(A) study on the effects of the curing behavior and viscosities of non-conductive films (NCFs) on flip chip solder joint morphology and reliability

Abstract

Recently, high speed and smaller electronic devices are demanded with the fast growing of mobile products market. For this reason, 3D - Through Silicon Via (3D-TSV) and ultra-fine pitch interconnection technologies have been rapidly developed in electronic packaging industry. To obtain highly reliable less than $30 \mu m$ fine pitch Cu pillar/Sn-Ag micro bump interconnection, non-conductive films (NCFs), pre-applied type of underfill material, have been investigated. For flip chip bonding processes using ultra-fine pitch Cu-pillar/Sn-Ag bump, there are several processing steps such as flux application, solder reflow, and underfill dispensing processes. However, conventional underfill material and process has several problems such as void between interconnection due to the limitation of capillary flow and flux residue causing interconnection corrosion. However, NCFs can achieve void free underfill function and no flux residue, and also have some advantages of reduced processing steps and lower cost because of the flux function included in NCFs. In order to realize stable Cu-pillar/Sn-Ag bump joint by the flip chip bonding method, it is necessary to investigate the behavior of solder and NCFs resin during the NCFs bonding processes. Especially, NCFs resin can be trapped on the solder joint during flip chip bonding process. In addition, concave shaped solder bump joint which can be high stress concentration area formed at the metallurgical bonding area between solder and PCB metal pads. Eventually, the poor Cu-pillar/Sn-Ag bump joint morphology can cause crack initiation and early failure during reliability tests. To control precisely NCFs resin flow and solder joint morphology during flip chip bonding process, the curing properties and viscosities of NCFs should be studied. Especially, the material properties of NCFs at the solder melting temperature determine Cu-pillar/Sn-Ag bump joint morphology during flip chip bonding processes. Therefore, it is important to optimize the curing properties and viscosity of NCFs at the solder melting temperature of $221^\circ C$. In chapter 2, the effect of viscosity and hardening properties of NCFs on Cu-pillar / Sn-Ag solder bump joints on solder joint morphology was studied. The shape of the solder joint was observed by examining the shape change of the solder joint according to the flip chip bonding temperature. In addition, the shape of the solder joint was observed by changing the viscosity of the NCFs according to the silica filler content of the NCFs. In this process, the mechanisms and conditions for the formation of traps and concave-shaped solder joints of NCFs were investigated. Finally, the viscosity approximation was used to derive the change in the hardness and viscosity of NCFs during the flip chip bonding process, and the conditions in which NCFs trap and concave shape solder joints were generated. In chapter 3, the effect of the viscosity and hardening properties of NCFs on solder bump flip chip joints on solder joint morphology was studied. By controlling the flip chip bonding time, the shape change of the solder joint according to the flip chip bonding temperature was observed to investigate the formation mechanism of the solder bump flip chip joint. And in this study, the degree of cure and Viscosity approximation were derived to derive the curing and viscosity values of NCFs that change within a short bonding time within 5 seconds. Did. Finally, the curing conditions and viscosity conditions of NCFs that generate concave shape solder joints were investigated. In chapter 4, reliability results of solder joint geometry were evaluated at Cu-pillar / Sn-Ag solder bump joint and solder bump flip chip joint. In addition, the effects of thermo-mechanical properties of NCFs and the shape of solder joints on thermal cycle reliability were compared and analyzed, and what was the dominant factor.