Conformal Coating Selection: Why Choose Two Part Conformal Coating Systems for Selective Coating

Phil Kinner, Business and Technical Director for Electrolube’s Conformal Coatings Division, explores the company’s new generation two-part conformal coatings and explains why they are becoming an essential system for manufacturers to increase reliability in the harshest of environments.

The applications for electronic systems continue to develop at a fast pace, with electronics becoming more and more a part of daily life. The emphasis has become increasingly about miniaturisation, with systems expected to perform more operations, quickly and reliably, whilst becoming smaller and lighter with every design iteration. As assemblies become ever more densely populated, and housing /casing designs become increasingly permeable to save weight, the use of conformal coating is becoming essential to protect the assembly from its operating environment and ensure acceptable reliability for the application intended, especially when operating in hostile environments.

Conformal coatings are traditionally thin polymeric coatings designed to protect assemblies from corrosive conditions experienced in their operating environment, such as salt-spray, high humidity and/or condensation, conductive particles, corrosive gases etc. Conformal coatings are also used to provide additional dielectric insulation to enable components to be placed in closer proximity than would be possible in air, and to provide mitigation against the growth of tin whiskers.

In order for the coating to provide high levels of protection against corrosion, or to provide the necessary insulation to prevent arcing, as well as provide a sufficient physical barrier to mitigate against tin whiskers, it is essential that the coating material covers all metal surfaces at a sufficient thickness to provide these barrier properties. In the forthcoming IPC TR-57 ‘Conformal Coatings: The State of the Industry’ whitepaper, which was brought about by the need to understand coating coverage with regards to potential tin whisker mitigation, hundreds of boards were coated with a variety of common coating chemistries and application methods, which were meticulously cross-sectioned for thickness. The results were quite surprising to many, with numerous areas of extremely low coverage being found, along with wide ranges of thickness on different component types. A typical result for a gull-wing device such as a QFP lead is shown below in figure 2.

Why did Electrolube develop the 2K range?

In addition to the coverage issues highlighted by the white-paper, conformal coatings are expected to operate in much higher temperature ranges and survive more thermal shock cycles than ever before. Simply applying the existing coating materials more thickly doesn’t necessarily achieve better coverage, but does increase the amount of stress on the coating and the assembly, which leads to major problems during thermal shock testing, especially with UV curable materials, which crack and then provide no protection at all. Many traditional materials are becoming obsolete because they cannot tolerate the newer maximum operating temperature norms, and are unable to achieve the coverage necessary to meet the condensation test needs of major global car manufacturers.
The 2K range was conceived to address these 3 main challenges. The thermoset, cross-linked 2K materials can survive wider temperature extremes and are formulated to be applied more thickly to achieve the coverage necessary for condensation resistance, dielectric reinforcement and tin-whisker mitigation functions, without placing undue stress on the assembly or the coating, which can lead to cracking. The 2K range is solvent-free, low-odour and low-hazard which enables users to reduce their VOC emissions and improve the safety of their process.

Why consider using a two part coating system?
Where an existing material fails to provide the required protection for the end-use environment, a 2K conformal coating material can provide significantly enhanced protection across the board, when compared to regular one component materials, whether it be condensation, immersion, corrosive gases or high-humidity environments. The additional thickness and coverage is especially beneficial with regards to tin whisker mitigation and dielectric re-enforcement.
In addition, the materials are solvent-free, low out-gassing and very low-odour, providing health and safety benefits with regards to other chemistries.

What are the key features and benefits of 2K products?
In general, 100% solids, no by-product materials providing increased thickness and coverage, formulated to survive typical automotive and aerospace thermal shock cycles at these thicknesses with minimal impact on solder joint reliability.

Fast chemical cure – 10 mins at 80°C to full cure. Improved throughput and reduction in WIP. Boards can be bagged after oven cure without residual solvent-odour or concerns about sufficient moisture being available to complete cross-linking process.

There is a UV/Chemical cure version available (2K850), which is instantly touch-dry when exposed to regular iron-doped microwave lamps or LED 365nm lamps. LED lamps require far less electrical power, have much longer lifetimes, produce less heat and do not interfere with factory communication systems. The overall oven lengths can be as short as 700mm due to the lower intensity and longer wavelength light being less hazardous.

Through conveyors can be repurposed for this purpose as shown below.

UV light can be shadowed by tall components, material coating the backside of leads or the underside of components, therefore a secondary cure process is required. The chemical cure process of the 2K materials is completed at a consistent and reliable rate (<6 hours), with no need for an additional thermal oven process and no ‘it depends on numerous factors’, as experienced with moisture secondary cure systems.

