Electro-migration evaluation between organic interposer and build-up substrate on 2.3D organic package

Abstract

In recent years, 2.3D packages using organic substrate are remarkable attention as a next generation high-end packages that realize to high speed and large-capacity computing system. i-THOP® (integrated-Thin film High density Organic Package) which is 2.3D type package, was developed. iTHOP® has structure in which organic interposer is stacked on build-up substrate. Transient Liquid Phase (TLP) technique using Sn-Bi solder has been employed for solder joints between organic interposer and build-up substrate. It is possible to raise the melting point by consuming the Sn phase in the solder to form the Cu-Sn inter-metallic compounds (IMCs). In this report, electromigration tests were performed on solder joints between organic interposer and build-up substrate. And the results were compared with the results of electro-migration test of the flip chip joints using Cu pillar / Sn-Bi solder. The current density was applied 38k A/cm2 at 125 degrees C. The resistance has rapidly increased to 4 % at 30 hours. And the resistance has slowly decreased to 0% at 1000 hours. Open failure was not observed at 1000 hours. Regarding to after assembly, Bi, Cu3Sn, Cu6Sn5 phase were observed at the solder joints between organic interposer and build-up substrate. Almost of Sn phase is consumed to form Cu- Sn IMCs. After electro-migration test, no segregation of Bi layer that caused an increase in resistance was observed. On the other hand, in the case of the flip chip joints using Cu pillar/Sn-Bi solder, the resistance has rapidly increased to 46 % at 100 hours. After that, the resistance has gently increased to 83 % at 2200 hours. And then, the resistance has slowly decreased to72% at 4840 hours. Bi atoms quickly migrated to the anode side by the electron flow. After electro-migration test, thick Bi layer was formed at anode interface. The amount of Bi layer formed on the anode side of Sn-Bi solder joints between organic interposer and build-up substrate is smaller than that of flip-chip joints using Cu pillar / Sn-Bi solder. In the case of Sn-Bi solder joints between organic interposer and build-up substrate, stable IMCs can be formed more quickly. These results indicated that applying TLP-like Sn- Bi solder joints between the organic interposer and the build-up substrate was stable.