Cellular to dendritic transition during transient solidification of a eutectic Sn–0.7 wt%Cu solder alloy

• Published in: Materials chemistry and physics 2012-01, Vol.132 (1), p.203-209
• Main Authors: Moura, Itamazeo T.L., Silva, Celina L.M., Cheung, Noé, Goulart, Pedro R., Garcia, Amauri, Spinelli, José E.
• Format: Article
• Language: English
• Subjects: Alloys; Cellular; Eutectics; Intermetallic compounds; Intermetallics; Microstructure; Morphology; Optical metallography; Solders; Solidification; Tin base alloys
• ISSN: 0254-0584; 1879-3312
• DOI: 10.1016/j.matchemphys.2011.11.033

► The microstructure of solder alloys depends on solidification thermal parameters. ► The final properties of solder joints depend on the alloy microstructure. ► A cellular/dendritic transition was observed to occur for the Sn0.7Cu alloy. ► The proposed experimental growth laws contribute to the control of soldering. The eutectic Sn–0.7 wt%Cu alloy is considered an important alternative to replace the classic eutectic Sn–Pb alloy, used to join metallic surfaces in electronic devices. The stable Sn–Cu eutectic is composed of a mixture of a tin-rich phase and fibrous Cu 6Sn 5 intermetallic particles. The morphology, size and distribution of stable and metastable intermetallic particles may affect the mechanical properties of the alloy. The distribution of these intermetallics is characterized by the interphase spacing, which depends on thermal parameters such as the growth rate ( v) and the cooling rate ( T ˙ ) during solidification. The aim of this study is to investigate the microstructural evolution of a eutectic Sn–0.7 wt%Cu solder alloy during transient solidification. The resulting microstructural morphology depends on v and T ˙ , and in the case of soldering processes the control of these parameters is essential for the design of the final microstructure. A gradual cellular to dendritic transition was observed to occur for growth rates ranging from 0.3 to 0.5 mm s −1 and cooling rates from 0.9 to 1.5 K s −1. The cellular region was shown to be characterized by aligned eutectic colonies, and experimental growth laws relating cellular, dendritic and interphase spacings to both v and T ˙ have been proposed.