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J Mater Sci: Mater Electron (2013) 24:20522057 DOI 10.1007/s10854-012-1055-4
Formation of SnBi solder alloys by sequential electrodeposition and reow
Yingxin Goh Seen Fang Lee A. S. Md. Abdul Haseeb
Received: 31 October 2012 / Accepted: 24 December 2012 / Published online: 4 January 2013 Springer Science+Business Media New York 2013
Abstract Eutectic SnBi alloy is gaining considerable attention in the electronic packaging industry because of its favorable properties such as low melting temperature, good wettability, and good mechanical properties. Miniaturization of electronic devices requires small solder bumps, a few tens of micrometers in diameter. Electrodeposition is a reliable technique for the deposition of small volume of solder. This work focuses on the formation of eutectic SnBi solder by reowing a metal stack containing sequentially electrodeposited Sn and Bi layers. The effects of layering sequence on the composition and microstructure of the resulting alloy is investigated. Irrespective of the layering sequence, a homogeneous microstructure is achieved after reow. The microstructure of the reowed samples is the same as that of a metallurgically processed SnBi alloy. Near-eutectic alloy with the composition Sn54.6 wt% Bi is obtained by the sequential electrode-position method.
1 Introduction
Tinlead(Sn-Pb) alloys have been dominating the electronics industry as solder materials for the past few decades. However, recent legislations restricted the use of lead (Pb) in electronic devices due to its inherent toxicity.
Attempts to develop alternatives to Pb-containing solders are being pursued currently. Most of these alternatives are Sn-based alloys. Examples include SnAg, SnBi, SnZn, SnIn, SnCu, and SnAgCu. The low melting temperature of Sn58 wt% Bi eutectic alloy (138 C) has made it a suitable candidate for solders intended for low temperature applications [17]. The eutectic SnBi alloy also possesses good wettability on Cu substrate and has high yield and fracture strength at room temperature [8]. A small amount of Cu or Ag addition has been shown to improve the thermal fatigue life and ductility, which is attributed mainly to the ability of such additional elements to rene the microstructure of SnBi alloys [912].
In recent years, the drive towards miniaturization has brought ip-chip technology into the forefront. Such miniaturized packages nowadays require ner solder bumps with pitch \50 lm. One way to achieve such small bump sizes is to employ through-mask...