Printing – Processes – Position or alignment
Patent
1996-05-16
1998-05-19
Yan, Ren
Printing
Processes
Position or alignment
101126, 1011271, 101DIG36, 348 95, 348132, B41L 302
Patent
active
057524467
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to alignment systems for use in aligning two objects in accordance with reference marks on respective surfaces of the objects. Such systems are employed, for example, in screen printers for printing solder paste onto printed circuit boards for surface mount assembly.
In screen printers for printing solder paste, a screen is positioned over a circuit board to be printed and the solder paste is applied to the board through apertures in the screen by means of a squeegee. To ensure that the solder paste is printed at the correct locations on the board for subsequent component placement, it is necessary for the screen to be aligned with the board prior to printing. This alignment needs to be performed to a greater accuracy than can be achieved by purely mechanical methods. Accordingly, the board and screen are provided with a number of corresponding reference marks, or "fiducials", such that alignment of corresponding fiducials on the board and screen indicates that the board and screen are correctly aligned, and alignment apparatus is employed to determine the relative positions of the board and screen fiducials. The position of the screen or board is then adjusted as necessary to eliminate any misalignment prior to printing.
The general arrangement of one type of alignment apparatus in a screen printer is illustrated in FIG. 1 of the accompanying drawings. In the system illustrated, a printed circuit board 1 to be printed with solder paste is supported by a base (not shown) of the printer in a generally horizontal plane parallel to the x and y axes shown in the figure. A screen 2 is supported by support means including a support frame 2a so as to lie above, and substantially parallel to, the board 1. The position of the screen 2 in the xy plane can be adjusted by means of screen positioning motors 3 the operation of which is controlled by a position controller 4. The support frame 2a and screen positioning motors 3 are mounted in a printhead portion (not shown) of the printer which can be pivoted away from the base to allow access when necessary, for example, to adjust the board support. The apparatus includes an imaging device 5 which is mounted on an XY table (not shown) for movement in a horizontal plane between the board 1 and screen 2. The position of the imaging device 5 is also controlled by the position controller 4.
The surface of the screen 2 facing the board 1 has at least two reference marks, or screen fiducials, 6a and 6b thereon. The surface of the board 1 facing the screen 2 has corresponding board fiducials 7a and 7b thereon. The positions of the board and screen fiducials are such that when each pair of corresponding fiducials 6a, 7a and 6b, 7b are in alignment, the screen is correctly aligned relative to the board for the subsequent printing operation to be performed after simply raising the board into contact with the screen. The mechanism for raising the board into contact with the screen is provided in the base of the printer.
The imaging device 5 is operated so as to acquire images of the fiducials 6, 7 and the acquired images are supplied to a vision processor 8 which is programmed to determine the locations of the fiducials in the images. The fiducial location data is then supplied to the position controller 4 for adjusting the position of the screen 2 to bring the corresponding fiducials 6a, 7a and 6b, 7b, and hence the board and screen, into alignment. A video monitor 9a allows an operator to monitor images supplied to the vision processor 8. An operator interface 9b is connected to the position controller 4 to allow operator control of the apparatus for preliminary programming and calibration purposes.
A known imaging device for use as the imaging device 5 in a system of the type described above is shown schematically in FIG. 2 of the accompanying drawings. The known imaging device, indicated generally at 10, is shown in FIG. 2 mounted between the screen 2 and board 1 in the apparatus of FIG. 1 as viewed along the +y direction. The imag
REFERENCES:
patent: 5060063 (1991-10-01), Freeman
patent: 5176078 (1993-01-01), Homma et al.
DEK Printing Machines Limited
Yan Ren
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