Ultra-high Vacuum Bonding at Room Temperature Aided by Ultrasonic Wave

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The ultrasonic bonding is a method to join materials by imposing ultrasonic waves on the bonding interface. It has widely been used as a wire-bonding tool in the surface mounting technology of printed circuits. A new attempt to use this method in bonding of metals to ceramics has been made and it has become clear that in the air, Aluminium can be joined to various ceramics like Si_3N_4,Al_2O_3,ZrO_2 and so on.But as for the bonding mechanism, a lot of points remains unknown. In order to clarify the mechanism, a bonding apparatus that can be evacuated up to an ultra-high vacuum and can control the bonding atmosphere was constructed. A metal sheet and a piece of ceramic are contacted together between a tip of horn and an anvil by a press device and bonded in the vacuum chamber. It was shown that Al can be joined to Si_3N_4 in the vacuum as well as in the air. Cu, on the other hand, could not be bonded to Si_3N_4 in the vacuum although the bonding was possible in the air. These facts are showing that nature of the bonding is affected by the atmosphere.Another important result is that there is the region for the time and pressure that makes the bonding possible. For example, Al can be joined to Si_3N_4 with the time longer than 0.5s and with the pressure more than 30MPa, which shows the existence of thermal activation process in the ultrasonic bonding of metal to ceramic.