Curing Mechanisms For Electronic Protection

The protection of electronic components can be realized by thin-film coatings, thick-film coatings or encapsulation. For these solutions, according to the requirement profile, different chemical systems are available (Newsletter Edition 4, 2021: Selection of chemistry as basis for best electronic protection (elantas.com)). Equally important for the application is the choice of a suitable curing mechanism.

The two-component reaction is very often found in connection with potting applications. Mixing of resin and hardener initiates in a chemical reaction. The reactivity can be enhanced and the processing time reduced by raising the temperature. The separate components alone are relatively insensitive, so that less demanding storage conditions and in part longer shelf lives are possible than with single-component products. At the same time, less inhibitors are required for stabilization, which can facilitate excellent performance of the polymers. The materials for two-component reactions are simpler to produce and, in some cases, also less expensive. However, it should be mentioned above all that these have the disadvantage of more complicated process control and greater error proneness with the mixing process compared with one-component products.

Whereas the two-component reaction typifies the classical solution for potting applications, with thick-film and thin-film coatings the one-component reactions predominate. With thermal curing it is necessary to exceed a certain temperature in order to activate the hardening reaction. If this temperature is not reached in the hardening process, the reaction does not fully harden the material or does not harden the material at all. The challenge with the development of such products is to keep the required curing temperature as low as possible in order to enable energetically efficient processing and at the same time to obtain a stable material with a longer shelf life at room temperature. 90°C for 30 minutes are typical curing parameters in the area of electronic protection. However, the exact hardening conditions can vary considerably for products of different chemical composition and must always be taken from the respective technical datasheet. An advantage of thermal hardening is that there is no limitation of the coating thickness, as it is the case with moisture curing, for example. Humidity curing materials cross-link by reacting with the moisture in the air to form a protective polymer. Following the initial skin formation, the atmospheric moisture diffuses into the deeper material layers, as a result of which the time required for the hardening process correlates with the layer thickness of the material. Depending on the chemical composition, secondary products can also form during the moisture reaction (for example methanol with silicones), which then also have to diffuse out of the material in order to enable complete hardening. Accordingly, moisture curing products are frequently somewhat permeable for gases. However, their outstanding advantage is above all the simple hardening process, which requires no further processing equipment.

However, when fast processing times are required there is no alternative to UV curing or UV LED curing. UV curing enables extremely short processing times, particularly in the area of conformal coatings, providing hardening of the material within only a few seconds. Despite the fast UV-polymerization, though, in nearly all cases a second hardening mechanism is required in order to achieve complete hardening. Due to capillary forces, frequently part of the material penetrates underneath the components into the so-called shadow areas. The UV radiation cannot access these areas with sufficient intensity, so that a second hardening mechanism is necessary. This can take place either in the form of a reaction with moisture or be initiated thermally.

Besides classical hardening by polymerization, so-called thermoplastic materials are also used for  protection of electronic components. These are applied in the molten state and then solidify by simply cooling. The advantage of these products, just as with moisture curing, is that they require no additional processing equipment for the hardening process. The previous Newsletter Edition 2, 2020 „Best moisture protection with the new Bectron® MR melting resins (elantas.com)“ provides further information on melting resins.

In addition, protective lacquers in which the polymer is dissolved in solvents are frequently used for protection of Printed Circuit Boards. Such solvent-based lacquers are easily handled and have the advantage that the solvent often has a cleansing effect on the PCB, so that in most cases the lacquers exhibit good adhesion. On the other hand, nearly all solvent-based lacquers contain volatile organic substances (VOC). With solvent-based varnishes, the solvent evaporates after application. The exposure to heat can accelerate this step. The previously dissolved polymer then remains on the PCB as a protective conformal coating. According to the type of polymer, post-curing (e.g. with the oxygen) can take place following physical drying.

In summary, a range of products with different hardening mechanisms is therefore available for  protection of electronic components. The table below gives an overview of the usual hardening mechanisms allocated to examples of specific products.