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Material Analysis of Lead Free Solder Deposited by Electrochemical Deposition
Keywords: Lead free solder, Materials analysis, ECD
Lead-free solders have essentially replaced lead-tin solders for new microelectronic applications. At the same time, many products are continuing to go through miniaturization, so the behavior of the solder material is changing as a function of the connection size. As lead-tin solder has been replaced by lead-free alloys, electrodeposited SnAg has become the standard solder alloy used on wafers. Tin-based solders exhibit a complex material structure in the deposit, as recrystallization occurs in the deposit, and as intermetallic compounds form. Understanding the complex nature of these materials is becoming increasingly important to the performance of our electronic products. We have characterized ECD SnAg solder alloy deposits in terms of composition, grain size, phase, microstructure, and texture using EBSD analysis. We will discuss the effects of modifying the process parameters on the microstructure and the implications on the performance of the solder bump as a chip-level interconnect. We have seen that the metals deposit as β-tin with finely dispersed intermetallic grains. We have also seen evidence of recrystallization of the deposit subsequent to deposition, although the recrystallization behaviour is significantly different than the well-characterized copper recrystallization. Finally, the metal at the interface below the solder reacts with Sn to form intermetallic compounds, the compositions of which are dependent on the materials present. The effects of the process parameters on the deposit properties are becoming increasingly important to understand in order to control the properties of the interconnection, and how it changes over its lifetime. We have characterized the incorporation of trace organic materials in the deposit as a function of plating parameters. The deposition of SnAg from an MSA-based plating bath is very different from the deposition of copper from a copper sulfate plating bath. We have noticed that the incorporation of organic additive materials, as well as the grain size, increases as the deposition rate is increased in the deposition of SnAg.
Tom Ritzdorf,
Applied Materials
Kalispell, MT

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