Circuit Board Conformal Coating with Silicones

Table of Contents

Introduction
Conformal Coating Material Requirements
Corrosion Prevention
Corrosion Resistance Improved Products: ECC3011 and ECC3051S Conformal Coatings
     Sulfur Resistance
     Corrosive Gas Resistance
     Salt Spray Resistance
Conclusion

Introduction

Recent trends in the electronics sector, that call for higher functionality and miniaturization, are resulting in increasingly greater demand for better reliability of printed circuit boards (PCBs). Various conformal coating materials are used to provide insulation performance and moisture resistance, and are usually organic polymers composed of solvents that form a coating layer as the solvent evaporates. While the application of solvents allows for easy attainment of lower viscosity, there may be concerns linked with several regulations applicable to the solvent, environmental impact and its potential effect on operator health.

Compared to organic polymers, silicone conformal coating materials have heat resistance, insulation and cold temperature resistance benefits, and because of their softer properties, excel in their ability to absorb stress. Silicone is also more conducive to formulation designs that do not contain solvents. It is for these reasons that solvent-less, low viscosity, silicone conformal coating materials are increasingly being sought.

Conformal Coating Material Requirements

Performance necessities for conformal coatings include:

  • Adhesion: Good adhesion to PCB and component surfaces, and prevention of moisture ingress
  • Durability: The ability to provide long-term dielectric property stability (maintain PCB insulation) under high humidity and high temperature environments
  • Flexibility: Elasticity of the coating layer to absorb stress related with expansion and contraction of components during temperature cycles.
  • Corrosion Prevention: Protection again corrosion of bonding wires, devices, etc.
  • Fast cure: Fast cure under temperatures that do not harm PCBs or components
  • Chemical resistance: Resistance to solvents and oil
  • Processability: Ease of application and curing using readily available equipment
  • Optical clarity: Transparency or translucence to enable visual observation of PCB components
  • Toxicity: Minimize damaging effects on operators and the environment. (Formulations without solvents are ideal)

Organic conformal coating materials such as polyurethane, acrylics and epoxy are said to have intrinsic drawbacks related to cure shrinkage, temperature resistance and toxicity. Silicone conformal coating materials consist of silicone resin that usually is solvent borne, or silicone rubber that is formulated without cures and solvents by either room temperature (moisture) cure or heat (addition) cure. Based on the performance needs of conformal coating materials, as long as cure time can be reduced, room temperature cure silicone rubber materials are thought to be good contenders because of their ease of use and ability to eliminate curing equipment from manufacturing procedures.

Over the years, Momentive Performance Materials has created a line-up of silicone coating materials that display the inherent characteristic of silicone for temperature resistance in low viscosity, solvent-less room temperature (moisture) cure formulations. The usual properties of these conventional coating materials are shown in Table 1. Based on end-use application necessities, products are available in a range of moderate to low viscosities and used extensively in the electronics sector.

Table 1. Typical Properties of Coating Materials.

Typical Properties Conventional Momentive Coating Grade Names
TSE399
Encapsulant
TSE3995
Encapsulant
TSE3991
Encapsulant
TSE3996
Encapsulant
TN3705
Encapsulant
ECC3010
Coating
ECC3050S
Coating
Appearance translucent translucent translucent translucent translucent transparent~ translucent transparent~ translucent
Viscosity (23 °C) Pa.S 2.5 2.5 1.5 1.5 1.5 0.11 0.55
Tack Free Time*1 Min 10 10 10 10 7 3** 5**
Corrosiveness*2 none none none none none none none
Density (23 °C) g/cm3 1.04 1.04 1.03 1.03 1.01 0.99 0.98
Hardness Type A 28 25 19 23 13 35 22
Volume Resistivity MΩ.m 2.0 x 107 2.0 x 107 2.0 x 107 2.0 x 107 2.0 x 107 1.0 x 107 1.0 x 107
Dielectric Strength KV/mm 20 23 18 23 26 20 20
Dielectric Constant (60 Hz) 2.9 2.9 2.9 2.9 2.7 2.8 2.6
Dissipation Factor (60 Hz) 0.005 0.005 0.005 0.005 0.002 0.001 0.001
UL Flammability Class HB HB HB HB HB V-0 (746E) V-0 (746E)
RTI Rating °C 105 105 105 105 105 105 130

*1 23C, 50% RH
*2 MIL-A-46146B
**100 μm
Typical properties are average data and are not to be used as or to develop specifications.

