Elemental Redistribution and Related Phase Transformation between Solder Bump and Ni/Cu UBM in Flip Chip Technology

Autor: Chien-Sheng Huang, 黃建盛
Rok vydání: 2003
Druh dokumentu: 學位論文 ; thesis
Popis: 91
Flip chip technology with BGA interconnection has attracted a great deal of attention in today’s electronics packaging. One of the challenging issues is the material selection for under bump metallization (UBM). The Ni-based UBM acts as a wetting layer of solder and also as a diffusion barrier for Cu metallization. In this study, the elemental distribution and related phase transformation between solders and Ni/Cu UBM were investigated with the aid of microstructure evolution and deliberately quantitative analysis by an electron probe microanalyzer. The soldering-induced Cu diffusion and intermetallic compound (IMC) formation were revealed. The atomic flux of Cu diffusion during reflow was in the order of 1015~1016 atoms/cm2s. After one reflow, Cu atoms diffused through electroplated Ni and dissolved into the previous formed IMC Ni3Sn4 to produce another layered IMC (Ni1-x, Cux)3Sn4. The composition of (Ni1-x, Cux)3Sn4 IMC was homogeneous in each joint despite of different Ni thickness. After more than three times reflow, Cu atoms further diffused through the boundaries of (Ni1-x, Cux)3Sn4. As the concentration of Cu in Sn-Cu-Ni alloy is greater than 0.6 wt.%, another scalloped IMC (Cu1-y, Niy)6Sn5 would form. The amounts of (Cu1-y, Niy)6Sn5 IMC formation could be associated to the Ni thickness and reflow times. The values of y were evaluated to remain around 0.4, however, the values of x varied from 0.02 to 0.35. The elemental distribution of IMC in the interface of the joint assembly was further correlated to the Ni-Cu-Sn ternary equilibrium. In addition, the mechanism of (Cu1-y, Niy)6Sn5 formation was also probed. Through appropriate etching and sample preparation, IMCs of Ni3Sn4 and Cu6Sn5 were revealed with distinct morphologies in a solder/Ni-Cu UBM joint assembly. The grain size of (Ni, Cu)3Sn4 IMC formed in the Sn-Ag system after one reflow was larger than that in the Sn-Pb system, thus the pathways for Cu diffusion were reduced. As a result, only thicker (Ni, Cu)3Sn4 IMC was observed, and no (Cu, Ni)6Sn5 IMC could be detected between SnAg solder and Ni. The elemental redistribution in the edge of the solder bump is crucial for its correlation with the bump strength. Hence, the IMC formation in the edge of solder bump between UBM and eutectic Sn-Pb solder was also discussed. During reflows, only (Cu1-y, Niy)6Sn5 IMC was observed in the sideway of Ni/Cu UBM. After high temperature storage (HTS) at 150C for 1000 hours, both (Cu1-y, Niy)6Sn5 and (Cu1-z, Niz)3Sn were found in the sideway of Ni/Cu UBM. The (Cu1-z, Niz)3Sn IMC formed since Sn atoms were almost exhausted near the sideway of Ni/Cu UBM. Moreover, (Cu1-y, Niy)6Sn5 IMC was visible even in the interface of solder and UBM after HTS test due to the fast growth rate.
Databáze: Networked Digital Library of Theses & Dissertations
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