Photosensitive composition plays an important role in the fabrication of printed circuit boards (PCBs). Photosensitive compositions can be classified, based on their intended applications, into two main categories: photoresists for etching and solder resists. The former are used in the laying out of the copper circuit pattern using photolithographic techniques and will be removed after the copper circuit pattern is completed. The latter are used after the copper circuit pattern is laid out to protect certain portions of the copper circuit from being contacted by the soldering material during the soldering process. Both types of photosensitive compositions can be applied using either a liquid-type or a film-type technique. With the liquid-type technique, the photosensitive composition is applied onto the substrate using screen printing, roller coating, curtain coating, dip coating, etc. With the film-type technique, on the other hand, the photosensitive composition is first coated onto a transparent film, typically a transparent polyester film, which is then overlaid on the substrate and the photosensitive composition is then applied onto the substrate surface by hot-laminating or other suitable techniques.
Photosensitive materials, or photoresists in general, can also be categorized according to how they are developed. At the present time, there are two most common types of developments for photoresists: the solvent-development type and the alkaline-development type. The former uses a solvent, typically an organic solvent, to remove (for a negative type photoresist) the exposed portion of the photoresist. On comparison, if the photosensitive composition contains carboxyl groups, the exposed portions can be removed, and thus developed, using an alkaline solution such as sodium carbonate. Because the solvent-type development process can cause solvent exhaust and waste disposal problems which can cause significant increase in the overall manufacturing cost, the current industrial trend is to use the alkaline-development type processes. During the alkaline-development process, the carboxyl-containing polymer molecules will be neutralized by sodium carbonate to form a polymer salt. The unexposed portion of the photosensitive composition, which was not cross-linked, will be emulsified and caused to be dispersed in water. This thus completes the development process utilizing a negative-type photoresist.
The photosensitivity of a photosensitive composition is related to the degree of crosslinking after being exposed to UV light. Typically, a greater number of unsaturated double bonds (typically provided by the so-called photomonomers) in the photosensitive composition will result in a higher degree of cross-linking. However, excessive amounts of photomonomers would result in the photosensitive layer being too tacky, thus adversely affecting the image transfer process. With a dry film type photosensitive material, the photosensitive composition is protected by the transparent polyester film from being contaminated and the oxygen-induced polymerization-inhibition, and the requirement for the unsaturated double bonds can be entirely supplied by the photomonomers. Under these conditions, a alkaline-developable photoresist can be prepared by introducing only the carboxyl groups into the polymer binder, there is no need to add double bonds into the polymer binder.
On the other hand, when a solution-type photosensitive material is used, because no such protective polyester film is provided, the film thickness and the reactivity of the photosensitive film becomes important, and, as a result, the photosensitive composition can only contain a limited amount of photomonomers. In order to compensate for the decreased photosensitivity due to the reduction in the amount of photomonomers, unsaturated double bonds must be incorporated into the polymer binder so as to increase its post- UV exposure degree of cross-linking. This method has been disclosed in, for example, U.S. Pat. No. 5,055,378, which disclosed a photosensitive composition containing a polymer binder formed from the reaction of an epoxy resin with an carboxyl-containing unsaturated compound so as to impart unsaturated double bonds to the polymer binder. However, the polymer binder disclosed in the '378 patent does not contain carboxyl groups. As a result, it must be developed using a solvent and cannot be developed using an alkaline solution.
U.S. Pat. Nos. 5,100,767, 5,009,982 and 4,943,516, and Japan Patents 61-243869 and 1-54390 disclosed various photosensitive compositions whose polymer binders are imparted with carboxyl groups by reacting the polymer binder disclosed in the '378 patent with anhydrides. Such an additional step not only allowed the photosensitive composition to become alkaline-developable, it also improved the photosensitivity of the polymer binder by introducing unsaturated double bonds into the polymer binder. However, because the formation of such alkaline-developable photosensitive polymer binder requires excess (about 10 to 20%) amounts of unsaturated carboxyl group containing compounds (such as acrylic acid or methylacrylic acid), which are typically volatile and can cause health concerns, the final finished products can cause health hazards to human bodies, especially after long term exposure to these products. This is one of the main disadvantages of the photosensitive compositions disclosed in the above-mentioned patents.
U.S. Pat. Nos. 4,764,452 and 5,364,736 and Japanese patents 4-239070 and 2-47657 disclose photosensitive compositions containing polymer binders based on modified styrene-maleic anhydride resins, or the SMA resins. In these photosensitive compositions, partially-esterized SMA resins were reacted with saturated low-molecular weight alcohols in a ring-opening reaction by which carboxyl groups are introduced into the side chains of the SMA resins to form a carboxyl group containing polymer binder which consequently becomes base-developable. Because the reaction to prepare the base-developable polymer binder does not involve unsaturated carboxyl group containing molecules, it does not raise the health hazard problem mentioned above. However, these photosensitive compositions exhibited another type of shortcoming in that, because the polymer binder does not contain unsaturated double bonds, the developed, or hardened, final product does not provide adequate chemical resistance and hardness due to inadequate degree of crosslinking. In order to increase the degree of crosslinking for the post-exposure polymer binder, it has been proposed to add a mono-functional unsaturated alcohol, such as hydroxyethyl (meth)acrylate, in the ring-open reaction of the SMA resin. This method was disclosed in U.S. Pat. Nos. 5,087,552, 5,114,830 and 5,296,334. The use of mono-functional unsaturated alcohols not only imparts the carboxyl group into the SMA resin, it also imparts double bonds so as to increase the degree of cross-linking. However, while this action increases the photosensitivity of the polymer binder, it also returns the old problem of leaving low-molecular weight volatile, smelly and potentially health-hazard molecules in the polymer binder.