Fundamental Study on Adhesion of Epoxy Underfill Materials with Passivation Layer in Flip-chip Packaging
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Fundamental Study on Adhesion of Epoxy Underfill Materials with Passivation Layer in Flip-chip Packaging Shijian Luo and C. P. Wong Packaging Research Center School of Materials Science and Engineering Georgia Institute of Technology Atlanta, GA 30332, USA
ABSTRACT The influence of aging in an environment with a high temperature and a high humidity on the adhesion performance of underfill material (epoxy cured with acid anhydride) to the passivation layer in flip chip packaging is discussed. Adhesion of underfill to different passivation materials degrades after aging in a high temperature and high humidity environment. The extent of this degradation is dependent on the hydrophilicity of the passivation material. Hydrophilic passivation such as silicon oxide (SiO2) and silicon nitride (Si3N4) shows much more severe adhesion degradation than hydrophobic passivation such as benzocyclobutene (BCB) and polyimide (PI). The mobility of absorbed water and of polymer chains is studied with solid state nuclear magnetic resonance (NMR) spectroscopy. Higher mobility of absorbed water and of polymer chains in rubbery state polymers contributes to faster adhesion degradation during high temperature and high humidity aging. The adhesion stability of hydrophilic passivation can be successfully improved by use of a silane coupling agent that introduces stable chemical bond at interface. A flow micro-calorimeter was used to study the absorption of silane coupling agent onto glass surface. The difference in adhesion retention improvement between aminosilane and epoxysilane is discussed. INTRODUCTION In order for an integrated circuit (IC) chip to be used, it has to be packaged so that it can interconnect with the outside world. Flip chip on board is a low cost, yet high performance packaging approach in which an IC chip is connected onto a printed circuit board through solder joints with the active side of the IC chip facing toward the board [1]. The introduction of an underfill, a polymeric material, between the IC chip or die and the board can greatly improve the solder joint fatigue life by mechanically coupling the substrate and the die. In addition, underfill can also provide environmental protection. The adhesion of underfill to the passivation layer on the IC chip is crucial to the integrity and reliability of the flip chip assembly. Delamination (total loss of adhesion) at the interface between underfill and die can lead to cracking of the interconnection. Moisture can then diffuse through the delaminated area, leading to corrosion of metal pad, line, and interconnection [2]. It has been shown that high adhesion strength at the interface between die and underfill is strongly correlated with improved solder joint fatigue life and alleviated underfill fillet crack problem [3]. When the flip chip package is subjected to aging in an environment with a high temperature and a high humidity, such as in an environment chamber at 85°C 85% relative humidity (85/85) or a pressure cooker at 121°C, 2 atm., and 100% relative humidity, to
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