Temperature- and moisture-induced warpages of COG package with non-conductive paste adhesive

Abstract

The use of non-conductive paste (NCP) or film (NCF) adhesives to replace the ACF (anisotropic conductive film) in COG (chip on glass) packages may be a possible solution for the low cost and finer bump pitch application to the LCDs (liquid crystal displays). However, when the NCP is applied, reliability 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 fillet after cured in the chamber with 150degC for 2 hrs, under thermal and moisture loads. Twyman-Green interferometry is used for measuring the warpages (out-of-plane deformations) of the COG packages due to such loads. Three-dimensional finite element method (FEM) is used for calculating the warpage and the 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 (warpage) of the COG packages with and without fillet have been experimentally measured under thermal cycling and moisture loading. The results show that, right after manufacturing, the much apparently concave-up-shape warpages are found at the COG packages with fillet, but slightly concave-down shapes for those without fillet. And the warpages decrease with the heating temperature increasing. The warpages (fringe patterns) of the COG package, determined from experiment and FEM, are quite consistent for case 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 29degC /85%RH for 12 hrs has been obtained to be epsivm = 0.3 % by testing NCP/silicon bi-material plate with a combination of Twyman-Green experiment and Timoshenko's bi-material theory. This strain value has been incorporated into the FEM analysis, and the obtained warpage distributions of COG packages with and without fillet are compared with those from Twyman-Green experiment. It is shown that moisture-absorption expansion strain leads to dramatically reduce the warpage of the COG package with fillet, but not for that without fillet