Electroless Ni/Au bumps for flipchip-on-flex and TAB applications

Tape Automated Bonding (TAB) and Flip Chip Bonding (FCB) of bumped dice have been used for a long time already, but only in the most demanding microelectronics assembly operations. This paper reports a low cost electroless Ni/Au wafer bumping process, specially developed for general purpose, high volume Flipchip-on-Flex applications. The bump height, shear strength and other reliability data are given. An assembly method is suggested where such Ni/Au bumped dice are reflow soldered or gang bonded on to the flexible substrate. As the nickel bumps will not collapse during the solder reflow, they are specially suited for high density assemblies like in many recent single and multi chip modules (SCM, MCM). Essential to the low cost and high volume of the assembly process is the fast pick and place operation combined with the one-pass reflow soldering of all components. In TAB applications hard, non-compliant Ni/Au bumps set the bonding parameters which totally differ from those used for soft gold bumps. Various examples of Ni/Au bumps in TAB applications are discussed