Since the electrical conduction in the COG assembly using a non-conductive adhesive takes place through the connection of the bump and the electrodes, the contact resistance can be applied to the evaluation of the design quality as well as the overall reliability of the particular assembly. It should be further noted that as reported in the literature (e.g., see Liu, 1996; Kristiansen et al, 1998; Nicewarner, 1999; Timsit, 1999), the contact resistance between the bump and the electrode on the substrate strongly depends on the contact stress and the contact area. A higher reliability of the packaging somewhat relies on better contact stability as well as larger bonding stresses.
In order to explore the physical contact behaviors of a non-conductive adhesive type of COG assemblies, the contact pressure during manufacturing process sequences and during the temperature variation are extensively investigated using a three-dimensional nonlinear finite element model. The so-called death-birth simulation technique is applied to model the manufacturing process sequences. The typical COG assemblies associated with two types of micro-bumps that are made of different materials: metal and composite are considered as the test vehicle. The contact stress between the electrode and the bump is extensively compared at each manufacturing sequence as well as at elevated temperature in order to investigate the corresponding mechanical interaction. Furthermore, the adhesion stresses of the adhesive are also evaluated to further investigate the possibilities of cracking or delamination within the adhesive and in its interfaces with the die and with the substrate. At last, a parametric finite element model is performed over number of geometry/material design parameters to investigate their impact on the contact/adhesion stresses so as to attain a better reliability design.