Ultrasonic Energy Welds Copper to Aluminum Ultrasonic welding succeeds in joining dissimilar metals where traditional methods might fail.
Attempting to weld copper to aluminum by conventional means through the application of thermal energy to melt and fuse the two metals (fusion welding) can result in an unreliable weld. The oxide layer on aluminum is difficult to remove, the melt temperatures of the two metals are not close, the two metals exhibit high thermal conductivity and alloying of the two metals creates a brittle intermetallic that is mechanically and electrically unreliable.
Modern applications involving semiconductor heat sinks and transition joints require the reliable joining of these metals. The common technique to accomplish this metallic bond has been to plate aluminum with another metal that facilitates soldering. This practice involves many steps. And it can be costly and environmentally unfriendly.
An alternative for design engineers is to weld copper to aluminum by applying ultrasonic energy, which joins the metals without melting. The ultrasonic process creates a high quality weld both mechanically and electrically without forming a brittle intermetallic and without intermediate steps.
A number of interesting copper-to-aluminum welding applications have been very successfully put into production over the past few years. A brief description of a few are given below.
Copper pads welded to aluminum heat sinks. This application involves the welding of copper pads to the aluminum heat sink. The pads are necessary for the eventual soldering of rectifying diodes to the heat sink. The original process required that the heat sink be plated with an alloy that would reliably accept the soldering process. This multistep, multifacility process was reduced to a single operation.
Copper pads welded to aluminum iginition module base plates. In this application an ignition module uses an aluminum back plate for most of the circuit mounting and for heat dissipation. A copper pad was required to locate and solder mount a power circuit. The design called for excellent thermal conductivity. Ultrasonic welding was employed in an automated system, which produces 3000 parts per hour.
Copper-to-aluminum transition joints in automotive starter motor field coils. A transition to a copper conductor was necessary for a conventional electrical connection within the motor. The copper conductor eliminates problems associated with terminating aluminum due to the tough, nonconducting oxide layer that forms on aluminum and the tendency for aluminum terminations to fail as a result of thermal cycling.
Copper-to-aluminum transition joints in distribution transformers. This is a similar application to the motor field coil This application is found in some distribution transformers that have been wound with aluminum wire. A transition weld is reqiured to attach a copper conductor that will allow conventional terminating techniques to be employed.
Ultrasonic welding of copper to aluminum has been shown to be efficient and effective as demonstrated by a number of practical production applications. The problems of tough oxides, high thermal conductivity, high electrical conductivity, intermetallics and brittle alloys are not significant with the ultrasonic welding process. And similarly, the problems associated with pre and postweld cleaning, fluxes, hot metal and high energy costs are eliminated. Modern ultrasonic welding equipment is capable of monitoring energy and controlling the critical welding process variables.
Excerpted from "Ultrasonic Energy Welds Copper to Aluminum", Welding Journal, January 1997
A number of interesting copper-to-aluminum welding applications have been very successfully put into production over the past few years. A brief description of a few are given below. 
Copper-to-aluminum transition joints in automotive starter motor field coils. A transition to a copper conductor was necessary for a conventional electrical connection within the motor. The copper conductor eliminates problems associated with terminating aluminum due to the tough, nonconducting oxide layer that forms on aluminum and the tendency for aluminum terminations to fail as a result of thermal cycling. 
