Assembly in Action: Motion Package Cuts Cost, Minimizes Footprint
The low-profile leadscrew and rail minimized the footprint of the automated bonder. To further reduce the cost and improve the reliability of the machine, the company adopted a fully assembled motion control module in place of individual components.
The bonder's four-axis rotary head attaches aluminum wire leads to electronic devices for interconnects. When semiconductor manufacturers expressed interest in a high-volume production machine, engineers looked for a space-saving mechanism to feed leadframe magazines. "Floor space in electronics assembly areas is expensive," explains Bob Babayan, vice president of engineering. "There's a lot of emphasis on achieving a small footprint for assembly machines."
Semiconductor chips from 1 to 10 millimeters square are commonly bonded on leadframes packed in standard magazines. The automated feeder concept called for magazines to be loaded at the load-station of the machine. Leadframes are fed out of the magazine to the bond area, and wire-bonded leadframes are fed into empty magazines on the output side.
To minimize the impact on the machine design, company engineers looked for a simple linear motion system that could be integrated with the existing wire bonder. They also wanted an affordable package that could minimize the overall footprint of the machine. According to Babayan, "We really didn't know how small it had to be. We just knew the smaller, the better."
Engineers developed a linear guide feed mechanism with LOSTPED criteria--load, orientation, speed, travel, precision, environment and duty cycle. They calculated a 7-pound design load would accommodate a magazine full of TO-220 devices. Vertical and horizontal actuators could be used in tandem to elevate full magazines to working height and push them through the work area and out the front. Required speed for the elevator is 2 inches per second. Travel is 232 millimeters in the vertical plane and 305 millimeters for the horizontal slide.
With leadframes positioned for welding by the four-axis bonding head, the magazine feeder did not need exceptional positioning accuracy. However, accurate alignment was critical to ensure smooth feeds without jamming. A 1/2-inch stainless steel leadscrew with 0.2-inch lead per revolution provided the driving force for the recirculating ball slide. The Acme leadscrew has ±0.0001-inch deviation per inch of travel and less than 0.005-inch backlash. It is turned by a stepping motor at about 10 revolutions per second to drive the carrier block at 2 ips.
The electronics assembly environment is clean, so factory-sealed actuators would not need bellows or special auxiliary seals. With relatively short strokes and electronics assembly operations running 24 hours a day, the feeder mechanism also needed the reliability to withstand a demanding duty cycle.
Traditional linear bearings require a large, costly base plate to anchor the rail. Using the end plates as mounting points eliminated the base plate and cut the system cost by 25 percent. Overall, the package required about 20 percent less overhead space than recirculating ball-and-rail actuators. It measured just 1.25 inches high by 3 inches wide.
A dedicated motor mount was eliminated in favor of motor mounting holes. The result was a linear motion package with modest impact on the footprint of the lead bonding machine. The Model 360C automatic wire bonder without a leadframe feeder has a footprint 24 inches wide by 43 inches deep. The Model 360CH with a single bonder is 50 inches wide by 47 inches deep. The Model 360CHD with dual feeders accommodating different size wires on the same component, or increasing throughput by alternating feeders, is 76 inches wide.
With the semiconductor wire bonder in production, the company looked at ways to simplify assembly, cut the cost of the feeder and enhance reliability. The ready-to-install module also guarantees alignment of the miniature recirculating ball slide and Acme leadscrew drive.
A fully assembled module reduced the number of components to be installed, saving alignment and installation time. The company saved $500 per unit in assembly costs. The pre-aligned miniature ball rail and leadscrew is now bolted into the machine.
For more information on motion control, call Rexroth Star at 864- 967-2777, visit www.rexroth.com.