Bumping and Flip Chips
One of the simplest electrical connections between a chip and the circuit board or a substrate can be made with small balls of electrically conductive material, called bumps. A bumped die can then be flipped upside down and aligned so that the bumps connect with matching pads on the board or substrate. Flip chip bonding has several advantages over traditional wire bonding, including small package size and greater device speed.
Bumping can be performed by extending conventional wafer fabrication methods. After the chips are made, underbump metallization (UBM) pads are created to connect to the chip circuitry, and bumps are then deposited on the pads. Solder is the most commonly used bumping material, although alternative materials – such as gold, copper, or cobalt can also be used depending on the application. For high-density interconnects or fine-pitch applications, copper pillars can be used. While solder bumps spread during the joining process, copper pillars retain their shape, which allows them to be placed much more closely together.