Geometrical instruments – Gauge – Collocating
Patent
1993-10-28
1995-10-24
Will, Thomas B.
Geometrical instruments
Gauge
Collocating
33614, 101481, 101DIG36, B41B 1100
Patent
active
054599415
DESCRIPTION:
BRIEF SUMMARY
This invention relates to apparatus for and methods of accurate alignment of a screen and a subject substrate using a vision system especially for use in screen printing in the manufacture of printed circuits.
Early methods of screen printing, some of which are still being used, entailed the manual alignment of the screen over the subject substrate such that the registration markers, often called fiducials, on the screen and the subject substrate coincided, when viewed through the screen. As there was a high degree of subjective choice involved in this method, it was essentially a trial-and-error system and usually required several attempts to achieve the desired result.
The repeatability obtained from this method was dependant on the similarity of the substrates, the accuracy of placing each subject substrate on the print table and the capability of the printing machine to maintain the screen in the original set position.
The use of screen printing techniques for multi-colour graphics and for the deposition of etch resist and solder resist media in the printed circuit industry led to ever increasing demands for better registration techniques.
In response to these demands for closer tolerance registration, systems were introduced, comprising one or more CCD cameras having a suitable lens arrangement, which fed into a system, typically microprocessor based, which provided magnified images of the registration marks on a cathode-ray tube display device. Such devices, typically, further comprised electronically generated graticules, which allowed fine adjustment of the screen and substrate by means of micrometer adjustments. These adjustments were usually made manually but some semi-automatic or automatic systems have been tried. Patent Specification EP 204901 discloses a semi-automatic screen printing machine using stepping motors.
In these prior art arrangements the cameras are mounted directly or indirectly, via additional sub-frames, to the main frame of the machine. This gives rise to several sources of positional error, namely the mounting of the screen to the main machine frame; the repeatability of the print table "in" position, the repeatability of the print table "out" position; the repeatability of the substrate's position on the print table and the rigidity of the camera mounts.
In order to achieve the improved absolute registration required, the line of development in this field has concentrated on improving the mechanical integrity of the machine. This is especially aimed at reducing displacement errors in the moving parts. On pursuing this line of development, it has been found that if the three most significant parameters are controlled to a tolerance of 0.0025 cm which in the case of the print table movement, typically represents an accuracy of 0.003%, then the overall maximum accuracy of the registration of such a system will be 0.0075 cm. This is not sufficient for the degree of accuracy preferable for some applications.
Accordingly, in one aspect the present invention may provide apparatus for use in printing, comprising a print table mounted on a base frame of the apparatus, support means adapted to rapidly mount vision means on the table, disposed such that the vision means views a major face of the print table, means for securing a substrate to the print table, printing means, and means for effecting relative movement between the print table and printing means whereby to move the printing means and a substrate mounted on the print table into register, the vision means being positioned to view fiducial marks carried by the substrate and to view fiducial marks carried by the printing means whereby to facilitate the adjustment of the relative positions of the printing means and the substrate so that the fiducial marks on the printing means and the substrate are in register, thereby effecting registration of printing means and substrate. Preferably the printing means comprises means for mounting a screen for use in a screen printing process, and fiducial marks are carried by the screen.
Said
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patent: 5168805 (1992-12-01), Kasanami et al.
Miller Austin R.
Will Thomas B.
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