Corrosion Prevention

As described, corrosion prevention is a crucial performance requirement for conformal coating materials. A range of materials including silver, ITO and chromium are presently used in applications such as electrodes for flat panel displays, where moisture protection of these materials is essential. Protecting against electrode corrosion and migration under impressed voltage and high humidity / high temperature environments is vital for these applications.

Additionally, in applications such as air conditioners, automotive electronics and power tools, the PCB can be exposed to outdoor conditions where protection against dust, temperature variations and corrosive gases becomes increasingly important.

As shown in Table 2, silicones fundamentally have a higher level of gas permeability when compared to organic polymers, and this difference is frequently related to a higher level of water vapor and corrosive gas permeability of silicones. Furthermore, silicone resin displays approximately half the gas permeability of a silicone rubber but yet has higher gas permeability than organic polymers in the order of 1~2 units.

Table 2. Oxygen Permeability of Various Materials and Permeability of Silicone (dimethyl silicone) to Various Gases.

Oxygen Permeability of Various Materials
Material Type Permeability
Dimethyl Silicone 60
Natural Rubber 2.4
Low Density Polyethylene 0.8
High Density Polyethylene 0.1
Butyl Rubber 0.14
High Density Polystyrene 0.12
Nylon 6 0.004

1x10-9 (cc•cm/sec•cm2•cmHg)

Silicone Permeability to Various Gases
Gas Permeability
H2 65
N2 28
O2 60
H2O 3600
NH3 590
CO 34
CO2 325
H2S 1000
CS2 9000
NO 60
NO2 1500

1x10-9 (cc•cm/sec•cm2•cmHg)

Polymer Permeability (1 mm)
Polymer Type Permeability
Polyolefin 2
Polyurethane 25
Acrylic 16
Silicone (resin) 47
Silicone (rubber) 100

g/m2•25 h (JIS Z 0208)

When electrodes containing silver are exposed to environments having sulfur or hydrogen sulfide, sulfuration and conduction failure may take place. This is frequently cited as an issue in applications such as power tools and air conditioning outside units.

Corrosion Resistance Improved Products: ECC3011 and ECC3051S Conformal Coatings

As described, silicone displays higher gas permeability than organic polymers and is fundamentally less able to obstruct sulfur vapor compared to other polymers. However, it is possible to alleviate corrosion on substrates such as silver, even in sulfur vapor rich environments, through enhanced adhesion. The method to enhancing adhesion and mitigating corrosion concerns involves silicone polymer selection, formulation modifications for adhesion improvement, and consideration of a variety of additives such as catalysts.

Momentive’s ECC3011 and ECC3051S conformal coatings are room temperature (moisture) cure silicone rubber conformal coating products that offer corrosion resistance, and use the product design approaches stated above. Common properties of ECC3011 and ECC3051S conformal coatings are given in Table 3.

Table 3. Typical Properties of Corrosion Resistant ECC3011 and ECC3051S Conformal Coatings.

Typical Properties Corrosion Resistant Coating Grade Names
ECC3011
Conformal Coating
ECC3051S
Conformal Coating
Appearance Transparent ~ translucent Transparent ~ translucent
Viscosity (23 °C) Pa.S 0.11 0.55
Tack Free Time*1 Min 3** 5**
Cure Time (23 °C, 50% RH) Min 10** 30**
Cure Time (60 °C, 15% RH) Min 2** 2**
Density (23 °C) g/cm3 0.99 0.98
Hardness Type A 35 22
Volume Resistivity MΩ.m 1.0 x 107 1.0 x 107
Dielectric Strength KV/mm 20 20
Dielectric Constant (60 Hz) 2.8 2.6
Dissipation Factor (60 Hz) 0.001 0.001

*1 23C, 50% RH
** 100 µm
Typical properties are average data and not to be used as or develop specifications.

