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, expands on the company’s new generation two-part conformal coatings and explains why they are becoming an essential process for manufacturers to increase reliability in the harshest of environments.
With electronics increasingly playing a part in daily life, the applications for electronic systems continue to develop at a rapid pace. Miniaturisation is ever increasing, 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 more permeable to save weight, the use of conformal coatings 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.
Conventionally, conformal coatings are thin polymeric materials intended to protect assemblies from harsh conditions experienced in their operating environment, including moisture, condensation, salt-spray, corrosive gases, conductive particles etc. Coatings can also provide further dielectric insulation, allowing components to be placed in closer proximity than would be possible in air, and to provide mitigation against the growth of tin whiskers.
To fully protect against corrosion and provide an effective barrier to mitigate against tin whiskers, which can potentially cause failure in electronics by shorting the circuit, it is crucial that the coating material covers all metal surfaces at an appropriate thickness to provide these barrier properties. To understand coating coverage with regards to potential tin whisker mitigation, the IPC TR-57 ‘Conformal Coatings: The State of the Industry’ whitepaper investigates hundreds of boards, coated with a variety of common coating chemistries and application methods, which were meticulously cross-sectioned for thickness. The results were surprising, 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.
Fig 2. Typical cross-section showing difficulty achieving coverage (blue layer) with traditional coating materials.
Why was the 2K range developed?
The main reasons why Electrolube decided to develop a new breed of 2 part conformal coating systems simply results from the increasing expectations for conformal coatings to perform effectively in higher temperature extremes, for coating systems to withstand more thermal shock cycles and to address the coverage concerns addressed by the whitepaper. Many conventional materials are becoming obsolete because they are failing to meet new maximum operating temperatures or provide insufficient coverage required for successful condensation tests, which are needed by global automotive manufacturers. Electrolube also wanted to dispel any myth that applying a coating material more thickly achieves better coverage. Over-applying a coating, in fact, only increases the amount of stress on the coating and the assembly, which can lead to disaster during thermal shock testing, especially with UV curable materials, which crack and then provide no protection at all.
Addressing these challenges was the driving force behind the development of the 2K range. Electrolube’s 2K coatings have been designed to withstand high temperature extremes and have the application ease of a coating with the performance properties of an encapsulation resin. 2K coatings can be applied more thickly without placing unnecessary stress on the assembly or the coating, which eliminates any concerns about cracking. 2K coatings also accomplish the coverage requirements for successful resistance against condensation, tin-whisker mitigation and reinforcing dielectric functions. 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 would you use a 2K coating rather than a single coating system?
It is time to consider using a 2K conformal coating when a traditional coating fails to deliver the necessary protection for the end-use environment, a 2K coating can provide considerably increased protection across the board for resistance against condensation, high-humidity, immersion and corrosive. The additional thickness and coverage is especially beneficial with regards to tin whisker mitigation and dielectric re-enforcement. 2K materials are also low-odour and solvent-free, providing health and safety benefits in comparison to other chemistries.
What are the key features and benefits of 2K products?
The 100% solids materials provide increased thickness and coverage, in comparison to single coating systems. 2K materials have also been developed to successfully endure automotive and aerospace thermal shock cycles with minimal impact on solder joint reliability.
2K coatings also provide a fast chemical cure, with full cure achieved in only 10 mins at 80°C, significantly improving throughput and reducing work-in-progress. For instance, the 2K850 is a rapid UV cure coating system within the 2K range, which is immediately touch-dry when exposed to regular microwave lamps or LED lamps. Following oven cure, the boards can be bagged without residual solvent-odour or concerns about sufficient moisture being available to complete cross-linking process. The overall oven lengths can be as short as 700mm due to the lower intensity and longer wavelength light being less hazardous and through conveyors can be repurposed for this purpose as shown below.
Due to the fact that UV light can be shadowed by tall components, a secondary cure process is required. The chemical cure of a 2K material is achieved within 6 hours at a constant and reliable rate without any requirement for an additional oven process.
Unlike traditional single coating systems, 2K materials have a different flow, which can provide improved edge coverage and resolves any issues regarding areas to be left uncoated such as connectors and BGAs. 2K can be applied by selective coating spray and needle dispensing to provide maximum coating accuracy to users.
