Shortening the time and temperature of curing is a current issue that is being explored in order to conserve energy, increase production throughput, and reduce costs. One approach is through the use of photocurable coatings. Although photocurable coatings have been used for over a century as photoresists, primarily in photoengraving and printing processes,[44] they have only recently gained favor in the electronics industry. Rapid uv-curing coatings are now available from several suppliers and all the major polymer types have been modified or formulated so that they can be polymerized on exposure to uv light. Ultraviolet- curing coatings are largely based on acrylic or acrylic-modified polymers. They are generally one-part, 100% solids, thus avoiding solvent emissions. When stored properly, their shelf lives range from six months to several years.
Beneficial properties such as rapid cure, reduced solvent emissions, room temperature cures, and the ability to coat heat-sensitive devices and circuits make photocurable compositions attractive for many electronic applications. In addition, some photocured coatings have been reported to possess enhanced physical and mechanical properties,[45] and, because of their short cure times (seconds), lend themselves well to completely automated production lines.[46] Optimum curing occurs in the wavelength range of 250 to 400 nanometers with a radiant output of at least 150 milliwatts per square centimeter.[47] Generally, medium pressure mercury vapor lamps are used. Many companies that sell uv-curable coatings also sell compatible curing equipment ranging from portable, hand-held wands for spot curing coatings, adhesives, sealants, and encapsulants[48] to conveyorized-belt systems (Fig. 3.21) for curing conformal coatings and marking inks.
Ultraviolet and uv/visible radiation-curing equipment can emit light at specific frequencies matching the absorption bands of photosensitive groups in polymeric coatings and adhesives. The shape and intensity of the emitted light can be varied to flood a large area, to focus the beam for high-intensity high-speed curing, or to focus the beam on a small area (spot curing) which is beneficial in rework and repair.
A major limitation in using photocured coatings on printed wiring assemblies is the inaccessibility of light to cure portions of the coating that are shadowed by large components. To obviate this, a double cure mechanism is used, for example, uv curing followed by heat curing or moisture curing. Some new polymer resins have been synthesized that combine in one molecule both the photosensitive group and the moisture cure group, for example, a polyurethane that is uv cured, but can subsequently be moisture cured to assure that the shadowed areas are also cured.[49]
Photoinitiation. Most photocurable coatings require a photoinitiator to start the polymerization process. Photons from the ultraviolet source react with the photoinitiator to generate active catalysts that join and cross-link monomers and oligomers into high-molecular-weight polymer structures that are insoluble and resistant to chemicals. Thus most photosensitive coatings are negative-acting types.
Photo-induced polymerization may be of two types: free-radical or cationic. Free-radical polymerization requires high intensity uv radiation, results in rapid curing, and permits the application of a wide range of coating thicknesses. Although photocured coatings based on free-radical mechanisms are the most widely used, coatings that cure by a cationic mechanism have some unique benefits and are gaining greater attention. Among these benefits are:
1. Only low-intensity uv radiation is required
2. Curing continues after the radiation source has been removed and all the photoinitiator has been depleted.[48]
The cationic process allows areas not directly exposed to the light source to be cured, called “dark” or “shadow” cure, whereas the free-radical process requires complete exposure to uv light for effective curing.[50] The choice of free-radical or cationic-cure coatings depends largely on the application. For example, cationic curing is best for curing a die-attach adhesive while free-radical curing is best for defining precise patterns of tight tolerances.
In most cases, uv exposure should still be followed by a secondary heat cure because some coating is shadowed by an irregular topography or flow beneath devices. Such secondary heat cures are not as long a duration nor do they require as high a temperature as normal heat-curing systems. Ultraviolet exposure time to obtain a tack-free film can range from less than a second to several minutes depending on the specific formulation, the intensity and wavelength of the uv radiation, and the thickness of the coating.
Free-radical Photopolymerization. Free radicals used in photoinitiation are generated by one of two methods: intermolecular hydrogen abstraction or intramolecular photocleavage. Both methods require absorption of light energy, followed by excitation to produce the active free radicals. Examples of compounds that undergo intermolecular hydrogen abstraction are benzophenone, benzil, and 9,10-anthraquinone. Examples of compounds that undergo intramolecular photocleavage include benzoin ether and phenyl-1,2-propanedione-2-o-benzoyloxime. The free radicals react with monomers and oligomers to initiate polymerization. Competing with polymerization is the decay and recombination of the free radicals and reactions of the free radicals with the growing polymer chain and with air oxygen. A general reaction scheme is shown in Fig. 3.22. Oxygen inhibition can be avoided or controlled by adding oxygen scavengers to the coating formulation or by curing in an inert ambient.
Cationic Photopolymerization. In cationic photopolymerization, compounds that convert to Lewis acids* on exposure to uv light are used as photoinitiators. Among these compounds are aryldiazonium salts, diaryl-iodonium salts, and triarylsulfonium salts. The Lewis acids generated are long-lived and continue to catalyze polymerization after the uv source has been removed. This ensures curing in shadowed areas and under compo-nents.[51] A further benefit of cationic curing is that an inert atmosphere is not necessary during curing since the reactions are not inhibited by oxygen.