A Study of Process-Induced Deformations of Anisotropic Conductive Film (ACF) Assembly

The warpage of glass substrate in chip-on-glass (COG) assemblies were believed to be the main cause of the so-called "Mura" phenomenon in thin-film transistor liquid crystal display (TFT-LCD) panels. The main objective of the study is to characterize the process-induced deformations of an anisotropic conductive film (ACF) type of COG assembly during the ACF bonding process. For effectively modeling the process-dependent thermal-mechanical behaviors of the ACF assembly, a process-dependent simulation methodology that integrates both thermal and thermal-mechanical finite element (FE) analyses and a "death-birth" meshing scheme is proposed. In the investigation, two various types of mircobump bonding technologies, i.e., the Au alloy and composite bumps, are explored. To substantiate the validity of the proposed methodology, the modeled results are extensively compared against experimental data. Moreover, the alternative goal of the study is to provide a design guideline through the exploration of the individual or combined effect of some essential parameters on the process-induced deformations. Both the modeled and experimental results reveal that in addition to the coefficient of thermal expansion (CTE) mismatch between the die and substrate, the process-induced temperature gradient appears to be the key factor causing the deformation of the assembly, and the deformed shape of the substrate is totally opposed to that obtained from isothermal modeling. Evidence also suggests that a preheated substrate could effectively ease the maximum process-induced warpage of the substrate, while the ACF fillet would unfortunately enlarge it. Besides, it is not surprising to find that an increase of the die length would enhance the maximum process-induced warpage of the substrate, but what surprises many is that an increase of the die width would diminish it