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The purpose of this study is to investigate the reliability of flip chip joints made with anisotropic conductive adhesives (ACA) on flexible polyimide (PI) and liquid crystal polymer (LCP) substrates.

Six test series using two ACAs and an LCP substrate were made with varying bonding pressure. The ACAs had the same matrix and conductive particles. To lower the CTE of one of the adhesives silica had been added to it. The reliability of the test series was studied in a temperature cycling test. The purpose of these test series was to find the optimal bonding pressure for both the adhesives used. According to the results from these initial tests, further test series were made with both LCP and PI substrates. The reliability of these test samples was studied using a temperature cycling test and a constant humidity test. The adhesion strength of the joints was studied before testing.

Both substrates had excellent reliability during the temperature cycling test. However, the reliability of the PI substrates during the constant humidity test was markedly lower than that of the LCP substrates. Additionally, the adhesion strength of the adhesives on to PI substrates was clearly less.

The work shows how the substrate material used affects the reliability of flip chip joints. In addition, the work shows how the addition of silica to the ACA matrix affects the reliability of the joints.

The purpose of this paper is to investigate the effect of substrate material and thickness on the thermal cycling reliability of flip chip joints assembled with anisotropic conductive adhesives (ACA).

Four test lots are assembled using three different substrates. Two of the substrates are made of FR‐4. The thicknesses of these substrates are 600 and 100 μm. The third substrate is made of liquid crystal polymers (LCP) and is flexible. With the thicker FR‐4 substrate two test lots are assembled using both normal and two‐step bonding profiles to study how the bonding profile affects the deformation of the substrate. Four different bonding pressures are used to study the effect of pressure on reliability and the failure mechanism of the ACA joints. The reliability of the test samples is studied using a temperature cycling test.

The reliability of the test lot with the LCP substrate is considerably better than that of the test lots with the FR‐4 substrates. Additionally, the thinner FR‐4 substrate has better reliability than the thicker FR‐4 substrate. The failure mechanisms found varied among the test lots. The effect of the two‐step bonding process on the deformation of the substrate is found to be minor compared with the effect of the glass fibres.

The work shows that the thermal cycling reliability of ACA flip chip joints is markedly influenced by the thickness and material of the substrate. It is also seen that the substrate used influences the failure mechanisms formed during thermal cycling testing.