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 package size and reduced in interconnection distances, resulting in a high performance and cost-competitive packaging method. This paper describes the investigation of alternative low cost flip-chip mounting processes using electroless Ni/Au bumps 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 characterized for plating speed, surface roughness, and elemental analysis as a function 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 conditions were determined by controlling complexing agents in electroless Ni solution. Annealing effects on Ni bump characteristics showed that the formation of crystalline Ni with Ni₃P precipitation above 300°C causes an increase in hardness and intrinsic stress, resulting in reliability limitation. As an interconnect material, modified ACFs composed of Ni conductive fillers for electrical conductor and nonconductive inorganic fillers for modification of film properties such as CTE and tensile strength were formulated for improved electrical and mechanical properties of ACF interconnection. The thermal cycle life of ACAs flip chip on organic boards was usually limited by the CTE mismatch between chip and board. However, flip chip assembly on FR-4 boards using modified ACAs almost doubled the thermal cycle life