A metal mask has been employed as a screen in printing of a solder paste onto a wiring pattern of a circuit board. As the circuit board has increased in size and mounting density, the metal mask has been required to be large with a precise opening pattern. Recently, the metal mask used is 600 mm long, 600 mm wide or larger, with a thickness of 0.1-0.15 mm and, the opening pattern is processed precisely with the use of a laser or the like.
In order to form a fiducial mark in the metal mask, a through hole is opened at a position of the fiducial mark simultaneously when the opening pattern is processed. Then the through hole is filled with a resin or an aluminum adhesive tape is attached to the through hole from the rear side to prevent solder paste from entering and adhering to the circuit board. The through hole is opened at the position of the fiducial mark in this case, because the laser technique affords only to form the through hole in the metal mask.
As such, processing accuracy for the opening pattern is required to be enhanced furthermore so as to improve the mounting density of the circuit board. However, the laser technique or any other methods cannot eliminate the issue of a peripheral part of the processed opening pattern being tapered in the case of the metal mask, that is, there is a limintation with respect to the improvements of the processing accuracy. At the same time, even when the metal mask is thinned to 0.1-0.15 mm, the metal mask of stainless steel or the like is hard and will not fit well to the circuit board, which can adversely influence the printing accuracy.
For solving the above-described issues, a resin stencil has been used. The resin stencil can be similarly made 600 mm wide, 600 mm long or larger and 0.1-0.15 mm thick. The peripheral part of the opening pattern can be processed perpendicularly by an excimer laser, without generating tapers, and therefore the processing accuracy is improved. Moreover, because of the resin, the stencil is soft as compared with a metal mask of stainless steel or the like and thus it can be conveniently fit to the circuit board.
When a fiducial mark is to be formed in the conventional resin stencil, a through hole is formed at a position of the fiducial mark simultaneously when the opening pattern is processed in the resin stencil, in the same manner as in the metal mask. Subsequently the through hole is filled with a resin 11 as shown in FIG. 7 or an aluminum adhesive tape 12 is attached to a rear face of the resin stencil as shown in FIG. 8 to prevent the solder paste from penetrating the through hole and adhering to the circuit board.
In other words, one additional step of filling the through hole with the resin 11 is necessary according to the prior art method, which is troublesome and results in higher costs.
Meanwhile, although attaching the aluminum adhesive tape 12 from the rear side is inexpensive, a gap due to the thickness of the aluminum adhesive tape 12 is formed between the resin stencil 10 and the circuit board. This adversely influences the printing accuracy and is consequently not recommended.
USPTO Backgrounds
USPTO Backgrounds