Electronics Assembly Knowledge, Vision & Wisdom
Electroplating of Cu in TSV
Electroplating of Cu in TSV
The research objective is to estimate metallurgical and mechanical properties of low alpha solder. Cu-filling property to TSV was also investigated.
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Authored By:
Dohyun Jung, Myung Hoon Roh, Jun Hyeong Lee, Soonjae Lee, and Jae Pil
Jung University of Seoul
Seoul, Korea

Hyunkyu Lee and Jeonguk Kwak
Duksan Hi-Metal Co. Ltd.
Yonamdong, Ulsan, Korea

Summary
Three-dimensional (3-D) packaging is a popular candidate to meet the demands of high density packaging and miniaturization of electronics. Especially, 3D packaging with TSV (through-Si-via) has advantages of shorter electric path and smaller bonding area. In TSV electrical conductive material is filled, and Cu-filling by electroplating is most popular one. Meanwhile, the soft error, the temporary malfunction of electronic devices caused by the effect of radiation, became a momentous issue in high density electronics packaging. Solder was found to be one of the major sources of radiation in electronic devices.

In three-dimensional packaging, the solder bumps are very close to the active Si devices, where even the low energy alpha ray can induce soft error, which needs to choose the low alpha solder in packaging. For high density packaging, Cu electroplating to TSV and characteristics of low alpha solder bumps were investigated in this study. A straight via with a diameter of 60 im and depth of 120 im were drilled in a Si wafer by deep reactive ion etching (DRIE) process. Cu was filled to the via by electroplating where the current waveform of a periodic pulse reverse (PPR) was applied.

The LC3 class of low alpha solder bump having a composition of Sn-1.0Ag-0.5Cu (SAC105) and a diameter of 80 im was formed by reflow on a UBM (under bump metallurgy) and on a Cu-filled TSV. For estimating the shear force of the low alpha solder bump, high speed shear tester (Dage 4000HS) was used, and for the fracture mode analysis, scanning electron microscopy (FE-SEM) was employed. As experimental results, Cu filled into TSV showed a typical bottom up filling.

The shear force of low alpha SAC105 bump increased with increasing shear speed from 10 to 1,000 mm/s, which depends on kinds of bump pads. Brittle fracture tendency increased with increasing shear speed. The properties of low alpha SAC105 solder were comparable to those of normal solder.

Conclusions
Cu was electroplated in TSV by PPR current wave form, and it showed a typical bottom up filling. The Cu-filling didn't show serious defects like void or seam. Low alpha solder bumps were produced on the Cu surface of TSV. Similarly to normal solder, Cu6Sn5 and Ag3Sn IMCs were found in SAC105 solder. The high speed shear property was also evaluated.

Initially Published in the SMTA Proceedings

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