Flip chip assembly on organic boards using anisotropic conductive adhesives(ACAs) and nickel/gold bumps

Flip chip assembly directly on organic boards offers miniaturization of pac kage size as well as reduction in interconnection distances resulting in a high performance and cost-competitive packaging method. This paper describe s the investigation of alternative low cost flip-chip mounting processes us ing electroless Ni/Au bump and anisotropic conductive adhesives/films as an interconnection material on organic boards such as FR-4. As bumps for flip chip, electroless Ni/Au plating was performed and charact erized in plating speed, surface roughness, and elemental analysis as a fun ction of plating condition. High plating rate and surface planarity of the electroless Ni were considered as requirements for ACA flip chip bumps. In order to obtain high plating rate and low surface roughness, plating condit ions was determined by controlling complexing agents in electroless Ni solu tion. Effect of annealing on Ni bump characteristics informed that the form ation of crystalline nickel with Ni3P precipitation above 300 degreesC caus es an increase of hardness and an increase of the intrinsic stress resultin g in a reliability limitation. As an interconnection material, modified ACFs posed of nickel conductive fi llers for electrical conductor and non-conductive inorganic fillers for mod ification of film properties such as coefficient of thermal expansion (CTE) and tensile strength were formulated for improved electrical and mechanica l properties of ACF interconnection. The thermal cycle life of ACAs flip chip on organic boards was usually limi ted by the CTE mismatch between the chip and the board. However, flip chip assembly on FR-4 boards using modified ACAsalmost doubled the thermal cycle life.