The viability of anisotropic conductive film (ACF) as a flip chip interconnection technology

The use of anisotropic conductive film (ACF) to form the interconnection between the die and the substrate is one potential variation of flip chip technology. Its appeal comes with the ability to make very fine pitch interconnects not feasible with solder, avoidance of lead (Pb), certain desirable electrical and mechanical properties, and simpler and lower temperature processing. The principal concerns with ACF are its long-term reliability and stability, and consistent electrical performance of the particulate interconnects. In this paper, results of rigorous process development and reliability testing using a set of test vehicles covering both CSP and PBGA package formats are discussed and analyzed. Two different low-cost bump structures on the die side and a few different substrate materials and designs were investigated. Since the application of the ACF material and thermo-compression attachment of the chip are two unique steps in the package assembly process, extensive design of experiments (DOE) was performed. Developmental reliability testing was done to gain insight into the failure mechanisms of these type of interconnects. Systematic refinements in the assembly process, the material choice and design rules for the die and substrates were made based on the understanding developed from these investigations.