Wire bondingThere are two conventional ways of bonding wires to chips. These are thermocompression bonding, and ultrasonic bonding. Commonly fine (25um diameter) aluminium wire is used, but gold wire as also quite often used.In thermocompression bonding, the die and the wire are heated to a high temperature (around 250 Celsius). The tip of the wire is heated to form a ball, the tool holding it then forces it into contact with the bonding pad on the chip. The wire adheres to the pad due to the combination of heat and pressure. The tool is then lifted up and moved in an arc to the appropriate position on the frame, dispensing wire are required. The process is repeated to bond the wire to the frame, but this time a ball is not formed. Ultrasonic bonding is used when the device cannot or should not be heated. In this case, the wire and bonding surface (pad or frame) are forced together the tool, and ultrasonic vibration is used to compress the surfaces together to achieve the desired bond. These two techniques can be combined to achieve a lower temperature form of thermocompression bonding. Flip Chip bonding is another emerging technique, again associated with microengineering. Here small beads of solder are formed on bonding pads on the die. The die is then mounted face- down on the base, and heated until the solder melts and forms a contact between metal tracks on the base and the bonding pads. [Advanced Packaging and Interconnection Technologies from MCNC.] |