FREQUENTLY ASKED QUESTIONS (FAQ) ON CONFORMAL COATINGS
VIDEO describing the unique moisture blocking and non moisture-water absorption properties of AIT PRIMA-PROTECT conformal coating in contributing to the unparalleled performance in protecting printed circuit board from damages in salt mist and/or in industrial environment with corrosive ions dissolved in the moisture and acid rains.
FREQUENTLY ASKED QUESTIONS (FAQ) ON CONFORMAL COATINGS FOR ELECTRONIC CIRCUIT BOARD PROTECTION AND PROTECTION OF ELECTRONIC DEVICES AGAINST AGGRESSIVE HUMID AND SALT MIST LADEN ENVIRONEMENTS
PRIMA-PROTECT™ Conformal coating (CC7090-E and CC7130-E) from AI Technology, Inc. has been proven to far outperform conventional conformal coatings of epoxy, acrylic and silicone. AIT CC7090-E and CC7130-E combine the hydrophobic repelling of water and molecular capability in blocking moisture and salt fog penetration.
1. What are the primary functions of conformal coating?
- The primary function of conformal coating is to prevent the electronic circuit boards from long-term corrosions due to moisture condensation.
- Other desirable features include a better prevention of electrical shorts in high humidity days.
2. If I have protected my electronics in a sealed plastics and metal boxes but the electronics are used outdoor, do I still need to conformal coat my circuit boards inside the sealed box?
- Having a sealed electrical box is good for rain. Moisture can still pass through the plastic boxes and rubber gasket.
- In fact, sealed boxes do not “breathe” and thus trapped moisture that cannot escape and will condense to become sitting water inside the box at night or colder weather. Condensed with potentially contaminating ionics from the air is one of the most potent corrosive ingredient.
3. How thick should the conformal coatings be for optimum protections?
- Traditional conformal coatings such as epoxy, polyurethane, acrylic and silicone recommend 50-75 micron thickness. Other than silicone, epoxy, acrylic and polyurethane conformal coating are generally hydrophilic and more susceptible to moisture penetration. However, because of the relative high Tg of acrylic, polyurethane and epoxy conformal coating of around 30-90°C, thickness beyond 75 microns will induce excessive stress on components mounted on the PWB and causes dramatic shorter life of the electronic devices.
- AIT conformal coatings are engineered to be super hydrophobic. AIT CC7090-E and SC7130 are effective from 25-50 micron thickness. Thicker coating thickness is not necessary but will not have any negative effect.
- AIT CC7090-E and CC7130-E are recommended to have 20-600 micron thickness. With the molecular structure of flexibility down to -55°C (glass transition temperature, Tg) Thicker coating thickness is not necessary but will not have any negative effect. That is, dip coating with finished thickness of 100-300 micron thickness will not causes stresses on the components populated on the PWB.
- When the conformal coatings are hydrophobic, the thickness requirements are not as important. In addition, AIT PRIMA-PROTECT™ CC7090-E and CC7130-E are one of the few that block moisture from penetrating that may otherwise carry with it the corrosive elements such as sodium, chloride, sulfur dioxide, etc. to condense on the surfaces that are being protected.
4. Why is silicone being one of the most hydrophobic molecular species of plastics but not necessarily the best of conformal coatings?
- While silicone molecular structures are generally hydrophobic, because of their low surface energy nature and form film form monomers that generally can migrate to undesirable areas to cause bonding and soldering problems if reworks or repairs are needed.
- Also silicone coating structure tends to have larger “free space” and allow fast moisture penetration. Any defects in coating will results in pot-hole of water retention areas that may cause corrosion over long period of time.
- Silicone is known to allow almost 10 times or more moisture to penetrate than typical acrylic conformal coating to allow the corrosive elements such as sulfur dioxide, sodium and chloride ions to be carried along onto the surfaces that the conformal coating supposed to protect. This high moisture permeability is a big problem for silicone conformal coating.
Water condensation can cause corrosion and failures of electronics if printed wiring boards are not protected with protective conformal coating. However, AIT CC7090E and CC7130-E are the very few conformal coatings can be used to protect electronic devices in humid salt fog and industrial polluted conditions.
