In steps for producing wiring circuit boards such as, e.g., flexible wiring circuit boards, a solder resist layer (covering insulating layer) is formed in order to protect the wiring pattern (conductor layer) and prevent solder bridging between connection terminals.
Screen printing has hitherto been used for forming solder resist layers. However, with the trend toward higher wiring pattern densities, photographic techniques capable of coping with finer wiring pitches (smaller spaces between wiring patterns) have recently come to be generally used in place of screen printing.
FIGS. 4 are diagrammatic sectional views illustrating an example of procedures of a conventional process for producing a wiring circuit board.
First, a conductor layer 90b having a predetermined pattern is formed on an insulating layer 90a as shown in FIG. 4(a). Thereafter, a solution of a photosensitive solder resist is applied on the surface of the insulating layer 90a including the conductor layer 90b. The solution of a photosensitive solder resist on the insulating layer 90a is then dried in a circulating hot-air oven to form a photosensitive solder resist layer SR as shown in FIG. 4(b).
Subsequently, as shown in FIG. 4(c), the photosensitive solder resist layer SR is irradiated with ultraviolet ray (UV) through a mask MK having a predetermined pattern. Thus, the photosensitive solder resist layer SR is selectively exposed to the light.
The exposed solder resist layer SR is then developed as shown in FIG. 4(d) to form a photosensitive solder resist layer SR having a predetermined pattern. The photosensitive solder resist layer SR is then subjected to a curing treatment to thereby give a wiring circuit board 900.
In FIG. 4(d), the central conductor layer 90b corresponds to, for example, a wiring part and the other two conductor layers 90b corresponds to, for example, connection terminals.
The photosensitive solder resist generally contains low-molecular ingredients such as, e.g., a polymerization initiator and a sensitizer so as to impart photosensitivity. However, due to the presence of the low-molecular ingredients such as the polymerization initiator and the sensitizer, the unexposed surface of the photosensitive solder resist layer SR has tackiness at room temperature even after drying in a circulating hot-air oven.
Consequently, in the conventional process for producing a wiring circuit board 900, it is impossible, after the drying of the photosensitive solder resist layer SR in a circulating hot-air oven, to stack up cut sheets of the resultant intermediate product or to wind the intermediate product into a roll. This is because if the intermediate product is superposed one on another or wound into a roll, the photosensitive solder resist layer SR, which has tackiness, undesirably adheres to the overlying insulating layer 90a. 
On the other hand, in processes for producing a flexible wiring circuit board having flexibility, a roll-to-roll operation is generally conducted frequently.
In the roll-to-roll operation, a long sheet (long sheet substrate) which has been wound around a roller is drawn out from the roller, subjected to predetermined treatments or processings, and then wound around a roller. This operation improves the efficiency of production of flexible wiring circuit boards and can realize a reduction in production cost.
However, if the above-mentioned intermediate product is wound into a roll, the photosensitive solder resist layer SR comes into contact with the insulating layer 90a and hence adheres thereto because the surface of the photosensitive solder resist layer SR has tackiness at ordinary temperature. Therefore, in the case where the above-mentioned photosensitive solder resist layer SR is used, the roll-to-roll operation cannot be conducted.
A technique for overcoming the problem described above has been proposed which comprises applying a release agent, e.g., a silicone resin, on the photosensitive solder resist layer SR to form a coating in order to avoid the tackiness of the photosensitive solder resist layer SR (see, for example, patent document 1 specified below).
In this case, even when the photosensitive solder resist layer SR comes into contact with the insulating layer 10a, the two layers are easily separated from each other by virtue of the release agent.
Patent Document 1: JP 05-243715A
However, the technique in which a coating film of a release agent, e.g., a silicone resin, is formed on the photosensitive solder resist layer SR has a drawback that there are cases where the application of the release agent on the photosensitive solder resist layer SR gives coating unevenness. Since the photosensitive solder resist layer SR is exposed to light through the release agent, the coating unevenness causes, in the exposure shown in FIG. 4(c), the photosensitive solder resist layer SR to suffer exposure unevenness. As a result, the exposed solder resist layer SR comes to have, for example, dissolution rate unevenness with respect to the developing solution in the development step thereof. There are hence cases where the resultant solder resist layer SR has thickness unevenness.
Furthermore, there are cases where uneven application of the release agent results in insufficient drying of the release agent itself and this results in sticking in intermediate product superposition.
In addition, since a coating operation and a drying operation each should be conducted twice, a large and expensive apparatus is necessary in the case where a continuous processing operation is to be conducted with a coating machine comprising the corresponding film coaters and dryers. Use of such an apparatus undesirably results in increased production cost.