Experimental and Numerical Studies of Warpages of ACF-Bonded COG Packages Induced From Manufacturing and Thermal Cycling

Abstract

Regarding the application of the chip on glass (COG) with anisotropic conductive film (ACF) to the liquid crystal displays (LCDs), the problems with the warpage of COG packages, interfacial delamination, and increasing contact resistance of bumps, during or after thermal and moisture loading, are major reliability issues encountered in the industry. The goal in this paper is to investigate the effect of the parameters, such as bonding pressure and temperature during manufacturing, thermal and moisture expansion of the ACF, and its elastic modulus and fillets, associated with thermal cycling (from room temperature to 85 ), on the warpages of the ACF-bonded COG packages. The full-field Twyman-Green interferometry is used for measuring the warpages of the COG packages due to the fabrication with the various bonding pressure and temperature and during thermal cycling. Three-dimensional finite-element models (FEM) are used for calculating the warpage in terms of such parameters, and those results are compared with experimental observations in order to understand the mechanics. The results show that the tremendously saddle-warped shapes are found at the COG packages after manufacturing, and their maximum warpages increase with the bonding pressure but are insensitive to the bonding temperature. However, these large warpages have been obviously relaxed as the temperature goes beyond 50 during the first thermal cycling. After the first cycling, the warpages of the COG packages become much less severe and remain stably, then slightly increase with the temperature. The deformed behaviors of the COG package under the thermal loads have been resolved by comparing the FEM results with the experimental observations. The moisture-induced expansion strain of the saturated ACF in the condition of 29 /85%RH was determined to be 0.147% by means of the combination of experimental and theoretical analyses. It is also found that, for relatively thin fillets, the coefficient of thermal expansion (CTE) and moisture absorption of the ACF do not have any obvious influence on the warpages of the COG packages, but for relatively thick fillets they do somewhat. Moreover, the warpages of the COG package affected by ACF fillet thickness, elastic modulus, and CTE have been thoroughly studied by the finite-element analysis and the results have been interpreted in detail. Overall, for reducing the warpage of the COG package, two approaches are suggested: giving the packages at 85 preheated after manufacturing and precisely controlling low mismatch of CTE between chip and glass substrate, since both play significant roles.