2K Conformal Coatings Address Performance Challenges in the Harshest Environments

Introduction

The performance requirements of coating materials continue to become ever more challenging, as electronics are subjected to ever more hostile operating environments, including higher temperatures along with increased levels of salt-spray and condensation. As OEMs try to save weight from their designs, the use of robust metal assembly housings are being replaced by less hardy designs, transferring increased performance requirements to the assembly, such as continued operation under full immersion conditions. Simultaneously, environmental legislation continues to evolve, with an emphasis on reducing solvent usage and VOC emissions. In response to these ever increasing performance challenges, a new generation of two-part 2K conformal coatings have been developed by Electrolube to provide even greater protection, compatible with the protection offered by an encapsulation resin, however, with the easy application of a coating. The 2K series of coatings have been rigorously tested in comparison to UV and silicone coatings in a variety of harsh tests, which have included thermal shock, powered salt-spray, condensation and mixed flowing gas (MFG) testing, which will be explored further in this article.

Firstly, let’s examine the driving forces for using conformal coatings in harsh environments. Given that the conformal coating is often providing secondary protection (housing normally provides the main barrier properties), it is useful to consider the main threats that must be mitigated against and other unique demands caused by the long service lifetimes of the assemblies, when selecting a suitable conformal coating material for harsh environments.  The vast majority of decisions to use coatings traditionally falls into one of the following 4 categories:

Protection against corrosion in end operating environment (Humidity, Corrosive gases, Salt-spray etc)
Protection against condensation in end operating environment
Reduction in component spacing to meet safety or design criteria
Tin whisker mitigation

By considering the requirements of each application independently, some common themes emerge.

Protect against corrosion in end operating environment
Corrosion is a complicated, diffusion controlled, electro-chemical process that takes place on an exposed metal surface. Despite the vast variety of potential mechanisms and causes, in the majority of cases, there are 3 requirements that must be fulfilled in order for corrosion to take place.

Intrinsically electrochemically dissimilar metals (e.g. Gold/Silver and Nickel/Tin), or the creation of an anode and cathode by application of applied bias.
The presence of an ionic species (usually Salts, Halides, Hydroxides etc).
The presence of mono-layers of condensed water to dissolve the ionic species, resulting in an electrolyte solution.

In order to prevent the possibility of corrosion, it is necessary to remove one of the pre-requisite conditions. The choice of metals is limited to those used in the solder and solder finish chemistries (which are dissimilar), and there will always be areas of potential difference due to the nature of an electronic assembly. Cleaning can help remove ionic species, but cannot prevent the re-deposition of ionic species from the operating environment.

Conformal Coatings help prevent the formation of electrolytic solutions by acting as moisture barriers. Given the 3D topography of metal surfaces on a PCB, all metal surfaces need to be sufficiently well coated to prevent exposure of the metals surface to a potentially corrosive environment, in order to provide the maximum degree of protection. Small voids in the coating that expose the metal surface can actually accelerate corrosion under the right environment. The conformal coating challenge is achieving the required level of coverage on the three-dimensional, complex topography of exposed metals.

In addition to perfect coverage, the coating also needs to be a good barrier against moisture and provide good adhesion to the substrate to prevent delamination. Once the coating is delaminated, moisture can eventually collect in this ‘pocket’ and form an electrolytic solution with any pre-existing ionic contamination. This is the reason that cleaning prior to conformal coating is recommended, to provide a powerful synergistic elimination of two of the three pre-requisite conditions for corrosion.

Protect against condensation
Whenever there is a significant level of humidity, there is always the opportunity for parts of the assembly to drop beneath the dew point, resulting in the formation of condensed water on the surface of the assembly, which can significantly reduce the insulation resistance of the boards surface, resulting in the malfunction of electronics.
Whilst pure water is not a great conductor of electricity, any ionic impurities (e.g. salts from manufacturing or end use environment) will become solubilised and will form a conductive pathway, leading to corrosion as described above. Higher levels of liquid water can rapidly accelerate this process.

In addition to corrosion, condensation severely tests the insulation resistance of the coating. Considering this is essentially an immersion application, water will very quickly find weak spots or voids in the coating. Whether there is no coating, or the coating is thin, the insulation provided will be negligible or less than optimal. The conductive solution can carry the current from one weak spot to another very quickly, resulting in either immediate failure, which may be reversible when the board dries out again, or which may be permanent if conductive corrosion products, dendrites (metallic filaments) or other permanent sources of current flow are deposited on the surface of the coating.

