The figure illustrates selected aspects of a manually actuated apparatus for use in coating two small objects with suitable metals, then forcing the two objects together at the coated flat surfaces with uniform pressure to cold-weld them to each other. The design of this apparatus provides for all steps of the coating and cold-welding processes, including intermediate steps of manipulation between these processes, to be performed in a vacuum, with no need to break vacuum between the processes as in older methods of coating and cold welding. By maintaining vacuum through all processing steps, one prevents the formation of surface oxides, which interfere with cold welding. Maintaining vacuum also prevents the formation of pockets of trapped gas, which render the bond nonuniform.

In the original application for which the apparatus was designed, the objects to be joined are components of an ultrasonic transducer; namely, a piezoelectric (e.g., lithium niobate) wafer as thin as 0.001 in. (25 µm) and a round sapphire rod with a flat surface (to mate with the wafer) at one end and a concave focusing surface at the other end. The faying surfaces of the rod and disk must be coated with thin layers of chromium, then gold, then indium, by sputtering or vapor deposition in a vacuum. Then the wafer is pressed onto the end of the rod with a pressure of about 300 kg/cm2 (about 0.1 MPa) to cold-weld the rod and disk together at the indium surface layers. Thinness and uniformity of the bond layers and uniformity of the cold-weld joint are necessary for proper acoustic performance.

The Wafer Can Be Positioned facing upward toward a metal-deposition source, the turntable can be turned to bring the rod and disk to different metal-deposition sources, and the metal-coated wafer can be pushed down onto the rod for cold welding, all without breaking vacuum.

The apparatus includes a collet that holds the sapphire rod and a rotating arm that contains a swivel disk, on which the wafer is held by a layer of wax. The arm enables the initial pickup of the prealigned wafer, the orientation of the wafer facing a metal-deposition source, and the repositioning of the wafer for subsequent bonding to the rod. During coating with metal, both the rod and the wafer are oriented with the bonding surfaces to be coated facing the source of the metal. The chromium, gold, and indium layers are deposited from three different sources. The apparatus includes a turntable so that the rod and wafer can be positioned below each of the three sources in sequence.

The collet is mounted in a spring-loaded holder, the springs of which are preadjusted to provide the appropriate bonding force. The springs are compressed and restrained by a cam-actuated lever. The cam is driven by a linear-and-rotary vacuum feedthrough that enables actuation without breaking vacuum. The feedthrough can be made to engage a cam socket when the angular position of the rotary table is such that the cam socket and the feedthrough are aligned with each other.

A similar feedthrough and cam-actuation mechanism is provided for the positioning arm. The arm is initially set in the coating position. After deposition of the three metal layers, the arm is rotated, by use of this mechanism, to push the wafer down onto the rod in the collet. The swivel disk provides limited freedom of tilt to allow the wafer to align itself with the tip of the rod when the wafer and rod are pushed together. A latch locks the arm into position for bonding. A restraining cam is rotated to unload the spring compression from a restraining lever, allowing the spring to press the wafer and rod together.

After the pressing, the vacuum system can be opened. Spring clips that hold the swivel disk in place and the collet that holds the rod in place are loosened, making it possible to remove the bonded parts with the swivel disk still attached to the wafer by wax. The swivel disk is released by gently heating the parts to melt the wax.

This work was done by Richard Oeftering and Floyd Smith of Lewis Research Center. For further information, access the Technical Support Package (TSP), free on-line at www.techbriefs.com under the Manufacturing/Fabrication category, or circle no. 106 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
Mail Stop 7-3
21000 Brookpark Rd.
Cleveland
OH 44135

Refer to LEW-15922.


NASA Tech Briefs Magazine

This article first appeared in the June, 1998 issue of NASA Tech Briefs Magazine.

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