
Conformal Coating Introduction
1. Conformal coatings protect electronic printed circuit boards from moisture and contaminants, which cause short circuits and corrosion of conductors and solder joints.
2. Moisture accelerates the electro migration of metal between conductors, damaging the circuitry of uncoated PCBs. Thus conformal coating is to be applied on PCBA in order to avoid:
b. Mechanical and thermal stress
c. Moisture and Solvents
d. Dirt and dust
e. Insects and rodents
3. Improve the electrical insulation resistance of the circuit board.
Application of Conformal Coating
Military & Aerospace systems
As Electronic Assemblies continue to be placed in harsher environments,
the need for selective coating will continue to grow.
Coating Material and Cured Thickness Check
Coating Material Introduction
Ideal Conformal Coating
- One part product
- Low viscosity
- Spray, dip, select, or flow-coat
- Long pot life
- Rapid cure
- Sufficient under component cure
- Wide range of operating temps
- Non – toxic
- Low cost
- Robust adhesion
Common Conformal Coating Materials
Coating Material and Cured Thickness Check
1. Coating material viscosity:
2. Cured coating material thickness :
Method: Spray the coating material on the copper board with normal coating program, and measure the thickness after it’s fully cured by curing oven.
Standard Range: 25 um~125 um
Coating Process Flow
Machine Capability And Settings
- Automated programmable spray equipment
- Moderate to high volume
- Often eliminates masking, <1% waste (over spray)
- Higher capital cost than hand spray or dip
- Less labor cost than hand spray or dip
- Application bulletins for major equipment available (Nordson, PVA, SCS, Asymtek )
- Parameters affecting thickness
- Material characteristics (viscosity, self leveling, cure rate)
- Dispensing pressures
- Head design
- Head travel speed
- Capability to dispense non flow bead for keep out area
1. Program edit panel
2. Coating working panel
Thermal cure schedules are generally a function of heat and time. For example, a typical thermally cured coating may require 10 minutes exposure at 100°C, or 30-40 minutes exposure at 65°C, complying with manufacturer’s recommendations.
Note: some material added the fluorescence for better inspection with UV light.
Coating Inspection Guideline
Coating PCB area:
–This area usually includes: soldering pads and circuit traces.
— Edge tolerance of coating deposit is up to ±1.5mm. Increasing coating area improves edges accuracy due to better definition of coating deposit along stripe length.
— Wherever is possible, build the coating program using long stripes for improved varnish deposit accuracy alongside coating stripes.
— Coating areas should be kept square or rectangular (90° corners) due to dispensing process limitations.
— Via holes have the potential to transfer coating material from coated side to opposite uncoated side. In many cases, this should be allowed as long as the material does not flow into a keep-out area or interfere with the design. The flow of material will create a visual imperfection: coating deposit will show “thinning of material” around the via hole and the opposite side will show an excess of material on the via hole.
— Usually component’s insulating body (plastic, ceramic, etc.) is not required to be coated. However, chip components with a height lower than 1mm are recommended to be coated entirely.
— Component’s corners and tip of pins whose height is larger than 1.8mm can not be considered covered with conformal coating due to physical properties of coating material that flows down to reach mechanical stability.
— Thickness of coating deposit should range between 25µm and 125µm and might not be uniform due to board’s relief (components, vias).
— Components and PCB areas masked by large components, covers, shields, daughter boards, etc. cannot be coated.
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