Tensile fracture behaviors of solid-state-annealed eutectic SnPb and lead-free solder flip chip bumps
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The tensile fracture behavior for solid-state-annealed eutectic SnPb and lead-free solder flip chip bumps was examined. The annealing temperatures were in the range of 125–170 °C for 500 h. Prior to solid state annealing, the eutectic Sn–37Pb (SnPb) and Sn–0.7Cu (SnCu) solders showed fracture through the bulk solder. Brittle interfacial fracture occurred in the Sn–3.5Ag (SnAg) solder. After solid-state annealing, the fracture behavior changed dramatically. For eutectic SnPb solder, the fracture modes gradually changed from cohesive solder failure to interfacial fracture with increasing annealing temperature. The fracture mode of the SnCu solder showed greater change than the SnPb and SnCu solders. After annealing at 125 °C, the SnAg solder had a ductile taffy pull fracture, but an increase in temperature resulted in brittle interfacial fracture again. The SnCu solder maintained the same ductile taffy pull mode up to 170 °C annealing, independent of the under bump metallization (UBM) type. Microstructure analysis showed that the interfacial fracture of the SnPb and SnAg solder bumps was ascribed to Pb-rich layer formation and Ag embrittlement at the interface, respectively. The bulk solder fracture of SnAg annealed at 125 °C appeared to be a transient phenomenon due to the abrupt breakdown of the hard lamella structure. The eutectic SnCu solder bumps had no significant change in the interfacial structure, except for interfacial intermetallic growth.
I. INTRODUCTION
The integrity of solder interconnections is a challenging issue in the packaging of very large scale integration devices due to the critical need for a large number of input/output (I/O) connections.1,2 To achieve the challenging I/O needs, flip chip technology is believed to be a key solution. The Sn–Pb alloy has been the most widely used solder alloy for interconnects because of its excellent soldering properties over a wide range of compositions. However, the toxicity of Pb has driven many countries to introduce legislation to ban Pb-containing materials, including solders.3 A significant number of studies on lead-free solders have been performed.4,5 These works have shown that the high content of Sn with several percents of alloying elements would be the good candidates. Even small alloying elements to Sn greatly modify the microstructure and mechanical properties of the solder alloys. Eutectic Sn–3.5Ag (SnAg) and Sn–0.7Cu (SnCu) solders are typical examples.6,7 The bulk properties of various lead-free
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0235 1826
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 6, Jun 2004 Downloaded: 03 Apr 2015
solders have been studied extensively8,9 whereas the interfacial mechanical properties of the solder joint [between under bump metallization (UBM) and solder] with a variety of lead-free solders have not been thoroughly studied. In particular, solder joint properties after solidstate aging requires additional study because the device operation temp
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