| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2014) | |
| Nanomaterials | Monday Sessions |
| Session NM-MoM |
| Session: | Nano Fabrication |
| Presenter: | Min-Jung Son, Sungkyunkwan University, Korea, Republic of Korea |
| Authors: | M.J. Son, Sungkyunkwan University, Korea, Republic of Korea I. Kim, Korea Institute of Machinery & Materials (KIMM), Republic of Korea T.M. Lee, Korea Institute of Machinery & Materials (KIMM), Republic of Korea S.S. Yang, Korea Institute of Materials, Republic of Korea H. Lee, Sungkyunkwan University, Korea, Republic of Korea |
| Correspondent: | Click to Email |
For flip-chip packaging technologies, which gain popularity in semiconductor packaging, forming fine solder bumps with a high aspect ratio at a low cost is an integral part. A promising alternative to the conventional methods (screen printing and electroplating) is reverse offset printing owing to its high throughput.
In the present study, we developed Sn-Ag-Cu paste customized for the reverse offset printing process to use as solder bump with a high aspect ratio.
In fabrication of the pastes, we controlled the particle size distribution using Sn-Ag-Cu particles with two different sizes (2 μm and 100 nm in diameter) and the particles were mixed with various ratio (100 : 0, 75 : 25, 50 : 50, 25 : 75, and 0 : 100) to control the rheological properties without using viscosity-increasing agents, which usually degrade the electrical properties. Various tools were used for measurement of rheological properties such as viscosity, thixotropy index (TI), storage modulus (G’), loss modulus (G”), crossover point of G’ and G”. The TI and crossover point of G’, G” gradually increased with the ratio of the nano particles increasing. In other words, if the initial viscosity increases with the increase of the amount of the nano particles, the shear thinning behavior and elasticity of the paste are dominant and the paste becomes suitable for the formation of the fine solder bumps with high aspect ratio. TI, which is related to the shear thinning behavior, increased from 0.08 to 0.53, and crossover point of G’, G”, which is related to the shear thinning behavior, increased from 0.15 to 148 Pa. In addition, we measured the electrical resistance of the paste to check the influence of the particle size on it. The resistance increased with the increase of the ratio of the nano particles. As a result, we obtained the lowest resistance in the paste made from only micro solder particles. Considering these results in terms of printability and electrical resistance, we used the paste with the particle ratio of 75 : 25 to fabricate solder bumps. We obtained 6 ~ 30 μm height, 100 μm pitch Sn-Ag-Cu solder bumps.