Conformal coatings for component protection: a new thick-film lacquer provides short process times and improved protection from climatic, chemical and mechanical loads.

Printed circuit board assemblies (PCBAs) are often exposed to severe climatic conditions and mechanical stresses. The possible climatic loads that may occur during electronic assembly include high or low air humidity, high or low temperatures, low air pressure, fast climate changes, condensation, microbiological loads and contamination. Therefore, protection and insulation are becoming more important to warrant properly functioning PCBAs, in particular when stringent demands regarding quality and service life have to be met.

Conformal coatings are lacquer systems designed to protect PCBAs from failures, even under unfavorable ambient conditions. In particular, conformal coatings provide protection from moisture, dirt and other contaminants including organic solvents and other chemicals (Figure 1).

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Because the aggressiveness and loads of climatic conditions under which PCBAs operate are increasing, most conformal coatings may be overtaxed regarding their protection capabilities, especially when condensation, of the PCBA occurs. This overtaxing must not be attributed to the inadequate efficiency or quality of the polymers or binding agents used in the coatings, but rather to the coating thickness used.

In this context, the edge coverage of the areas to be protected is important. However, improved edge coverage cannot be realized with the standard ink systems available without some disadvantages.

Conformal Coating Options

In accordance with IPC Standard 2221 Generic Standardization on Printed Circuit Board Design, the recommended layer thicknesses for conformal coatings are 30 to 130 [micro]m for acrylic, epoxy and urethane resins and 50 to 150 [micro]m for silicone resins. The layer thickness substantially influences the coating's protective effect.

The layer thickness is almost directly proportional to the migration resistance; doubling the layer thickness almost doubles the migration resistance and thus the protective effect. The solution of the problem appears quite simple. Using currently available conformal coatings, simply increase their layer thickness and their protective effect is improved. However, this approach has some disadvantages.

With most conformal coatings, the drying rate of lacquer films depends on their thickness. Thicker films dry slower because their solvents have to migrate further to leave the wet lacquer film. With lacquers that dry oxidatively, the oxygen in the air also has to penetrate further to fully cure the lacquer film.

These delays in the drying process are not just linear; a double layer is not equivalent to doubled drying time. Instead, the drying rate is exponential; a double layer corresponds to about quadruple drying time. Thus, the time required to reach the desired ultimate properties, such as adhesion and electrical insulation, becomes much longer, and the coating is also more sensitive to layer thickness fluctuations that are inevitably encountered on a PCBA. In addition, these coatings are particularly sensitive to an early encapsulation or climatic load, because of solvent retention.

A second approach is to duplicate the lacquer coating, which is practiced frequently. Generally, the same factors also apply to this approach. Process times become much longer. Bad process control can result in the additional danger of wrinkling or pulling.

Another approach is to use a lacquer system with a higher solids content. However, these lacquer systems require extended drying periods even though less solvents are used. A higher solids content at comparable viscosity means a low molecular resin. Thus, a slower drying cycle is needed because more resin molecules have to be interconnected for filming. Again, the greater layer thickness delays the emission of the solvents from the coating.

Solvent-free coating agents constitute a technically and ecologically reasonable solution. Their chemical basis is generally known from casting compounds and casting resins. In this context, the limits between casting resins, casting compounds and conformal coatings become blurred. The disadvantages of the casting resins and casting compounds are known. These mainly two-pack systems have significantly higher viscosities, and, due to their two-pack character, more expensive processing is needed as compared to conformal coatings.

Another possible solution would be powder-coating, but these coatings require high temperatures of more than 130[degrees]C for curing. Furthermore, the wetting of fine structures and, particularly, the underfilling of components are inadequate.

Moisture-curing one-pack polyurethanes are another possible solution. But, their disadvantages include an equally slow drying process, sensitivity to moisture on the PCBAs, and limited storage stability. This last factor becomes particularly apparent when the containers have to be opened several times.

Finally, conventional ultraviolet-curing lacquers stand out because they have very low viscosities, are solvent-free and are particularly fast drying. However, they have a considerable disadvantage: ultraviolet (UV) lacquers only dry in those areas on the PCB where the UV radiation directly reaches the binding agent. In shadow zones, especially underneath components, no curing occurs at first. So the lacquer system in those areas remains sticky and cannot provide the required protection. These areas constitute a considerable weakening of the protective potential and can even lead to the failure of the entire PCBA, particularly with more stringent climatic loads such as condensation.

With UV-curing lacquers, a thermal post-treatment at temperatures above 100[degrees]C frequently must be performed. This treatment initiates film formation in the shadow zones. However, such post-treatment can substantially restrict or even prevent the use of some lacquers.

An Alternative Lacquer

A new thick-film lacquer has been developed that may eliminate the numerous problems related to the thick-film coating of PCBAs. Based on a copolymerisate of polyurethane resin (PUR) and polyacrylate resin, this lacquer uses two different chemical curing mechanisms that complement each other in the drying process.

In the first phase, the lacquer is subjected to a UV drying process, and, after a very short time period, the lacquer has dried to such an extent that it can be handled without reservation. The UV crosslinking very quickly generates a protective film that is mechanically loadable and, regarding its electrical insulation properties, complies with the demands on a conformal coating.

Curing can be performed in standard UV-curing equipment. A good dispersion of the UV rays is advantageous to ensure the sides of the components are adequately exposed and cured by preventing shaded areas adjacent to the components (Figure 2).

