Evaluation of flip chip bonding using ACA on polyester substrates

The use of polyester as substrate material became more and more important recently for chip card and smart label applications. The material is economically and ecologically reasonable compared to PVC or ABS. As one of the main disadvantages is the low temperature stability, mainly isotropic conductive adhesives (ICA) have been used for interconnection of bare dies and single chip modules to the antennas. The use of anisotropic conductive adhesives (ACA) instead of ICA can potentially shorten the process time in manufacturing of chip card inlays or smart labels. This paper summarises our efforts to introduce the ACA technology to low temperature materials in several past and ongoing projects. Ni/Au bumped dies have been bonded to different coil materials like aluminium, copper and silver ink. As the characteristics of these antennas are completely different to each other, special treatments were necessary to ensure a stable interconnection. We characterised the flip chip assemblies to different metallisations in terms of adhesion, electrical interconnection, and manufacturability. Long term behaviour has been examined using high and low temperature storage, humidity exposure and temperature cycling. The study has shown that the selection of ACA has to be adjusted to the antenna material carefully. Thus mechanically and electrically stable interconnection could be established.