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This paper presents the results from the evaluation of different types of flexible substrates for high‐density flip chip application. In this work two different types of base materials were used, epoxiglass (EG) and polyimide (PI). According to previous tests the type of conductive particles in the adhesive seems to be one of the key factors in high‐density interconnections. The adhesive selected for these tests was a composite of epoxy matrix and high content of isolated soft metal‐coated polymer particles. Two different test structures with contact areas of 50 × 50μm and 50 × 90μm were compared. The total amount of contacts in one IC was approximately 200 and the effective pitch size was 80μm. The contact resistances were measured by four‐point method and the continuity by daisy chain structure. The reliability of the flip chip interconnections was tested in thermal cycling and humidity tests.

The increasing demand for fine pitch interconnections has led to a growth of interest in anisotropically conductive adhesives (ACAs) as an alternative to solder joints in high density applications. The understanding of the conduction mechanisms for ACAs is of vital importance when choosing the right adhesive for a specific application. In the conductivity model, a formula has been created that can be used to estimate how the degree of deformation of the particles effects the resistance, especially in the case of soft metal‐coated polymer particles. Using this model, it is possible to estimate the total contact resistance. Some comparisons are made with real measurements for gold and indium‐tin‐oxide (ITO) surfaces, using gold‐coated polymer particles and gold bumped chips. For gold surfaces, the measurements have shown reasonably good correlation with the model. In the case of the ITO surface, the interface resistances seem to be the major part of the total resistance.

To achieve good reliability and high performance flip chip interconnection, process parameters and materials used in the flip chip process must be optimised. In this paper reliability of Sn37Pb solder bumped flip chips on an FR5 board, using different fluxes and underfill materials, are tested in a temperature cycling test. Also the contact pad geometry used on the FR5 board had a great influence on the reliability of the joint. The solder bumps were grown on the well‐known TiW/Au under bump metallurgy (UBM1) with an extra layer of nickel (UBM2) which gives a reliable and high performance solder joint.