5. Is there an advantages offered by conformal coating that are physically stronger and harder?
- Conformal coating is not designed as physical protection media.
- General mechanical strength is not required. As long as the conformal coating remains intact at low and high temperature, low and high humidity, resists water and common chemical and solvent, they will be effective conformal coating.
- Typically, higher strength or hardness means high modulus of elasticity. These stronger coating such as epoxy and acrylics may impart excessive interface stresses that may cause the components to malfunction or delaminate from the circuit board and render them not effective.
6. What are dielectric constant and loss and their relevance in conformal coating?
- Dielectric constant is a measurement of how the conformal coating responds to electric field. Higher dielectric constant than 3.0 means that the molecule is responding to the alternating field and current and could cause the circuit speed to be affected. It is not a desirable characteristic for higher frequency devices.
- Dielectric loss represents the internal molecular friction of the conformal coating. Higher number than 0.1 will general some heat inside the molecule and not a good property to have in conformal coating.
7. What are dielectric strength and its relevance in conformal coating?
- Dielectric strength is a measurement of insulation strength of the coating. Generally higher the better.
- However, the circuit boards are supposedly designed to have adequate protection against normal operation. As long as the conformal coating do not negatively affect the dielectric strength in air, they generally will be adequate.
- Generally, more hydrophobic and the lower the extractible ionic impurities the conformal coating, the better in terms of not attracting water that generally enables the ionic impurities to become mobile and potentially negatively affect the dielectric strength.
8. What are the different methods of applying conformal coatings and their relative advantages and disadvantages?
- Brush coating is used for smaller volume production. It has the advantage of applying conformal coating only on those needed areas. But the cost of labor with such method is higher.
- Spray coating is used for larger volume production and generally faster. However, areas that must not be coated must be properly masked before coating. The masking process cost time and money.
- Dip coating is used for even larger volume production and are generally the fastest method. Again masking will be required.
- In some special conformal coatings such as parylene coating, they are coated using specialized vacuum deposition technique that are costly and required high capital costs.
Moisture block and water repelling coatings engineered with UV blocking capability of CC7133-UVB, can also be used to coat on edges of solar panels and structural parts to protect them against salt water and salt fog ingress and corrosive effects of such moisture penetration.
9. How do I mask the areas of circuit boards that cannot be coated over with conformal coating?
- Typical masking method is taping with dots and stripe. They are more suitable for flat areas. They are generally polyester or polyimide carriers with pressure sensitive adhesive on them. They are not generally totally anti-static even when the pressure sensitive material may be anti-static.
- In some special cases, “rubber boots” that fit the connectors to be protected are used. Obviously, the mold and molding of such boots that are customized are costly and are more suitable for high volume applications.
- AIT provides masking tapes (MT100-S) that use proprietary flexible carrier that are intrinsically anti-static. The ability to conform to smaller curvature makes them superior for most applications.
- AIT also provides self-boot forming liquid gel (ML150-S) that can be applied onto pin and connector to form protection from being coat over. They can be removed by simple peeling.
- In all cases, whenever possible, remove the masking materials after applying the conformal coating before the conformal coating is allowed to cure. This will prevent tearing of the conformal coating when the masking materials are removed and also allow easier cleanup and reuse in case “boots”.
10. Should I conformal coat the edges of the circuit boards?
- Most circuit board edges are cut and thus are not protected with any coating.
- Whenever possible, the edges should be coated over with conformal coating to prevent easy moisture and water migration between the board solder mask and the board itself.
With properly molecular and UV blocking engineering, SC7053-UVB is proven effective in protecting structural parts from salt water and salt fog attacks. SC7050-UVB is the transparent counterpart coating that has the same characteristics and performance in blocking UV and protection against salt fog.
11. Why is it important for conformal coating to have low Tg and controlled coating thickness when coated on the printed circuit boards?
Traditional acrylic-based and epoxy conformal coating that has glass transition temperature around 10-20°C and induces excessive stresses during low temperature cycles in the usages and reduces the reliability of the electronic devices (http://www.slideshare.net/CherylTulkoff/how-conformal-coating-can-kill).
