The Effects of Introducing a Palladium Surface Finish on The Interfacial Reactions Between Sn-Zn Solders and Cu Substrate

Autor: Po-Hsun Lee, 李博燻
Rok vydání: 2012
Druh dokumentu: 學位論文 ; thesis
Popis: 100
Electroless Ni/electroless Pd/immersion Au (ENEPIG) had been developed since mid-1990s. It wasn’t extensively seen in commercial use because it was incompatible with eutectic Sn-Pb solder. “Restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS)” published on July 1, 2006 by European Union was carried out due to the concern of environmental protection. Pb-containing electronic products have been prohibited so that the lead-free solders were used to replace Sn-Pb solder in electronic package. In addition, ENEPIG has many advantages such as improvement of black pad problem, enhancement of wire bonding ability, and cost decrease. Sn-9Zn (in wt.%) alloy is considered as a promising Pb-free candidate solder because its melting point (198°C) is very close to that of eutectic Sn-Pb solder (183°C) and also the cost is low. A uniform layer of the Cu5Zn8 phase is formed at the Sn-9Zn/Cu interface after reflow at 230 °C but fractures very severely during subsequent solid state aging at 170 °C. Such severe fracture of an interfacial intermetallic compound deteriorates the integrity of joint interface and leads to excessive atomic interdiffusion as well as intermetallic compound growth. By inserting an electroless palladium (Pd) layer with a various thickness (0.2, 0.5 and 1.0 μm) at the Sn-9Zn/Cu interface, a Pd2Zn9 phase is formed between Cu5Zn8 and Sn-9Zn solder. This Pd2Zn9 phase exhibits higher microstructural stability at the interface during solid state aging and effectively retards the fracture of the Cu5Zn8 phase. The integrity of the Sn-9Zn/Cu interface is improved remarkably. Mechanical tests reveal that the shear strength of the Sn-9Zn/Cu joint is enhanced by inserting an electroless Pd layer at the interface. Finally, because the Zn atoms play an important role in this interfacial reaction, we change the Zn composition in the solder to observe the differences of the overall interfacial reaction. According to the experimental results, when the Zn concentration is low (1wt.%), two intermetallic compounds, CuZn and Cu6Sn5, were formed at the interface. When the Zn concentration is increased to 3 wt.%, Pd2Zn9 and Cu5Zn8, can be found at the interface, and the Pd2Zn9 phase gradually transforms into PdSn4 phase with increasing reaction time. When the Zn concentration is high (6 and 20 wt.%), the results are similar to Sn-9Zn/Cu.
Databáze: Networked Digital Library of Theses & Dissertations
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