Reduce component spacing
Although air is normally an excellent insulator, it can begin to break down when stressed by a sufficiently high voltage (an electric field strength greater than about 3 kV/mm), becoming partially conductive. Across gaps, breakdown voltage in air is a function of separation. If the voltage is sufficiently high, complete electrical breakdown of the air will culminate in an electrical spark or an electric arc that bridges the entire gap. Conformal coatings provide additional insulation resistance and can be used to enable designers to make their designs more compact, by placing components closer together than would otherwise be possible.

Tin whisker mitigation
The application of conformal coating is a tin whisker mitigation strategy. Conformal coatings mitigate the risk of tin whiskers generating a short in one of three ways.
A tin whisker eruption needs to puncture and penetrate out through the coating. Research shows that this is unlikely.

In order to short, the protruding tin whisker must either:
meet another protruding tin whisker from an alternative polarity
penetrate back through the coating at a place of opposite polarity to create a short.

All of these situations for forming a tin-whisker short are unlikely (but not impossible) events. Research and modelling show that as long as you have adequate coverage and thickness of conformal coating on conductive surfaces leads, it is unlikely for tin whiskers to penetrate through the coating once, and almost impossible to do so twice. That leaves the only real potential failure mechanism as two protruding tin whiskers meeting and forming a short, which is a statistically insignificant probability.

By considering these 4 case scenarios, it should be clear that the coverage of the applied conformal coating must be as close to perfect as possible. In addition, it should be recognised that the greater the thickness of conformal coating achieved, the greater the protection will be from the environment.

For conventional liquid-applied materials and application processes, achieving good coverage and thickness has been shown to be problematic in the IPC’s ‘state of the industry assessment’, with literally thousands of cross-sections analysed, showing zero (for all practical considerations) coverage on many common component leads and bodies for all material/process combinations.

Cross-sections courtesy of Rockwell-Collins.

Given the importance of edge-coverage and thickness and the challenge in achieving both, while still maintaining other performance requirements (e.g. temperature resistance, thermal shock resistance, cycle-time etc.) a new coating concept was developed to reliably meet these increased performance demands.

The Two-Component (2K) Approach

Electrolube developed the 2K conformal coatings series by way of providing a solution to common problems faced by manufacturers with performance issues of current coatings in harsh environments. The 2K series of coatings is significantly different in its protective capabilities due to its capacity for greater coating thickness and enhanced edge coverage. 2K coatings are VOC-free, fast-curing, high performance two-part conformal coatings that provide a solvent-free alternative to both UV and silicone materials, require less capital investment than UV materials and improve on the performance of most silicones in harsh environments. The majority of coatings in the 2K range are hydrophobic, giving excellent protection against water immersion, salt mist and humidity, making them ideally suited for automotive electronics.

2K materials are commonly used in high performance industrial applications such as adhesives, paints and even potting materials. The systems consist of a resin portion and a cross-linker portion, that are reasonably stable when kept separate. However, once mixed in the correct ratio, an unstoppable chemical reaction begins which continues until a solid polymer is formed. By adjusting the nature of the resins and the hardeners, a wide variety of polymers can be produced from soft-rubber like materials to high-strength glass-like materials. Many of these materials are diluted with solvents, which increases the usable life of the system and enables the use of existing application methodologies. However, with the restrictions on solvent-usage, it was decided to focus on generating a solvent-free solution.
Moving to a solvent-free 2K system required the development of a special applicator to facilitate this technology as shown below.

Images courtesy Nordson Asymtek and Precision Valve and Automation (PVA)

The approach uses twin progressive cavity pumps to control the metering of the two reactive components, keeping the materials separate until the mixing process, which occurs in a small, disposable static-mixer, immediately prior to application. Utilising this approach of mixing immediately prior to application, enables the use of more reactive materials, resulting in shorter cure times. The atomised spray application enables a wide range of material viscosities to be selectively applied within a normal cycle time. The use of larger pumps can enable faster cycle times, and the all metal design of the applicator provides the opportunity to heat the material to improve process consistency and enable the use of more viscous materials.
This applicator technology enables all of the 2K range of materials to cure within 10 minutes at 80°C, the typical dry-time and temperature for a solvent-based acrylic. Indeed, the latest addition to the 2K range cures primarily with UV radiation (conventional microwave or arc-lamp systems (UVA) or the latest LED curing technology), whilst the chemical cure ensures complete-curing within shadowed areas in just a few hours, compared with weeks or months for moisture initiated secondary cure mechanisms.

