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Programmable pins support PCBs during screen printing
System automatically scans board and adjusts the pin pattern to resist deflection
By Brian J. Hogan, Managing Editor -- Design News, February 20, 1995
Franklin, MA--Electronics manufacturers create solder pads on PC boards using high-speed screen printers. In MPM Corp.'s new high-volume, Ultraprint-3000-Series precision printer, the downward force exerted on a board by the squeegee can range from 1 lb to 50 lbs.
To keep boards from bowing under that load, users typically use blocks fixed to the printer's bed beneath the board. Workers must place the supports manually, changing the block set for each new board.
Design engineer Pat McCormick, Product Manager Steven Hall, and their colleagues at MPM found a better way to support boards in the new 3000 Series. Their system employs a set of magnetic pins stored in the screen printer. A gantry, positioned by "H'-Block and Power Block rod-cylinder slides from Tol-O-Matic Inc., Hamel, MN, picks the pins out of their storage position and locates them on a steel plate beneath the PC board.
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When a board first enters the 3000-Series screen printer, it takes up a position on a queuing conveyor. A scanner reads a bar code on the board, and transmits that information to a 486-based motion-control board. Capable of simultaneously controlling seven of the 22 independent axes on the 3000 Series, the motion controller checks a lookup table, determines the pattern of support pins needed by the board, and instructs the gantry to deliver those pins to the printer's workbed. (A second on-board computer handles documentation and diagnostic data.)
The 3000 Series can store as many as six stencils, automatically selecting the one needed by the incoming board. After the support pins and stencil are in place, the board passes onto a process conveyor that locates it beneath the stencil. An optical sensor provides feedback that allows precise adjustment of board location. Paste is placed on the stencil by a dispenser positioned by an Axidyne stepper/ball screw assembly and a band cylinder (both from Tol-O-Matic). The printer's squeegee then delivers the paste to the fully supported board in a two-stroke sequence. A laser system then uses triangulation to measure the height and location of as many as 14 solder pads per second.
If 45-degree printing is required to ensure accuracy or make some downstream operation easier, a gearbox can position the printer's bed at 0 to 90 degrees relative to the squeegee's centerline. After the printing operation, the board moves to a pickup conveyor, and remains there until downstream equipment can receive it. Able to process as many as five boards per minute, the 3000 Series can also work with lot sizes as small as a single piece.
Additional details, 3000 Series...Contact Robin Alan Rhodes, MPM Corp., 10 Forge Park, Franklin, MA 02038, (508) 520-6999.
Additional details, Tol-O-Matic Automation products...Contact Douglas C. Moore, Tol-O-Matic Inc., 3800 County Road 116, Hamel, MN 55340, (800) 328-2174.
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