What types of products feature within the 2K range
Electrolube’s 2K, two-part conformal coating range fundamentally combines the tough, resistant properties of an encapsulation resin, with the easy application of a coating. Examples of products within the range include 2K550, 2K850 and 2K750 conformal coatings.
2K550 is a tough, flexible, high performance coating, which has excellent clarity and resistance to discolouration, humidity and condensation. The UL94V-0 compliant polyurethane coating is characterised by greater coating thickness and enhanced edge coverage and shows improved adhesion, hardness and scratch resistance. 2K550 also features a fluorescent trace to aid inspection.
The 2K850 two-part, UV cure coating combines the speed and convenience of UV cure with the benefits of Electrolube’s 2K system. The 2K850 offers a faster processing time while still delivering high levels of protection. The polyurethane coating is a significant development to help speed up production, providing an immediate tack-free coating and enabling complete cure at room temperature within 24 hours, compared to the industry moisture cure average of 8–14 days. Featuring a wide operating temperature range of -40 to +130°C, with good thermal shock performance, 2K850 provides a consistent chemical cure and strong protection against water, moisture, salt mist and condensation, making it ideal for tough electronic applications exposed to harsh conditions.
Electrolube has also developed a new silicone 2K conformal coating, 2K750. The UL94V-0 compliant coating provides a wide operating temperature range of -40 to +220°C, with a high continuous use temperature of 200°C. The tough, low-stress coating has an extremely fast thermal cure time of 10 mins at 80°C, in addition to curing at ambient temperature, and provides excellent adhesion to most solder resists and metals. The transition to implementing Electrolube’s 2K conformal coatings range couldn’t be simpler with a straightforward valve modification such as PVA’s VPX-2KS or Nordson Asymtek’s AM range.
What testing procedures have the 2K products been through and what are the results?
All of the 2K materials have been tested to comply with the BMW GS95011-5 automotive standard and to meet the requirements of the IPC-CC-830. The testing procedures for the BMW GS95011-5 automotive standard includes sequential thermal shock, salt-spray, corrosive gas, 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.
Following intensive powered condensation testing on automotive and aerospace assemblies, Electrolube’s 2K300 demonstrated outstanding protection compared to the next best coating. The 2K materials have also undergone the BMW dewing test and the NPL’s new condensation test on SIR test boards, again with particularly positive results and with little difference 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).
Which applications are most suitable for 2K?
2K materials are ideally suited for harsh and demanding applications. The coatings are particularly suitable for 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.
However, 2K materials have also been beneficial to military and aerospace customers, as well as industrial controls and white goods manufacturers, with 2K improving resistance to condensation, damage resistance from insects, liquid water-ingress and corrosive gases.
Do I need to make any changes to my existing selective coating equipment?
Electrolube has collaborated closely with popular selective coating equipment suppliers such as ANDA, PVA and Asymtek to ensure that the thicker 2K formulations are fully optimised for application using commercially available dispensers. Together they have developed and tested 2K applicators that provide customers with a retrofit upgrade to their existing and new equipment. The minor modification 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 precision work, it is possible to apply via a needle dispenser also.
For instance, PVA’s new two-component spray valve, VPX-2KS, has proved to be a highly successful modification for users of selective coating systems, adapting easily into their existing equipment to ensure excellent control of material flow, improved edge definition and easier regulation of dispensing starts and stops.
In comparison to single coatings, 2 part conformal coatings can achieve impressive full coverage of components at higher film thicknesses, which results in unparalleled resistance to condensation, corrosion, tin whisker mitigation and dielectric re-enforcement. 2K coatings are also hydrophobic, which gives improved humidity and moisture protection, including saltmist, making 2K an excellent choice for automotive electronics as well as other tough outdoor applications.
Some 2K materials can be reworked with comparitive ease, despite their greater thickness and enhanced chemical resistance. 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. 2K coating materials will still give significantly improved protection at lower film builds, compared with traditional materials, due to their improved ability to cover component leads and other sharp edges.
The solvent-free range of 2K materials significantly 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.