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In the second, slower drying phase, a chemical cross-linking by air humidity occurs, particularly in the shadow zones in which UV light-initiated cross-linking cannot be implemented. By means of this cross-linking mechanism (PUR curing), the air humidity diffusing into the polymer is "intercepted" and used for a polymerization and further curing of the lacquer in the shadow zones. This reaction proceeds much slower and is terminated after about eight to 14 days, depending on the ambient conditions.

The UV-functional group and the isocyanate functional group, responsible for the shadow zone curing, are attached to the same polymer molecule. This double function feature guarantees maximum resistances and quality because no noncross-linked binding agents remain in the film as soft resins. The layer thicknesses that can be achieved with this lacquer in one coating cycle range from about 200 [micro]m to 2000 [micro]m (Figure 3).

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This thick-film lacquer offers increased resistances and process-friendly drying. Depending on the film thickness applied, this lacquer can also promote component fixing to avoid the effects of vibration; for example, loss of contact. Because of its dual cross-linking mechanism, this lacquer provides lower shrinkage in the cross-linking process than comparable conventional UV systems. Because it is solvent-free, the lacquer may be ecologically reasonable, in conjunction with better protection for PCBAs.

The condensation resistance of the new lacquer is higher than that of conventional conformal coatings. This result is due to the comparatively high degree of cross-linking and the high coat thickness. Moreover, any moisture penetrating before the curing completes is intercepted and used for further cross-linking, thereby improving the system.

The new lacquer also provides higher chemical resistance to concentrated acids, alkalis and other aggressive media. In addition, overall good electrical properties and a high tracking resistance can be expected. Because of the binding agent's special chemical structure, a very good adhesion to the usual electronics substrates is achieved after the combined curing process.

In addition to the residual stresses from chemical shrinkage that occur during cross-linking, with thick-film lacquers in particular, the thermal residual stress and its effects must also be considered. Due to the higher coating thicknesses applied and the possibility of partial filling, thick-film lacquers can, for example, convey greater forces than conventional conformal coatings that are only applied in thin layers. Conventional conformal coatings are more likely to crack rather than destroy the PCBA.

Thermal residual stress is inevitable on coatings where materials with varying coefficients of thermal expansion (CTE) are combined and subjected to thermal shock. Here, the different performances of the materials involved with respect to Young Modulus and the CTE play a major role. In such cases, solvent-free ink systems that can be cured at room temperature and with minimum shrinkage are advantageous. This type of thick-film lacquer with a delayed cure using ambient air humidity displays relaxation of the residual stresses that are an unavoidable side effect of UV curing. The result is first-rate adhesion, even to metal substrates.

The residual stress, [sigma] *, caused by the varying CTEs can be expressed by the following formula:

[sigma] * [approximately equal] [E.sub.Polymer] ([alpha].sub.Polymer] - [[alpha].sub.Substrate] [DELTA]T

where [E.sub.Polymer] represents the Young Modulus, [alpha].sub.Polymer] represents the CTE of the polymer coating and [[alpha].sub.Substrate] represents the CTE of the substrate; for example, the base material, solder joints or plastic coating of the component.

If transition states are passed through above the glass transition temperature, significant changes in the material characteristics can occur. Consequently, a lacquer that displays as few fluctuations as possible with respect to the Young Modulus and CTE over the required temperature range (usually -40 [degrees]C to 120[degrees]C) is beneficial. A low Young Modulus is equally favorable to avoid residual stress. The new thick-film lacquer, for example, offers this capability as it can be formulated with a high elasticity that is maintained even at lower temperatures (Figure 4).

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Due to a glass transition state that lies in the minus temperature range, the predictable temperature-dependent deformations can easily relax. The result is minimum tension, which is demonstrated by a thermal shock resistance.

Regarding a product of higher viscosity, special attention must be paid to the wetting and underfilling of components. The new lacquer is a surface-active system that ensures complete underfilling of even large integrated circuits (ICS). Figure 5 shows the thick-film lacquer (fluorescent light-blue) flowing underneath the component due to the capillary effect, whereas the lacquer's physical flow stops at the upper end of the component.

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Figure 6 depicts a mound of thick-film lacquer that was applied to an IC with an edge length of 25mm. While the lacquer on the PCB flows about 1 cm, the IC is completely underfilled (Figure 7). To demonstrate this effect, the IC was broken off and tipped up to show the underfill. Thus, a complete underfill of components is feasible, providing safe and complete encapsulation (Figure 8).

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Conclusion

A new thick-film lacquer based on a copolymerisate of polyurethane and polyacrylate resin offers several advantages over conventional conformal coatings. Short process times can be realized despite high coating thicknesses due to the optimally synchronized curing mechanisms. The new thick-film lacquer provides fast UV curing and cross-linking by further reaction even in shadow zones. This one-pack system also provides the resistance of a two-pack system.

The feasible film thicknesses range up to 2000 [micro]m. The thickfilm lacquer combines the advantages of the increased resistances with those of a process-friendly drying cycle. It has maximum efficiency regarding possible climatic, chemical and mechanical loads. Depending on the film thickness, this lacquer can provide improved protection of PCBAs and the under filling of components to prevent damage from vibration.

Dr. Manfred Suppa is the manager of research and development for Lackwerke Peters GmbH, Kempen, Germany; e-mail: [e-mail].