Capillary-action base anchors surface-mount terminals

Mount Kisco, NY--It started with a single pin, a terminal that could be randomly placed on a PC board for manual circuit testing. Those pins had always been through-hole devices for stability. But with the rapid adoption of surface-mount technology for every other board component, keeping separate through-hole assembly equipment just for pins became an economic encumbrance. Could someone, finally, create a surface-mount pin?

The problem isn't trivial, says Janos Legrady, manager of research and development at Zierick Manufacturing Corp. A single pin, without the multiple legs of other surface-mount devices, would float out of place during reflux soldering, the standard PC board assembly technique. Creating a geographically stable surface-mount pin involves the statics, dynamics, and fluid mechanics that, on a small scale, resemble getting a telephone pole to float upright.

Fortunately, the test-pin project gave Legrady and his team some leeway. Because connecting leads to test pins is done manually, some floating doesn't matter as long as the pin remains close enough to be electrically connected to the board's lead pad. Nevertheless, says Legrady, "We used the project as an opportunity to look further at a self-centering pin." That is, one that could be put in place with automated equipment and hold its position, so that downstream automation could find the pin for subsequent as-sembly procedures.

The result is a 0.025-inch square pin, 0.250-inch long with a 0.090-inch square base. It's stamped from 0.012-inch-thick full-hard brass and folded to create the pin shape. The microscopic channel between the folded pin halves is the secret to its performance. In tests, as long as the solder-paste pad prepared for the pin remains close in area to the area of the pin base (not more than 0.150-inch square), the pin displacement after reflow did not exceed 0.001-inch in any direction.

Placed in a reflow oven, the paste turns into liquid solder and flows by capillary action up the pin's channel. "Instead of floating, the pin sinks to within 0.001 inch of the board surface," explains Legrady. Low pressure beneath the pin base and residual, viscous solder around its periphery help hold the pin in place until the solder solidifies.

"It's a new kind of solder joint," says Legrady. In addition to its precision, the joint delivers mechanical advantages as well. It has nearly ten times the strength of conventional soldered connections in tension and in shear. As a result, the joint may eliminate the need for mechanical connectors in high-stress applications.

Apart from the pin itself, the team had to ensure the ability of the new design to fit into the high-volume, high-speed world of automated electronics assembly. Here, Zierick's 75-year history in the metal-stamping business held it in good stead. The team developed stamping dies and a progressive-forming process that produces the pins in a continuous chain.

In turn, the chain can be loaded into a reel-type magazine for the pin-feeding mechanism that forms the final part of what the company calls the Surf-Shooter(TM) system. Designed for easy incorporation into any of the major surface-mount-device automated assembly machines, the pin-feeder mechanism automatically shears off pins from the chain one at a time.

Additional details...Contact Janos Legrady, Zierick Manufacturing Corp., Radio Circle, Mount Kisko, NY 10549, (914) 666-2911.