AIT PRIMA-PROTECT™ CC7090-E and CC7130-E are engineered to have glass transition temperature, Tg, at -55°C. That is, the PRIMA-PROTECT™ conformal coating is not sensitive to the thickness of coating applications and will not exert stresses on the components on the printed wiring boards. The unparalleled performance and reliability demonstrated in the parallel side-by-side testings are attributable to the incorporation and combination of moisture penetration blocking and molecular flexibility to allow wide latitude of coating thickness to provide the best protection of electronic devices against heat and moisture, salt fog and even salt water immersion. CC7090-E and CC7130-E are free of silicone and will be contaminate circuit boards for rework and soldering.
The following chart is a summary of various properties that are critical to the performance of conformal coating in protecting the printed circuit boards in electronic devices:
|TYPE||PRIMARY AND CRITICAL PROPERTIES FOR PRINTED CIRCUIT BOARD (PWB) CONFORMAL COATING|
|Material Costs||Ease of Application||Costs of Repair||Tg (°C)||Thickness for Stress Control||Moisture Absorption-Retention (%)||Moisture Penetration (gm/m2/day/25µ)||Salt Fog Tolerance||Sulfur Dioxide and Acid Rain Penetration||Temperature Range|
|Acrylic||Low||Brush, dispense, spray, (dip-coating is not recommended)||Low||10 to 60||40-60µ (Thicker coating will shorten MTTF)||0.3||550||Fair||Fair||Up to 125°C|
|Epoxies||Medium||Brush, dispense, spray, (dip-coating is not recommended)||High||50 to 90||40-60µ (Thicker coating will shorten MTTF)||0.5-1.0||20-37||Fair||Fair||Up to 150°C|
|Polyurethanes||Medium||Brush, dispense, spray, (dip-coating is not recommended)||High||50 to 90||40-60µ (Thicker coating will shorten MTTF)||0.25-0.80||37-57||Fair||Fair||Up to 125°C|
|Silicone||High||Brush, dispense, spray, dip-coating||High||<-55||Not Critical (Flexible and stress-free)||0.01||70-1872||Poor||Poor||Up to 200°C|
|Parylene||Very High||Vacuum||High||?||<5µ||?||16-217||Good||Good||Up to 125°C|
|Medium||Brush, dispense, spray, dip-coating||Low||-55||Not Critical (Flexible and stress-free)||<0.01||<10||Outstanding||Outstanding||Up to 85°C and 125°C respectively|
|http://www.academia.edu/1902960/Coating_materials_for_electronic_applications_polymers_processes_reliability_testing (Source for data of moisture penetration and absorption for acrylics, epoxies, polyurethanes, silicones and Parylene)|
The critical properties of PRIMA-PROTECT™ CC7090-E and CC7130-E have been proven to outperform even the most proven conformal coating such as Parylene in the most stringent environment where blocking moisture laden with corrosive sulfur dioxide in industrial pollutant, sodium and chloride ions in salt fog are critical.
|TYPE||Secondary Properties for PWB Conformal Coating|
|Solvent Resistance||Dielectric Constant and Insulation Resistance||Abrasion Resistance||Pot Life|
|Acrylic||Poor to Good depending on solvent types||>3.0/Good Dielectric Strength||Good||Not Limited|
|Epoxies||Oustanding||>3.0/Good Dielectric Strength||Good||Limited|
|Polyurethanes||Good||>3.0/Good Dielectric Strength||Good||Limited|
|Silicones||Good||<3.0/Outstanding Dielectric Strength||Fair||Limited|
|Parylene||Outstanding||>3.0/Outstanding Dielectric Strength||Good||Limited|
|PRIMA-PROTECT™ CC7090-E, CC7130-E||Poor to Good depending on solvent types||<3.0/Outstanding Dielectric Strength||Fair||Not Limited|
For more information on recent developments of conformal coating and other protective coatings:
AIT technical sales and service department can also be reached at: 1-609-799-9388 or 1-800-735-5040 (EST) and Fax: 609-799-9308