A conformal coating is a protective film of between a minimum of 25µm to (usually) 150µm thick – which 'conforms' to the shape of the populated PCB it is applied to.
These conformal coatings help provide protection from the possible effects of the working environment that the assembly will be used in – these can typically include harsh environmental weather conditions where moisture, humidity, condensation and extremes of temperature can adversely affect the performance and life of an assembly.
Other harsh working environments can include the use of solvents, fuels, fume generation, heat and vibration and marine applications where salt laden atmospheres can cause major corrosion problems leading to mould growth, arching, contact failure, current bleeds and shorting where operational life expectancy of the equipment is seriously and expensively compromised.
Conformal coatings are used on assemblies for use primarily in the automotive, marine, aerospace, lighting, defence and renewable energy industries, however with the expansion of the electronics industry, where equipment life expectancy demand has now increased, there has been a growth in the requirement to conformal coat substantially more consumer and domestic applications.
Application Methods
Conformal coatings can be applied either by manual processes or automatic. The different application methods are described below:
Brushing
What is Brush Coating?
This is the application of conformal coating using a brush, "painting" coating onto the printed circuit boards. The conformal coating technician applies the coating selectively, working around masked off areas as per requirements. This is a relatively skilled, manual process and can be used for low-volume, high-quality finish. This is a cost effective method at low volume requirements and is a technique used to finish and rework PCBs.
What are the main advantages of brush coating?
- Low set up costs
- Ideal method for low volume work and coating rework
- Relatively simple process
What are the main disadvantages of brush coating?
- Quality is not uniform due to different technician ablilities
- It can be a slow process and labour intensive
- Difficult to maintain coating thickness
Spraying
What is Conformal Coating Spraying?
Spraying conformal coating is used to coat low, medium and medium to high volumes of PCBs. This process can provide good coverage with usually high-quality finishes. The conformal coating is pre-blended using thinners to suit specific spray kit requirements and is applied using a manually operated spray gun. The requirement for a skilled trained operator is essential. Any components which must not be coated are masked prior to spraying in order to protect them during this process from coating ingress and contamination.
What are the main advantages of spray coating?
- Relatively low set up costs
- Method allows for low, medium and medium to high volume work
- Provides good coverage of the PCB
- With expertise and good training, coating thicknesses can be maintained within industry standard specifications
What are the main disadvantages of spray coating?
- Requires training for skilled operator
- Suitable, reliable equipment, spray guns, spray booths required
- Operator health and safety considerations – PPE, full air/fume extraction facility
- Blending/mixing area required – secure material handling
- Competent material batch record keeping required.
Dipping
Generally, dip coating is a process whereby a PCB assembly is submerged, either fully or partially, into a tank or suitable container of conformal coating.
The assembly can be either manually dipped (by hand) or for where larger volumes and/or greater controls are required, the use of a semi or fully automated controlled dip coater unit or machine is required.
Dip coating an assembly generally ensures complete and full coverage, except for any masked components. By using either a semi or fully automated dip process, functions such as emersion, dwell and withdrawal speeds and times can be fully controlled. Once set, this arrangement provides a fully repeatable process usually guaranteeing a known coating thickness with excellent aesthetic appearance.
Manual or hand dipping is suitable for quickness, convenience and extremely low volumes i.e. one or two assemblies – and of course where no controls or repeatability is a concern and appearance doesn’t matter either.
Semi-automated dip systems are suitable for almost all volume requirements, from one-off to one-thousand off. Generally, if the requirement is exceeding this figure then a fully automated system may become necessary and a more complex design criteria may need to be considered.
What are the main advantages of dip coating?
- Manual process – very low set up costs – but no control and labour intensive
- Semi-automated process - allows for low, medium and medium to high volume work
- Provides full coverage of the assembly
- Repeatable control, including coverage, thickness and appearance
- Relatively simple dipping jigs, fixtures etc can be employed.
- Safer level of health and safety issues and concerns – i.e. greater control over coating fumes etc
What are the main disadvantages of dip coating?
- Manual dipping – no control, no repeatability, labour intensive and high health and safety issues.
- Semi-automatic process – cost of good suitable dip coating system.
Other essential elements to consider:
- Extraction and racking systems required to post cure assemblies
- Adequate control of coating materials with good housekeeping and coating viscosity controls essential to maintain coating process
- Comprehensive training for operator required to cover equipment, materials handling and assembly set up for suitable dip procedure.
- Operator health and safety considerations – PPE, full air/fume extraction facility
- Blending/mixing area required – secure material handling
- Competent material batch record keeping required
Selective Coating
As the name possibly suggests, conformal coating is applied to selected areas on the assembly.
A selective coating machine is programmed to 'spray' coat the exact areas required on a repeatable basis. However, the same machine can be programmed to completely 'spray coat' the whole assembly if required.
Generally, there is a reduction in the amount of masking required and this can vary between completely to a little depending upon the complexity of the assembly, coating requirement, and volume of assemblies to be coated.
Of course, if there is little or no masking required there is less manual labour in comparison to other methods of coating, however, there is not only the cost of the selective coating machine to consider, there are set up costs including programming which point to this process as only being cost effective for large volume PCB coating work.
What are the main advantages of selective coating?
- Automated process - allows for medium to high volume work to be undertaken
- Repeatable process/control, including coverage, thickness and appearance.
- Much safer level of health and safety issues and concerns – i.e. greater control over coating fumes and material handling
What are the main disadvantages of selective coating?
- Not suitable or cost effective for low volume work
- Initial cost of Selective coater – can be expensive
- Training of both operator and/or technician to run, program and maintain equipment
Other essential elements to consider:
- Good coating machine fume/solvent extraction required
- Racking systems required to post cure assemblies
- Competent control of coating materials with good housekeeping and coating viscosity controls essential to maintain coating process
- Comprehensive training for operator required to cover equipment, materials handling and assembly set up for suitable selective coater.
- Operator health and safety considerations – PPE, full air/fume extraction facility
- Blending/mixing area required – secure material handling
- Competent material batch record keeping required
Aerosol
A pre-prepared conformal coating in a tin. A consistently repeatable mixed and blended conformal coating presented in aerosol format for the immediate facility to conformally coat a suitably prepared PCB assembly.
A conformal coating blended with suitable solvents and propellants which allow for a reasonably good quality coating atomised spray to be produced that can be directed on to the surface of a PCB assembly.
All application methods and subsequent coating results are dependent upon the skill and ability of the sprayer.
Acrylics, Urethanes, silicones and other derivative coatings are available in various aerosol formats and sizes. Convenience in a tin.
What are the main advantages of aerosol coating?
- Relatively inexpensive
- Relatively easy to use and convenient
- Relatively safe to use – simpler fume extraction facility can be accommodated depending upon usage.
What are the main disadvantages of aerosol coating?
- Not suitable or cost effective for medium to high volume work
- Not a truly repeatable process
- No real control over coverage, thickness and appearance
- Much safer level of health and safety issues and concerns – i.e. greater control over coating fumes and material handling
Other essential elements to consider:
- Good fume/solvent extraction still required
- Racking systems required to post cure assemblies
- Competent control of coating housekeeping and solvent use for cleaning essential to maintain coating process
- Operator health and safety considerations – PPE, full air/fume extraction facility
- Competent material batch record keeping required.
We Have The Solution!
Chiltern Connections has been a leader of conformal coating services for over 25 years. We can create the optimal solution for your business requirement, drawing upon our extensive abilities and resources.