Hygro-thermal warpages of COG package with nonconductive paste adhesive

The use of nonconductive paste (NCP) or film adhesives to replace the anisotropic conductive film in chip on glass (COG) packages may be a possible solution for the low-cost and finer-bump-pitch application to the liquid crystal displays. However, when the NCP is applied, the reliability-related issues, such as excessive warpage, interfacial delamination, and increasing contact resistance of bumps, occur in the COG packages. The goal of this paper is to experimentally and numerically study the warpage of NCP-bonded COG packages with and without fillets after being cured in the chamber with 150°C for 2 h, under thermal and moisture loading. Twyman-Green interferometry is used for measuring the warpages (out-of-plane deformations) of the COG packages due to such loading conditions. A 3-D finite element method (FEM) is used for calculating the warpage and the obtained results are compared with experimental observations in order to provide an insight to it in terms of mechanics. The full-field out-of-plane deformations (warpages) of the COG packages with and without fillets have been measured under thermal cycling and moisture loading. The results indicate that, right after manufacturing, the very (apparently) concave-up-shape (smiling-shape) warpages are found at the COG packages with fillet, but slightly concave-down (crying) shapes are at those without fillet. And the warpages decrease with the heating temperature increasing. The warpages of the COG package, determined from the experiment and FEM, are quite consistent for the case either with fillet or without fillet. Prior to investigating the moisture effect on warpages of the COG packages, the moisture-induced strain of the NCP at the condition of 29°C/85%RH for 12 h has been obtained to be ε^m = 0.3% by testing a NCP/silicon bimaterial plate with a combination of Twyman-Green experiment and Timoshenko's bimaterial theory. This strain value has been incorporated into the FEM analysis, and the obtained warpage distributions of COG packages with and without fillets are compared with those from Twyman-Green experiment. It is found that moisture-absorption expansion strain leads to dramatically reduce the warpage of the COG package with fillet, but not for that without fillet. The results regarding the maximum warpages of the COG in terms of the parameters, such as fillets, the coefficient of thermal expansion, and elastic modulus of the NCP are also thoroughly discussed in this paper.