Electroless Nickel-Gold Reliability UBM, Flipchip, and WLCSP (Part I of III)

摘要

The demand for solder bumping has increased dramatically in the last several years. A majority of all the Flip Chip (FC) and Wafer Level Chip Scale Packaging (WLCSP) devices use thin film sputtering or electroplating technologies to depositing the Under-Bump-Metallurgy (UBM). As the markets for bumping products continue to grow, cost pressures are forcing the industry to look at different alternatives to these thin film manufacturing technologies. A well known alternative is the electroless nickel/gold (E-Ni/Au) process to produce a thicker UBM structure. This technology does not require any high vacuum or photolithography equipment to form the metal stack on top of the bond pads. The proliferation of electroless nickel/gold for UBM applications has been limited in part due to a number of early programs which failed and left a negative perception within the industry towards electroless nickel. Many of these programs did not have adequate process controls in place and several were trying to plate on semiconductor devices which were incompatible with the various electroless processes being used at the time. This paper addresses several of the process variables that affect the quality and reliability of electroless nickel/gold plating on semiconductor devices. These include: aluminum thickness, composition of the aluminum bond pad (Cu alloy), electric potentials setup within the circuitry, chemistries and process controls, and interactions with post plating processes.