Warpages of ACF-bonded COG packages induced from manufacturing and thermal cycling

As the application of the COG (Chip on Glass) with ACF (Anisotropic Conductive Film) to the LCDs (Liquid Crystal Displayers), 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 the present study 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 fillets, and thermal cycling (from room temperature to 85 °C), 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 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 beyond 50°C during the first thermal cycling. After the first cycling, the warpages of the COG packages become much less severe and stably and slightly decrease with the temperature increasing. 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 °C /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 CTE and moisture absorption of the ACF have not obvious influences on the warpages of the COG packages, but for relatively thick fillets they do somewhat. Overall, for the improvement of reducing the warpage of the COG package, two approaches are suggested: giving the packages at 85°C preheated after manufacturing and the precisely-controlled low mismatch of CTE between chip and glass substrate, since they play a significant role.