Eyelets are hollow rivets used in the production of printed-circuit boards (PCBs) that help to form a mechanical solder joint that increases circuit reliability, especially where high currents are involved. They also offer significant economies over through-hole-plated techniques. Besides the increase in mechanical strength, eyelets also offer an overall saving in the solder required to make the same joint.
"Typically, PCB boards need about 100 eyelets inserted for through-hole soldering after electronic components are mounted, " says John Channing, founder and managing director of Channing Automation (Plymouth, England), a manufacturer of eyelet-insertion machines. "These boards have to be produced in volume, so the speed of insertion required is high," he says. Earlier versions of these machines could insert eyelets in about 0.75 s. In recent designs, the insertion process is less than 0.5 s.
Eyelets of 2 and 3 mm in diameter are in common use for PCBs, and most current machines insert them into boards to an accuracy within 0.3 mm. At these tolerances, physical insertion accuracy degrades and copper can be lifted off the PCB, which leads to poor soldering, low conductivity, and failed joints. Such boards must then be rejected.
"Our goal is a consistent accuracy within 0.1 mm," says Channing. Boards are fed into the insertion area, and an integrated F150-2 machine-vision system from Omron (Schaumberg, IL) inspects the eyelets. "For system setup," says Channing, "drop-down menus are superimposed over 512 x 484 x 8-bit images to control such functions as filtering and camera shutter speed and the center of gravity, area, and edge position of the eyelet."
In operation, the vision system searches for the first hole, which is used as a datum or reference for the remaining holes, whose position is derived from computer-aided-design data. Because the image is backlit, the captured image appears as a silhouette. Using the filtering functions of the F150-2, edges of the image are sharpened, and the center of the hole is calculated.
If the hole is displaced from the target position, the image processor generates the necessary correction signal for the Omron 402 motion controller mounted inside a programmable logic controller. With the center of the datum hole perfectly positioned, the remaining holes in the board should line up accurately. However, the vision system is used to check each hole before insertion of the eyelet.