Machine Design

Bellows let chip testers run hot and cold

The S-170, made by Aseco Corp. in Marlboro, Mass., accepts chips loaded on two, four-track aluminum magazines, then singulates them into a large thermal soak chamber. Corrosion-resistant nickel bellows by Servometer, Cedar Grove, N.J., handle extreme temperatures in the chamber and are arrayed along the chip track. A spring in the center post of each bellows actuator retracts when air pressure is removed.

Developers of an IC tester realized they had a problem when they looked for actuators able to function over the equipment's –76 to 302°F test range.

Pneumatic actuators in the S-170 automated chip test handler extend and retract pins and probes. The pins knock flashing off plastic chip packages, function as test probes, and position chips for testing and sorting. Trouble was, temperature extremes in the thermal soak chamber raised havoc with many otherwise viable actuator candidates. Pistons, for example, would have seized up. "We'd have had to mount them outside the chamber on a shaft and that would have complicated things," explains Aseco Corp. manufacturing engineer Dan Pareene. Corrosion was another concern. Rapid cooling in the test chamber creates condensation and would have increased the chance of moisture infiltration.

Corrosion-resistant nickel bellows turned out to be the solution. Manufactured through an electrodeposition process by Servometer, Cedar Grove, N.J., the seamless, nonporous bellows will not collect moisture. Electrodeposition also maintains high chemical purity and preserves nickel's high yield strength (110 kpsi min) and high tensile strength (125 kpsi min). Extreme temperatures, moreover, were no problem strengthwise. Nickel retains toughness and strength at up to 350°F and gets stronger at cryogenic temperatures far colder than in the test chamber.

Servometer assembles customized bellows incorporating adjustable return springs and end caps that Aseco supplies. Depending on end-user specs, each S-170 has up to seven bellows actuators arrayed along the chip track. The spring in the center post of each bellows actuator retracts when air pressure is removed. Bellows in the device range from 38 to 0.75 in. diameter. Stroke lengths range from 0.2 in. for the stopping pins to just 0.07 in. for the test probes.

Electrodeposited bellows are manufactured by forming a mandrel to the shape of the inside of the bellows, depositing the proper thickness of spring quality metal on to the mandrel, trimming the plated mandrel to define the ends, and finally dissolving the mandrel. This leaves behind a thin convoluted tubular shell of plated metal. The electroplating process allows the fabrication of an extremely thin wall (down to 0.0003 in.). This makes the devices useful for applications requiring a high degree of sensitivity and where large deflections are desirable with only minute forces. The bellows are designed for up to 100 million-cycle life expectancy.

TAGS: Technologies
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