2K can be applied by selective spray and needle dispensing to provide maximum coating accuracy to users. The materials do not flow like traditional coating materials, providing improved edge coverage and minimising issues with keep-out coating areas such as connectors, BGAs etc.
Examples of products from the 2K range

2K300 uses a unique high-temperature range (long term performance at 150°C), low modulus elastomeric polyurethane with exceptional insulation-resistance, even when submerged or powered up in liquid/salt-water. 2K300 has been shown to provide exceptional thermal shock (-65° to +150°C), condensation, liquid/water and chemical resistance to numerous solvents, oils and transmission fluids. 2K300 also provides exceptional edge coverage, ensuring tin whisker mitigation and strong dielectric re-enforcement.

2K850 is a UV-curable coating material with a chemical cure mechanism that ensures full-cure in shadowed areas. The material is opaque, and available in a bright red colour to ease inspection and maximise AOI contrast. The material meets the requirements of UL94 V-0, is low modulus and elastomeric to ensure sufficient flexibility to survive thermal shock, has a thermal conductivity of 0.7WmK-1 to prevent hot-spot formation by spreading heat generated throughout the coating layer. The material provides an extremely good level of chemical resistance and gives improved condensation and humidity protection.

How have these products been tested and what are the results?

In addition to the bare bones requirements of IPC-CC-830, all of the 2K materials have been tested to ensure compliance with the BMW GS95011-5 automotive standard which includes sequential thermal shock, corrosive gas, salt-spray and temperature and humidity cycling. In addition to the BMW standard, the materials have been subjected to a minimum of 1000 thermal shock cycles (-40° to +130°C) on populated, no-clean assemblies to ensure there is no tendency to cracking at the higher film builds.

Many boards have been cross-sectioned after selective coating to ensure coverage and thickness requirements are achieved.

2K300 in particular has been subjected to powered condensation testing on automotive and aerospace assemblies, with outstanding protection achieved compared to the next best coating. The materials have also been subjected to the BMW dewing test, as well as the NPL’s new condensation test on SIR test boards, again with extremely positive results, and with little difference seen between damp and dry conditions in the cycling test as shown below. Higher insulation resistance values are desirable, as is recovery to similar levels between wet and dry conditions. Low insulation resistance values which remain low are indicative of corrosion or electrochemical migration (ECM).

What applications are 2K best suited to?

With their fast throughput and single-piece flow, 2K materials are especially suited for demanding automotive applications where the combination of high temperature resistance, long-term flexibility to survive thermal shock cycling and an unsurpassed ability to withstand corrosive gases, aggressive media such as transmission and brake fluids, powered salt-spray and condensation/dewing tests takes their protective capabilities to the next level. The materials also provide resistance to total immersion in salt-water whilst under load, for at least 30 hours.

In addition to automotive applications, the materials have proved popular with military and aerospace suppliers due to changes in aircraft manufacturers’ requirements for functionality during condensing conditions.

Industrial controls and white goods manufacturers are also benefitting from improved resistance to condensation, liquid water-ingress, corrosive gases and insect damage resistance.

Are there any modifications required for existing selective coating equipment?

Electrolube has worked hard with popular selective coating equipment suppliers such as ANDA, PVA and Asymtek to develop and test 2K applicators that provide customers with a retro-fit upgrade to their existing equipment (or new machine purchases).

The upgrade essentially consists of metering pumps (and controllers) to ensure the correct volumetric mix-ratio of the materials, and a new spray applicator that mixes the materials immediately prior to spraying. For detailed or precision work, it is possible to apply via a needle dispenser also.

Conclusion

When used at the higher film thicknesses recommended, it is possible to achieve 100% coverage of components resulting in unsurpassed corrosion, condensation and immersion resistance, tin whisker mitigation and dielectric re-enforcement, the most common reasons for using conformal coatings. Despite their greater thickness and enhanced chemical resistance, some of the materials can be reworked with relative ease. There are materials within the range that are formulated to be opaque and coloured, to both improve machine AOI recognition and manual inspection, but which can also be used to hide board designs.

At lower film builds, the coating materials will still give significantly improved protection compared with traditional materials, due to improved ability to cover component leads and other sharp edges.
The solvent-free range of materials improves the reliability of assemblies in automotive, aerospace and industrial applications, whilst simultaneously offering faster curing characteristics, lower odour and improved health and safety characteristics than other 100% solids technology such as UV curable acrylated materials.