The polyurethane pre-polymer, 100% solids material performed well in SIR and thermal shock testing.

Moisture-cure and 100%-solids chemistries have been used for years in a range of applications; however, previous attempts to develop moisture-cure conformal coatings have been unsuccessful. Moisture-cure coatings tend to be fast-reacting and could block machines, particularly spray equipment where a fine needle dispenser is used to apply the coating. These 100%-solids materials are high-viscosity materials that can cause problems in spraying and dipping applications. They can be applied only in thick films and therefore can take a long time to cure. In most cases, however, the viscosity is so high that successful coating of a delicate PCB cannot be achieved.

A novel non-VOC conformal coating uses a polyurethane pre-polymer as the base resin, a 100%-solids material in the form of a high viscosity liquid. The pre-polymer is pre-reacted polyurethane and isocyanate where some of the functional groups are blocked. Upon exposure to moisture, the reaction continues to completion, forming a coating.

The non-VOC coating is created by adapting polyurethane pre-polymer with a blend of carefully chosen diluents to ensure all materials are reacted within the system. The diluents chosen offer a massive reduction in viscosity, creating a 100%-solids material at a sprayable viscosity, similar to that of solvent-based materials, without VOC emissions (sidebar). The cure profile starts with the polyurethane pre-polymer reacting with moisture in the air. This is then followed by a further reaction with the diluent blend, releasing only CO2 from the reaction. This cross-linking is designed to enhance mechanical strength and abrasion resistance.

The common environment that a coating is subjected to is standard atmospheric conditions. Initial tests generally are conducted to evaluate electrical and mechanical performance. Following these tests, the environment can be altered to assess coating performance under more severe conditions. Such conditions can include salt mist, high humidity, high temperature and thermal changes, either as a gradual rise or decline in temperature or an immediate thermal shock. After exposure to such environments, the coating then can be retested for thermal and mechanical properties, determining its suitability for various applications.

UV cure and water-based technologies have been discussed as possible replacements for solvent-based coatings, and some meet industry standards. Still, solvent-based coatings remain the favored technology on performance and application grounds and therefore provide a good benchmark for the required properties of a conformal coating. Among the various types of coatings used to benchmark were a pair of proprietary materials: DCA, a modified silicone conformal coating and HPA, a transparent acrylic conformal coating, which between them have military, defense standard and UL approvals.

Environmental testing often consists of elevated levels of humidity or salt mist and general temperature changes. A humid or salt atmosphere is created in a corrosion testing chamber, typically at around 85% humidity and 5% salt. The atmosphere is kept constant within the chamber for a set time. This can vary from 24 hrs. to a number of weeks, depending on requirements. Surface insulation resistance (SIR) is measured before and immediately after environmental exposure. In order to meet industry standards, a coating’s SIR should be greater than 108Ω following environmental testing. SIR is also measured after a recovery time, typically around 15 min. This is where the coating is removed from the test environment and left to recover in atmospheric conditions before testing. After this time, the coating should have almost completely recovered to its original SIR value.


Conformal coatings may also be applied to PCBs forming part of a device subject to changes in temperature. Automotive applications call for extended temperature ranges, especially under-the-hood. Military and aerospace applications can demand even more extreme upper and lower temperatures. Thermal testing is designed to simulate all possible scenarios. Thermal cycling tests are carried out in one chamber, where temperature is changed at a set rate. The highest specification in industry standards is a rate of change of temperature of 12°C/min., covering a range of approximately -25° to +125°C.1 Thermal shock tests are slightly different, however. A shock test is harsher than a cycling test. Consisting of two separate chambers, one at the lowest temperature and one at the highest temperature required, the test board is passed through from one chamber to the next. This results in an immediate change to the surrounding temperature of the board. The flexibility, appearance and SIR are evaluated following a set number of cycles through either the gradual or shock method.

The novel coating was tested in high humidity and 5% salt environments at room temperature and at 40°C for 21 days, in accordance with IEC 60068-2-78 and IEC 60068-2-11, respectively. SIR was measured immediately after testing and again after 15 min. recovery, to identify coating performance. The results (Figure 1) showed the performance of the novel coating in such environments is comparable to solvent-based coatings. In some cases the coating exhibited superior performance with faster recovery times (Figure 2).


In thermal shock tests, the test was set to 15 min. at -70° and +130°C, and 20 complete cycles were performed in accordance with IEC 61086-2 section 4.2. (It should be noted thermal shock tests were carried out at a wider range than specified in IEC 61086-2: 70°C instead of -55°C.) Long-term temperature extreme tests – thermal aging – were also performed for 500 hr. at -60° and +125°C, in accordance with IEC 61086-2 section 4.3. Flexibility and adhesion testing was carried out in accordance with ISO 1519 and BS EN ISO 2409, respectively. The novel coating showed no reduction in SIR, adhesion or flexibility in either test. Finally, the coating was exposed to an environmental cycling test per MIL-46058C. This consisted of 24 hr. immersed in water, 24 hr. at 105°C, 96 hr. at 95-100% humidity and a final 8 hr. at -70°C. The appearance and SIR were evaluated following the cycle and again remained unchanged (>1 x 1013 Ω).


