Printing – Stenciling – Stencil and work support
Reexamination Certificate
2000-02-07
2002-02-19
Grohusky, Leslie J. (Department: 2854)
Printing
Stenciling
Stencil and work support
C101S114000, C101S123000, C356S237100, C348S131000, C382S149000, C382S112000
Reexamination Certificate
active
06347583
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a screen printing machine and particularly to the art of preventing a defective printing caused by clogging of the screen.
2. Related Art Statement
A screen printing machine is used in a wide range of art. For example, a screen printing machine is used for applying an adhesive or a solder paste to a printed wiring board (“PWB”) before electric components are mounted on the PWB to produce one or more electric circuits on the PWB. This screen printing machine includes a screen which has a plurality of openings and on which a printing material such as an adhesive or a solder paste is placed, and a squeegee for forcing the printing material into the openings of the screen and thereby printing the material through the openings onto predetermined places on a PWB which is positioned below the screen. Thus, the printing material is printed on the PWB, at respective places thereon corresponding to the openings of the screen, and with respective shapes and volumes corresponding to respective shapes and capacities of the openings. However, if one or all of the openings of the screen is or are clogged (hereinafter, referred to simply as “the clogging of the screen”), only an insufficient amount of printing material or substantially no printing material is printed on the PWB.
In order to avoid the above-identified problem, a screen printing machine for printing a solder paste on a PWB has been used in such a way that an image taking device is employed to take an image of the solder paste printed on the PWB and image data representing the taken image is used to estimate about the clogging of the screen. If the clogging of the screen is estimated, or an early sign of the clogging is found, the screen is cleaned, or replaced with another screen. However, for some reasons, in particular, the reason that the color and lightness of surfaces of the PWB are near to those of the solder paste, it is not easy to take, with an image taking device, an image of the solder paste printed on the PWB. In addition, it is not easy to determine or measure the shape and/or size (three-dimensional, in particular) of the solder paste printed on the PWB. Moreover, there are some cases where the shape and/or size of the solder paste printed on the PWB may not accurately reflect the clogging of the screen. Thus, even though the screen is actually clogged, the clogging of the screen may be overlooked, which may result in producing one or more defective electric circuits on the PCB.
While the case where the solder paste is printed on the PWB has been described as an example, the above-identified problem may occur to not only the case where an adhesive is printed on a PWB but also the case where a printing material is printed on an object other than a PWB.
SUMMARY OF THE INVENTION.
The present invention provides a screen inspecting method, a screen inspecting apparatus, and a screen printing machine that have one or more of the technical features that are described below in respective paragraphs given parenthesized sequential numbers (
1
) to (
11
). Any technical feature that includes another technical feature shall do so by referring, at the beginning, to the parenthesized sequential number given to the latter technical feature. Thus, two or more of the following technical features may be combined, if appropriate. Each technical feature may be accompanied by a supplemental explanation, as needed. However, the following features and the appropriate combinations thereof are just examples to which the scope of the present invention is by no means limited.
(1) According to a first feature of the present invention, there is provided a method of inspecting a screen of a screen printing machine, comprising the steps of taking, with an image taking device, an image of at least one opening of the screen, and judging, based on image data representing the taken image, whether the opening of the screen is clogged. The image taking device may take an image or respective images of only one or more pre-selected openings of the screen. For example, the image taking device may be used to take an image of such an opening that is likely clogged or must be prevented from being clogged. Otherwise, the image taking device may take respective images of all the openings of the screen. In the latter case, the image taking device may take the respective images of all the openings, all at once, or alternatively may divide the openings into a plurality of groups and sequentially take respective images of the openings of each of the groups. In a special manner belonging to the alternative case, the image taking device is used to take sequentially an image of each of the openings. Respective images of openings of a single screen may be taken by a single image taking device or the cooperation of a plurality of image taking devices. In the case where the image taking device takes an image of an entire opening of the screen, it can be judged, based on image data representing the taken image, whether at least a portion of the opening of the screen is clogged with the printing material. In addition, the present screen inspecting method can be used, as needed, to identify which portion of an opening of the screen is clogged to which degree. Thus, in the case where the present method is carried out, as needed, during a printing operation in which a printing material is sequentially printed on each of a plurality of objects, the clogging of the screen or an early sign of the clogging can be detected, and an operator can take a necessary action, e.g., cleaning the screen or replacing the screen with another screens. Meanwhile, in the case where the present method is carried out each time printing on each object terminates, and if clogging is detected for an opening for which no clogging had been detected at the previous time, it can be estimated that a defective printing would have occurred to the previous object.
(2) According to a second feature of the present invention that includes the first feature (1), the step of taking the image comprises lighting the screen on one of both sides of the screen that is opposite to the other side on which the image taking device is provided, and taking a silhouette image of the opening. Since a silhouette image is not influenced by the color and/or lightness of the screen and/or the printing material, it can be accurately judged, based on image data representing the silhouette image, whether the opening of the screen is clogged. Since generally a solder paste is opaque, it is advantageous to take a silhouette image of the opening. In the last case, the screen may be a transparent one.
(3) According to a third feature of the present invention that includes the first or second feature (1) or (2), the step of judging comprises comparing the image data representing the taken image, with reference image data representing an image of the opening without clogging. It is easy to identify clogging of the opening, by comparing actual image data representing the taken image of the opening, with reference image data representing an image of the opening without clogging. In addition, which portion of the opening is clogged to which degree can be identified by comparing the shapes and size of the opening that are represented by the actual image data, with those represented by the reference image data. Moreover, irrespective of the shape of a portion of the opening through which a light passes, clogging of the opening can be identified by comparing the area of the opening represented by the actual image data, with that represented by the reference image data. In the last case, each of the actual image data and the reference image data may consist of only area data representing an area of the opening.
(4) According to a fourth feature of the present invention that includes any one of the first to third features (1) to (3), the step of taking the image comprises taking a first image of the opening before at least one printing of the screen printin
Adachi Jun
Isogai Takeyoshi
Mizuno Manabu
Shimizu Toshinori
Fuji Machine Mfg. Co. Ltd.
Grohusky Leslie J.
Oliff & Berridg,e PLC
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