Effect of Zn on the intermetallics formation and reliability of Sn-3.5Ag solder on a Cu pad
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Varying amounts of Zn (1, 3, and 7 wt%) were added to Sn–3.5Ag solder on a Cu pad, and the resultant solder joint microstructures after a reflow and isothermal aging (150 °C, up to 500 h) were investigated using scanning electron microscopy, energy dispersive x-ray, and x-ray diffraction, which were subsequently correlated to the results of microhardness and drop tests. Zinc was effective in improving the drop resistance of Sn–3.5Ag solder on the Cu pad, and an addition of 3 wt% Zn nearly doubled the number of drops-to-failure (Nf). The beneficial role of Zn was ascribed to suppression of Cu6Sn5 and precipitation of Zn-containing intermetallic compounds (IMCs). However, the Zn effect was reduced as Cu6Sn5 and Ag3Sn precipitated in a joint IMC layer after prolonged aging. The interface between Ag5Zn8 and Cu5Zn8 was resistant to drop impact, but two other layered IMC structures of Cu6Sn5/Cu3Sn and Cu5Zn8/Cu6Sn5 were not. I. INTRODUCTION
Worldwide legislative moves to forbid the use of Pb in commercial electronics1 have spurred extensive searches for substitute materials for Pb–Sn solder, and candidate Pb-free solder materials include Sn–Ag, Sn–Cu, Sn–Ag– Cu, and Sn–Zn alloys.2–5 Lead-free solders bearing Zn have received particular attention, as Zn is known to improve the mechanical reliability of solder alloys.6,7 Zinc is known to improve the creep resistance of Sn– Ag–Cu solder5 and suppress the formation of large Ag3Sn platelets.8 The solder joint microstructure was shown to be significantly affected by the Zn content in the solder and in the type of underlying under-bump metallization (UBM).9–12 Over Cu UBM, ␥–Cu5Zn8 and –Cu6Sn5 intermetallic compounds (IMCs) were found for Sn–0.7Zn solder; however, only ␥–Cu5Zn8 was observed for Sn–2Zn solder.13 For Sn–9Zn over Cu, three sublayers of ⬘–CuZn, ␥–Cu5Zn8, and an unidentified Cu–Zn phase were found after a reflow and subsequent aging, and the thickening of ␥–Cu5Zn8 decreased the solder joint strength significantly.9 Recent packaging technology is driven by handheld microelectronics products such as cellular phones and personal digital assistants (PDAs). These devices are prone to accidental drops, and the resistance of solder joints to drop impact has become more important.14 Accordingly, extensive drop tests were conducted using
various Pb-free solders, and effects of surface finishes were investigated.15–21 However, relatively less work has been conducted regarding the correlation between the Zn content and drop reliability. In the present work,
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0234 J. Mater. Res., Vol. 22, No. 7, Jul 2007
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FIG. 1. Schematic diagram of (a) drop test specimen and (b) equipment. © 2007 Materials Research Society
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Y.K. Jee et al.: Effect of Zn on the intermetallics formation and reliability of Sn–3.5Ag solder on a Cu pad
FIG. 2. Microstructures of 0Zn(Sn–3.5Ag) solder over Cu UBM: (a) as-ref
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