Other conformal coating performance testing includes electrical properties evaluation. This is essential to all applications of conformal coatings to a PCB, regardless of the environment to which it is exposed. Typical tests, also outlined in many of the standards discussed, are dielectric strength, surface resistivity, dissipation factor and dielectric constant.

Surface resistivity, dissipation factor and dielectric constant tests were conducted per ASTM D257 and ASTM D150 (measured at 100Hz), respectively. The results were comparable to that of solvent-based and other available coatings. When cured at room temperature, good electrical properties are exhibited. However, when heat-cured, NVOC fully cross-links, offering superior electrical performance (Figures 4a and b). Solvent resistance tests were also carried out and showed similar results (Figure 5). The use of elevated temperatures to cure the coating, it was found, actually improves the cured film performance, without affecting flexibility.


In order to meet the majority of accepted industry standards, additional properties such as flame retardance and resistance to mold growth are required. The novel coating incorporates use of liquid flame-retardants and a microbiocide to achieve such properties. UL 94 provides four levels of approval. The highest rating, V-0, requires the test substrate to self-extinguish within 10 sec. following a 10 sec. exposure to a naked flame. Setup parameters are specific and final grading achieved by taking into account the performance of five separate samples. The novel coating has been tested according to UL 94 and fulfills the requirements for V-0 status. Sterility, in-can and film preservation tests have been carried out on the coating, and results show it prevents microbial contamination where applied. Microbial growth was measured according to ASTM 5590-94 and ASTM 5589-97.

Reference

1. IEC 61086-2, “Coatings for Loaded Printed Wire Boards (Conformal Coatings), Part 2,” section 4.1, 2004.

Bibliography

• ASTM D257-07, Standard Test Methods for DC Resistance or Conductance of Insulating Materials, 2005.
  
  
• ASTM D150, Standard Test Methods for AC Loss Characteristics and Permittivity (Dielectric Constant) of Solid Electrical Insulation, 2004.
  
  
• UL 94, Test for Flammability of Plastic Materials for Parts in Devices and Appliances, 2007.
  
  
• ASTM 5590-94, Standard Test Method for Determining the Resistance of Paint Films and Related Coatings to Fungal Defacement by Accelerated Four-Week Agar Plate Assay.
  
  
• ASTM 5589-97, Test Method for Determining the Resistance of Paint Films and Related Coatings to Algal Defacement, 2002.

 
Jade Bridges is R&D manager at Electrolube (electrolube.com); jade.bridges@hkw.co.uk.

About VOCs

Volatile organic compounds (VOC) are carbon-based compounds that vaporize easily at room temperature. They are more clearly defined by the EU Solvents Emissions Directive, which states a VOC is “any organic compound having, at 20°C, a vapor pressure of 0.01kPa or more, or having a corresponding volatility under the particular conditions of use.”

VOCs can occur naturally; for example, isoprene and monoterpene are two of the most common VOCs emitted by vegetation and are commonly termed Biogenic VOCs (BVOCs). Manmade VOCs come from industrial and domestic sources, including emissions from oil, gas and transportation, and general fuel consumption and solvent use. The latter is of most concern within the coatings industry.

The EU Solvents Emissions Directive covers defined operations such as the manufacture of coatings, coating activities (such as PCB conformal coating) and surface cleaning. The threshold limit value for solvent quantity is 5 tonnes per year for such activities as conformal coatings. Manufacturers whose solvent consumption falls below these thresholds fall outside the scope of the directive. Occupational exposure limits (OELs) in the workplace will still be apparent, however. It is therefore clear the Solvents Emissions Directive affects conformal coatings manufacturers and end-users.

Why all the fuss? VOC emissions have to be controlled due to their effect on the environment and human health. VOCs contribute to the formation of ground-level ozone, a major component of smog. Such pollution can have many detrimental effects on the environment, in particular, damaging forests and vegetation. When not managed properly, VOCs can also cause health problems. Overexposure causes them to act as irritants and, in worst cases, carcinogens. Therefore, it is fair to say the formation of ground-level ozone is a serious air pollution problem.

Ozone is not emitted directly, but is formed from the photochemical interactions of VOCs and nitrogen oxides. The only significant process that forms ozone is the photolysis of NO2; therefore, ozone is in a photostationary state. When VOCs are present in the atmosphere, they react to form radicals that either consume NO or convert to NO2. Increasing the level of VOCs in the atmosphere increases the conversion of NO to NO2 and in turn causes ozone levels to increase. The absence of VOCs would therefore significantly reduce the amount of ozone formed.

• Prev
• Next