Process-dependent contact characteristics of NCA assemblies

The physical contact characteristics of a nonconductive adhesive (NCA) type of flip-chip-on-glass (FCOG) assemblies during manufacturing process and temperature variation is explored by using three-dimensional (3D), nonlinear finite element analysis together with the so-called "death-birth" simulation technique. The contact mechanics of two typical types of micro-bump bonding technologies, i.e., the metal (i.e. Au alloy) and composite bumps, are extensively addressed, and substantially compared. The validity of the modeled contact characteristics is further verified by an electrical contact resistance measurement that adopts a four-point probe method and an equivalent circuit approach. Finally, through the parametric study, the dependence of the contact stress at the bumps and the peeling stress at the UV resin on a number of geometry and material design parameters is effectively identified. Both the modeling and experimental results show that the bump height uniformity is a key factor in the overall contact performance of the assembly, and should not be neglected from the analysis. In addition, it is identified that the composite-bump bonding technology outperforms the metal-bump assembly as a whole in terms of the contact consistency and stability due to its better bump uniformity and compliance. Furthermore, it is surprising to find that there is a full disagreement in the parametric results of the bump height and Al thickness between the shorter bump and the taller among those nonuniform bumps, and more importantly, an increase of the bump height or a reduction of the Al overcoat thickness would enhance the overall contact performance of the assembly.