Summary:
As a connection material, copper has some relevant advantages over aluminum, proven in power electronics applications. Because of the dissimilar material properties, ultrasonic welding parameters of copper differ significantly from those of aluminum. Most significantly, ultrasonic power and normal force are higher. In battery pack applications, normal force and/or vibration amplitude are limited by the sensitivity of the battery isolator ring and the cell surfaces to be connected are typically nickel-coated steel. Regarding copper wire bonding, it was not possible to create reliable connections under these circumstances, let alone reach the required process times for industrial battery pack assembly. Similar issues are typical in power electronics applications, where too high force and/or vibration amplitude can damage the die.
An effective solution to this joining problem is to utilize heat during the bonding process. Heating of the whole battery pack is impractical, as Li-ion battery cells can only withstand a limited temperature of about 60 °C. Instead, the new technique of laser-assisted thermosonic wire bonding is investigated. This technique uses a near infrared laser source integrated into the bonding setup. The laser radiation is used to heat the bond tool tip during or immediately before the bonding process. Wire and connection area are heated by heat conductance from the hot tool tip. The temperature is controlled by pyrometry to keep the bonding quality constant. This approach provides heat locally and only during the process without affecting surrounding materials. On test substrates, this technique enables bonds equivalent in strength to standard (room temperature) ultrasonic wire bonds, but with reduced force and/or bonding time, demonstrating its potential for copper wire connections on dies and battery cells.
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