Wire bonding is typically applied to make electrical connections between an integrated circuit die or chip and a carrier on which the die is mounted. Bonding wires are attached to bond pads on the chip and bonding leads on the carrier respectively by ultrasonic welding using an ultrasonic transducer which is integrated into a wire bonding apparatus. The ultrasonic transducer is an energy converting-device which converts electrical energy into ultrasonic vibrations and transmits the ultrasonic vibrations to a capillary at a tip end of the transducer to perform wire bonding.
FIG. 1 illustrates a side view of a conventional wire bonding apparatus 10 with a transducer 12 positioned over a bonding area for wire bonding. The transducer 12 has a long slender rod-shaped horn 13 and a capillary 16 located at a tip end of the horn 13. Wire is threaded through the capillary 16 and extends out of the capillary tip. The capillary 16 is raised or lowered with respect to a bonding area by moving the transducer 12 vertically.
An ultrasonic generator 17 is connected to an end of the horn 13 that is opposite to the tip end of the horn 13 where the capillary 16 is mounted. The ultrasonic generator 17, typically comprising a stack of piezoelectric elements, is housed in a transducer holder 14 mounted on a bond arm 18. The transducer holder 14 is supported by the bond arm 18, which is in turn connected to a sliding bar 22 at a pivot 20. Rotational motion of the bond arm 18 about the pivot 20 rotates the bond arm 18 up and down thereby raising or lowering the transducer 12 relative to the bonding area. The sliding bar 22 may further move and position the bond arm 18 and transducer 12 along the x and y axes.
A carrier 26, on which components such as semiconductor dice and bonding wires are attached, is held in position by a window clamp 24 on a wire bonding platform to perform wire bonding. The wire bonding platform includes a heater block 28 which provides heat to the carrier 26 to facilitate wire bonding conducted on it.
During wire bonding, the ultrasonic generator 17 produces ultrasonic vibrations which are transmitted along the horn 13. The long slender rod shape of the horn 13 is suitable for amplifying the ultrasonic vibrations transmitted to the capillary 16 while suppressing attenuation. In this way, ultrasonic vibrations may be transferred to the capillary 16 efficiently.
A pressing force is also applied to the capillary 16, and bonding is accomplished by applying the ultrasonic energy transmitted through the horn 13 onto the wire that is subsequently attached to bonding pads, typically on the semiconductor dice. Control of the positioning of the capillary 16 relative to the bonding area is essential to perform wire bonding accurately. This is particularly so when very fine wires and bond pitches are involved.
As can be seen in FIG. 1, the transducer 12 is positioned over the heating zone during wire bonding. Heat is transmitted to the immediately vicinity of the heater block 28, which means that the transducer 12 is exposed to heat from the heater block 28 by radiation. The heat may cause the transducer 12 and the transducer holder 14 to expand. This may therefore affect the relative positioning of the capillary 16 and a bonding position. As a result, wire bonding accuracy is affected.
A thermal shield may be used for reducing heat transmission from the heater block 28 to the transducer 12 and the transducer holder 14. FIG. 2 is a side view of the wire bonding apparatus 10 with a thermal shield 30 affixed to the sliding bar 22 of the wire bonding apparatus. The thermal shield 30 is operative to separate the transducer 12 from the heater block 28, thereby reducing radiation of heat from the heater block 28 directly to the transducer 12. However, the window clamp 24 needs to be lifted to unclamp the carrier 26 every time the carrier 26 is moved or when it is removed from the heater block 28. The presence of the thermal shield 30 limits the vertical distance A that is movable by the window clamp 24. It is desirable to increase the vertical distance that is movable by the window clamp 24 to ensure sufficient clearance of the window clamp from the carrier 26 and the components attached thereon when the carrier 26 needs to be moved.
FIG. 3 illustrates a front view of the wire bonding apparatus of FIG. 2 showing the thermal shield 30 between the transducer 12 and the heater block 28. While the thermal shield 30 may reduce the heat radiated from the heater block 28 to the transducer 12, it is noted that the transducer 12 is still exposed to ambient heat from the surrounding air above the thermal shield 30. The transducer 12 and transducer holder 14 would still expand such as to change the relative positioning of the capillary 16 and the carrier 26, resulting in decreased wire bonding accuracy.
FIG. 4 is a side view of the wire bonding apparatus 10 of FIG. 2 illustrating movement of the transducer 12 along the y and z axes. The thermal shield 30 is movable together with the sliding bar 22 along the x and y axes as it is attached to the sliding bar 22. However, when the transducer 12 moves along the z-axis by rotary movement of the bond arm 18 about the pivot 20, the thermal shield 30 cannot move correspondingly in the z axis. Thus, the transducer 12 is likely to receive more heat energy from the surrounding air as it moves further away from the thermal shield 30. For the aforesaid reasons, use of a conventional thermal shield in a wire bonding system does not effectively address the problem of thermal instability encountered by a transducer during wire bonding. It would be desirable to provide better thermal protection to the transducer 12 from the heater block 28.