Role of titanium on the reactive spreading of lead-free solders on alumina
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Eduardo Saiz Materials Sciences Division, Lawrence Berkeley National Laboratory (MSD-LBNL), Berkeley, California 94720
Velimir R. Radmilovic National Center for Electron Microscopy, Lawrence Berkeley National Laboratory (NCEM-LBNL), Berkeley, California 94720
Antoni P. Tomsia Materials Sciences Division, Lawrence Berkeley National Laboratory (MSD-LBNL), Berkeley, California 94720 (Received 20 June 2006; accepted 13 September 2006)
The wetting of Sn3Ag-based alloys on Al2O3 has been studied using the sessile-drop configuration. Small additions of Ti decrease the contact angle of Sn3Ag alloys on alumina from 115° to 23°. Adsorption of Ti-species at the solid–liquid interface prior to reaction is the driving force for the observed decrease in contact angle, and the spreading kinetics is controlled by the kinetics of Ti dissolution into the molten alloy. The addition of Ti increases the transport rates at the solid–liquid interface, resulting in the formation of triple-line ridges that pin the liquid front and promote a wide variability in the final contact angles.
I. INTRODUCTION
New low-temperature brazing alloys are required in many applications to integrate components that decompose or degrade above threshold temperatures. Traditionally, a key component in the design of brazing alloys for ceramic joining is the addition of reactive elements such as Ti, Cr, Zr, etc., to enhance spreading. The improved wetting resulting from the addition of reactive elements is usually associated with the formation of new compounds at the solid–liquid interface. However, it is unproven whether compound formation is actually necessary for enhanced wetting, or mechanistically, how the potential for compound formation translates into the capillary forces that specifically drive spreading of the fluid. Recently, an alternative reactive wetting mechanism that focuses on the adsorption of the reactive element at the solid–liquid interface before the nucleation of the reaction phase has been proposed as a critical step to reduce interfacial energy and drive spreading.1
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0393 3222
J. Mater. Res., Vol. 21, No. 12, Dec 2006
Tin-silver-based alloys have emerged as lead-free alternatives to the traditional solders used in the microelectronics industry.2,3 It is also recognized that tin and indium alloys can also be of interest for the brazing of ceramics at low temperature. However, most of the work with lead-free alloys concentrates on the soldering of metals, and not many papers discuss their wetting on ceramics, due in part to the difficulty in eliminating oxide layers from the liquid surface.4–8 Sn–Ag–Ti alloys are also of theoretical interest. It has been observed that the wetting of Sn-based alloys on ceramics can be greatly enhanced by the addition of titanium, but in our work, no continuous reaction products could be detected at the metal–ceramic interface as is commonly observed in other systems.1,9 These observations indi
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