| Flip chip assembly is widely recognized as the most common and promising packaging technology in high-density and high-end electronic packaging, where bump fabrication is one of the most important technologies. Three-dimensional packaging technology, which provides a path to miniaturization, high bandwidth, low power, high performance and system scaling, has been developed recently for system-in-package applications, and it also requires fine pitch and high density of solder bumps.At present, there are several technologies mainly used in bump fabrication, screen printing, ball placement and electroplating. As the bump pitch reduces constantly, especially to below 100μm, electroplating is the best due to its advantages of high deposition rate, mass productivity, total cost and the ability to define bump size and pitch.The current environmentally conscious manufacturing moves toward Pb-free schemes for electronic devices and components. Sn–Ag alloy system is an acknowledged candidate among Pb-free solders. As the standard reduction-potential difference of silver and tin is very large, a uniform co-deposition of tin and silver to make the proper eutectic-solder composition is very difficult. The two-step electroplating process using separate reactors for each element has been developed.In this paper, we propose a technique for bumping Sn3.5Ag. First, the technology of pure metal layer electroplating was studied and optimized in the tin mesylate-based tin plating bath and succinimide cyanide-free silver plating bath separatedly. Then tin layer and silver layer were deposited sequentially, and the stacked layers were alloyed by reflowing in the glycerol bath. Effect of reflow time on interfacial reaction of electroplated Sn/Ag layers with Cu substrate was carefully investigated. At last, area—array tin solder bumps each of size 60μm diameter on an 180μm pitch with very tight height variation were obtained and perfect spherical bump shape with smooth surface was achieved after reflow. |
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