Morphology of instability of the wetting tips of eutectic SnBi, eutectic SnPb, and pure Sn on Cu
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The Pb-based solder used in microelectronics industry is becoming an environmental issue. To understand the wetting behavior of solders with and without Pb, we have studied the surface morphology and wetting reaction of eutectic SnBi, eutectic SnPb, and pure Sn on Cu through the measurements of wetting angle change and wetting tip stability by SEM and EDX. The wetting angle remains constant after the initial spread, but the eutectic SnPb/Cu continues to react and forms a reaction band in front of the solder edge as well as intermetallic compounds at the interface. For eutectic SnBi/Cu, there is no reaction at the wetting tip, and the wetting angle does not change much; however, the interfacial reaction between eutectic SnBi and Cu forms intermetallic compounds at the solder joint; the wetting tip is not in a static equilibrium. A rough surface and edge was observed on the eutectic SnBi/Cu joint, but the eutectic SnPb/Cu has a smoother surface and edge.
I. INTRODUCTION In the microelectronics industry, two levels of solders are used to join the semiconductor chip to its packaging module and to join the module to a platedcircuit board.1'2 For example, the low melting point eutectic SnPb solder is most widely used in the bonding of TAB (Tape Automated Bonding) packages and PTH (Pin Through Hole) technologies in the circuit-board assembly. The high melting point Pb-rich SnPb solder is used in C4 (Controlled Collapsed Chip Connector) bonding to attach IC chips to their multichips module. The high-low combination allows a two-stage soldering operation in chip-joint and module packaging. However, Pb-containing solder is becoming an issue of conscious manufacturing due to environmental and health concerns. The anti-Pb bills3 in legislation have compelled the microelectronics industry to find Pbfree solders. The Environmental Protection Agency is requesting that the Pb-based solders should be phased out or greatly reduced from the microelectronics applications. Hence, how to cut down the use of Pb in soldering and how to develop Pb-free solders are urgent materials issues in advanced microelectronic technology. To remove Pb from solders, we must understand the role of Pb in soldering. During the soldering process of SnPb on Cu, the latter reacts only with Sn to form Cu-Sn intermetallic compounds. While Pb does not react with Cu, it controls the soldering temperature, limits the Cu-Sn reaction, retards the growth of Sn whiskers, and lends ductility to the joint.4 The key of the soldering process is the wetting of a liquid solder on a conductor surface. Although there have been many J. Mater. Res., Vol. 10, No. 3, Mar 1995
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studies on solder reactions on Cu,5"10 how Pb influences the wetting behavior and the Cu-Sn reaction affects the equilibrium condition at the wetting tip are not very well known. In this paper we address the instability of the wetting tip of eutectic SnBi and eutectic SnPb on Cu. We study the surface morphology of the solders after reflow. The wettin
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