Coverage and Thickness
The following cross-sections demonstrate the achievable thickness and coverage for a variety of common components
It should be clear from the following sections that the 2K materials enable a much greater thickness and perfect application coverage, to provide a higher level of protection. Performance advantages of the 2K materials can be demonstrated in three of the harshest tests employed by users today.

Thermal Shock
Coated automotive ECU assemblies subjected to 1000 thermal shock cycles from -40°C to +140°C without stress cracking.

Powered Condensation Testing
A populated Electrolube SIR test board was used to evaluate the NPL’s new condensation test method, prior to the launch of the current ongoing project.

The boards were tested uncoated, selectively coated with a solvent-based acrylic and selectively coated with a 2K material.

The board and test results for a BGA device and an SOIC are shown below. The results show that for both component types, the SIR results with the 2K coating remained 2 orders of magnitude higher and did not vary much whether the material was covered with condensed water or not. However, for the acrylic there was a fairly significant difference depending on whether the material was under condensing conditions or not. The BGA device showed evidence of dendrite formation in the uncoated evaluation.

 

Powered Immersion testing in salt-water
In order to assess the ability of the coating material to withstand powered immersion in salt-water conditions, two experiments were performed.

Firstly, standard IPC B-24 SIR coupons were spray-coated and cured at a variety of different thicknesses, achievable from a production process. The SIR coupons were placed in a horizontal position and powered up at 10V. Each SIR pattern was covered with 5% (w/w) saline solution, and the patterns were monitored to ensure they remained completely saturated with saline throughout the test duration. SIR measurements were recorded every minute. The experimental set-up is shown in Fig A.

Fig A. SIR experimental set-up
The SIR measurements for the OSP coated Cu, Reflowed SAC alloy, Ultra-thin conformal coating (3 micron) and Acrylic conformal coating (50 micron) are shown below in Fig B. Whilst the acrylic material showed very low SIR values, the results were at least consistent and significantly better than the Cu-OSP value. Interestingly, the SAC alloy showed reasonable resistance to Saline, whereas the UT coating was indistinguishable from the OSP coated CU, both of which showed evidence of electrochemical corrosion.


Fig B. SIR comparison of Acrylic, Solder Paste, UT and AR coatings

However, when we look at the results for the 2K materials during this same test we can see that within the intended thickness range (>100 micron), there is no impact from the salt-immersion throughout the duration of the 30 hour test. At 50 micron thickness, the SIR shows some impact of the saline immersion, but the results are still 3 orders of magnitude greater than the UT or AR materials.


In the second immersion study, IPC-B52 test coupons were coated with 2K300, 2K850, UT and AR type materials by a normal application process. The coupons were cured in accordance with supplier recommendations and left or 28 days prior to commencement of the immersion test.

The boards were immersed in saline solution and then immediately powered at 5V. The boards were filmed during their immersion. A significant degree of bubbling and the formation of corrosion products could be seen almost immediately with both the uncoated control and the UT coating material, especially on the components. The Acrylic coated board showed almost immediate bubbling, but the formation of corrosion products took approximately 2-3 minutes.

The 2K300 and 2K850 boards showed no evidence of bubble formation throughout the 30 hour duration of the immersion test.

Conclusion
The 2K range of materials provide sharp edge coverage and protective thickness, unrivalled by other liquid applied conformal coatings. The materials have been developed specifically for thick film applications and engineered to optimise this performance advantage, whilst mitigating many of the issues associated with thick coatings, such as cracking during thermal shock testing and solder-joint lifetime degradation, especially for very sensitive devices such as BGA packages. The rigorously tested two part systems can be applied thinly (50-75μm), however, they have been designed, formulated and tested to be applied at much greater thicknesses (150-300μm), to facilitate superior encapsulation of components and component leads.

The materials will drop into any existing selective coating process with a simple valve modification and minimal investment. The extreme protection provided by the increased thickness and superior coverage will be valued by many Automotive and Aerospace suppliers struggling with increased demands from OEMs for improved condensation resistance and even powered up sea-water immersion conditions.

Phil Kinner
Electrolube Technical Director (Coatings Division)