What is conformal coatings in electronics?
Read the following to find out:
For several decades conformal coatings have been used to protect PCBs from the environment, extend circuit life, ensure safety and improve reliability. A coating itself is a thin, flexible layer (typically 25 to 200Ám) of protective lacquer that conforms to the contours of the PCB and its components.
Conformal coatings protect circuits from: corrosive chemicals (salt, solvents, petrol, oils, acids); humidity and condensation; vibration; current leakage, electromigration and dendritic growth. In addition, conformal coatings allow closer track spacing as they prevent current leakage which, in turn, enables designers to reach new levels of miniaturisation.
As the demand for expensive, densely populated, compact assemblies increases, conformal coatings (originally developed for military applications) have expanded into the more commercial areas including household, retail, automotive, aviation, space, off shore, medical and the like.
Conformal coating chemistries
A number of basic raw materials are used in the formulation of conformal coatings. Polyurethanes (PUs) and acrylics are the most common components. However, they have limitations and are not able to offer the degree of protection that may be required in more hazardous environments such as aviation, aerospace and off shore. PUs and acrylics tend to be less expensive but, if not capable of fully protecting a PCB, could result in future service and maintenance costs.
Newer formulations use modified silicone technology which does not contain free silicones. These coatings, though initially more expensive, can be more reliable in the long term and therefore will reduce service and repair costs. Modified silicones can withstand a wide temperature range and have excellent chemical resistance
PU, acrylic and silicone coatings use solvent thinners which may require the use of extraction and protective clothing in production.
Single part epoxies are another type of base mater ial. Generally these types of coatings can provide a thicker film and greater degree of protection from moisture and humidity. However, they are fairly rigid and less able to withstand vibration.
The advent of more stringent health and safety requirements.and the potential requirements for removal of volatile organic compounds (VOCS) has resulted in a new generation of coatings. The two most popular types are materials which use water as a dituent, such as Electrolube Aquacoat, and 100 per cent solid materials that cure using a UV source. Such coatings are based on a complex emulsion of PU`s, acrylics and co-polymers and are able to offer advantages in terms of performance, environmental issues and production efficiency.
Some of the major benefits are: shorter cure/drying times, particularly in relation to modified silicones; improved thickness over sharp points and edges; non flammable nature eliminates special storage conditions; no requirements for special extraction systems and protective clothing; coating equipment does not need to be flame-proofed; and solvent resistance of the cured coating (even without heat cure) almost equals a cure modified silicone and is con- siderably better than PU or acrylic based materials.
Selecting conformal coatings
The selection of a conformal coating depends on a number of parameters and can be broadly broken down into three areas - the design of the board itself, the production requirements and the end user requirements.
Board design PCB geometry and population:
While not key to the selection of a specific conformal coating, these factors influence layer thickness and the dispensing method.
Cleaning, solder pastes and fluxes: PCB cleaning is a basic issue that could affect conformal coating adhesion as well as the type of solder paste and flux used. Coating over many no clean fluxes is possible providing there are very low levels of other con- tauninants. However, most manufacturers recommend cleaning: even no clean products may need some cleaning if a conformal coating is prerequisite. Particular attention needs to be paid to SMT components which may trap residues. Electrolube offers a variety of water based (Safewash) and solvent based (Fluxclene, SSS) cleaning technologies.
Temperature range and cycling:
Both these parameters are important as they define the performance of the coating at various temperatures. In general, silicones and epoxies are able to operate across a wider temperature range than PUs and acrylics. New water based coatings from Electrolube can protect at 150C continuously with peaks to 190C.
Production volume often defines the dispensing method. Capital equipment purchase may be necessary or, alternatively, application can be carried out by subcontractor coating or by renting machinery.
Generally four different ,dispensing methods are used to apply conformal coatings.
Brush application: a popular, cost effective application method but care must be taken not to over brush and leave a coating that is either to thick or so thin that it fails to protect the circuit. Also, it is difficult to coat under components and around edges and corners. In skilled hands this method is ideal for small scale production of simple assemblies.
Aerosol: this approach is common for PCB repair but the user must be experienced to avoid over spraying (accidental socket coating etc). This method is cost effective, convenient and easy, making it ideal for repair/maintenance situations.
Dipping: dip coating is excellent for medium volume production as it ensures a uniform coating thickness with complete edge coverage and under component penetration. Where flammable coatings are used, machines need to be either pneumatic (Electrolube`s DMC Dip Coating Machine) or flame proofed. This equipment controls dipping/withdrawal speed and dwell time. Enhanced thickness and protection is possible via double coating. However, a thickness beyond 200Ám does not increase protection and may lead to cracking.
Selective coating: this method, based on a programmable XY table, is ideal for high volume production as it eliminates the need to mask or tape areas that cannot be coated such as chip carriers and edge connectors.
Masking and repair/maintenance:
Different methods are used to protect components that cannot be coated. These include: tape masking (very timeconsuming); coating masks which can be peeled off after coating (they need to cure before the coating process); or preformed sleeves. All of these processes are time consuming and are the only altemative unless selective coating methods are used.
This is important when considering the production process and coating properties. Cross linking of the coating`s molecular structure is often necessary to guarantee the required solvent resistant characteristics. This normally requires an additional process. Many coatings dry at room temperature, eliminating the need for special heat cure equipment. However, many coatings are not cross linked and will, generally, not have such a high degree of solvent resistance. The new generation of water based coatings do offer near perfect solvent resistance without temperature cure since they cross link at room or low temperatures.
This is an alternative to using a dyed coating for QC purposes. A fluorescent trace in the coating glows blue/purple when exposed to a UV light source, thus identifying coating thickness and penetration under/around components.
End use requirements
This aspect of conformal coating is a double edged sword. The greater the solvent resistance the more difficult it is to remove for repair and maintenance. The key to success is often the solderability of conformal coatings.
Colour: Dyed or pigmented coatings can be used as security camouflague or to highlight certain components. Alternatively they may be used to ensurehat the coating has penetrated all areas of the board.
Conformal coating thickness in the range 25 to 200Ám is the standard requirement of the electronics industry. Double coats are possible but they are time consuming as the first coat needs to be touch dry before applying a second coat. This ensures good adhesion and avoids cracking and wrinkling. For 100 per cent moisture protection (immersion) potting resins should be used.
Many applications must conform to standards (eg MIL, IPC or UL) particularly for aviation and military applications.
Though the MIL standard will become obsolete in the year 2000 the ability for a coating to meet such a standard offers the peace of mind that it will continue to perform well. Many PUs and acrylics have been tested to meet this standard.
Extreme test parameters are the basis of the IPC and UL standards. UL746C has a very demanding test regime to meet electrical safety standards and only a few coatings are approved including Electrolube`s SCC3.