Flip chip, also known as
controlled collapse chip connection or its acronym,
C4, is a method for interconnecting
semiconductor devices, such as
IC chips and
microelectromechanical systems (MEMS), to external circuitry with
solder bumps that have been deposited onto the chip pads. The solder bumps are deposited on the chip pads on the top side of the wafer during the final
wafer processing step. In order to mount the chip to external circuitry (e.g., a
circuit board or another chip or wafer), it is flipped over so that its top side faces down, and aligned so that its pads align with matching pads on the external circuit, and then the solder is reflowed to complete the interconnect. This is in contrast to
wire bonding, in which the chip is mounted upright and wires are used to interconnect the chip pads to external circuitry.