Sulfur Resistance

Sulfur resistance of ECC3011 and ECC3051S conformal coatings was tested according to the technique described in Figure 1. As reference, uncoated specimens and specimens coated with silicone without the corrosion countermeasure were included. The results can be seen in Figure 2.

Sulfur Resistance Test.

Figure 1. Sulfur Resistance Test.

  • Place a silver sheet (90 μm) on glass-epoxy board (2 x 25 x 80 mm)
  • Apply conformal coating (100 μm thickness)
  • Place sulfur powder and the specimen inside a 100c glass jar
  • Store at 70 °C and observe the surface condition of the silver

Sulfur Resistance Test Results.

Figure 2. Sulfur Resistance Test Results.

The uncoated specimen turned black in one day, while the results for ECC3051S and ECC3011 conformal coatings after 7 days confirmed a sulfur corrosion benefit from the enhancement countermeasures. Silicone resin caused brownish discoloration, and upon conclusion of the test, the sliver sheet and its contact interface with glass epoxy was observed. A little delamination from the glass epoxy board was noticed with the silicone resin specimen, while ECC3051S and ECC3011 conformal coatings displayed no delamination. Good adhesion to the silver surface and no sulfur corrosion was noticed for ECC3051S and ECC3011 conformal coatings.

Corrosive Gas Resistance

A number of actual-use environments require protection against corrosive gases (NOx, H2S, SOx etc). To confirm performance against corrosive gases, a mixed gas corrosion test was performed using comb-shaped electrodes based on the conditions described in Figure 3.

Mixed Gas Corrosion Test.

Gas Concentration (ppm)
H2S 10 ± 5
NO2 200 ± 20
Cl2 10 ± 5
CO2 200 ± 20

25 °C, 75% RH, 21 days

Figure 3. Mixed Gas Corrosion Test. (IEC60068-2 Method 4)

Regardless of its lower gas permeability, the silicone resin type coating material resulted in some localized corrosion, while ECC3051S and ECC3011 conformal coatings displayed no corrosion. These results prove that the formulation approach used in ECC3051S and ECC3011 conformal coatings can have a positive advantage in preventing corrosion related to mixed gas exposure.

Salt Spray Resistance

Resistance to salt mist is a common worry with electronic appliances and components. A salt spray corrosion test was performed using comb-shaped electrodes based on the conditions stated in Figure 4.

Salt Spray Test

Figure 4. Salt Spray Test. (IEC60068-2-52, Severity 5)

  • Salt Spray Cycle: [(Salt Spray at 35 °C x 2 hours) + (40 C, 95% RH x 22 hours)] x 4 cycles (96 hours total)
  • Conditioning Cycle: After each Salt Spray Cycle, dwell at 23 °C, 50% RH x 72 hours (168 hours total)
  • Cycle Repetition: Repeat above process 4 times (672 hours, 28 days)

In spite of its lower gas permeability, testing of the silicone resin type coating material caused black colored corrosion, while ECC3051S and ECC3011 conformal coatings resulted in no corrosion. These results prove that the formulation method used in ECC3051S and ECC3011 conformal coatings can have a positive advantage in preventing corrosion related to salt spray exposure.

Conclusion

Conformal coating materials are used across the electronics industry. As the industry progresses and necessities continue to evolve, it can be estimated that the inherent advantages of silicone conformal coating will progressively be required to fulfill industry needs, and that through continued incremental efforts such as those stated in this article, silicone can continue to expand into applications where organic conformal coating materials are currently used.

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