Morphology of wetting reactions of SnPb alloys on Cu as a function of alloy composition
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Morphology of wetting reactions of SnPb alloys on Cu as a function of alloy composition C. Y. Liu and K. N. Tu Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, California 90095-1595 (Received 4 February 1997; accepted 8 October 1997)
We have investigated the wetting angle, side band growth, and intermetallic compound formation of seven SnPb alloys on Cu ranging from pure Sn to pure Pb. The wetting angle has a minimum near the middle composition and increases toward pure Sn and pure Pb, but the side band growth has a maximum near the middle composition. The intermetallic compounds formed are Cu6 Sn5 and Cu3 Sn for the eutectic and high-Sn alloys, yet for the high-Pb alloys, only Cu3 Sn can be detected. While no intermetallic compound forms between Cu and pure Pb, the latter nevertheless wets the former with an angle of 115±. The driving force of a wetting reaction, which may be affected by the free energy gain in compound formation, is discussed by assuming that rate of compound formation is fast.
I. INTRODUCTION
Solder technology is essential in electronic packaging. The Semiconductor Industry Association has projected a trend of increasing use of solder in semiconductor device manufacturing. This is due to the need of larger and larger number of input/output leads on high circuit density device chips. To meet the need, solder joints in area array configuration is superior than the periphery wire bond interconnect technology. The U.S. Congress and the Environmental Protection Agency, however, have expressed a concern for the use of Pb in the electronic packaging industry. There are four antiPb bills pending in the Congress. As a consequence, the lead (Pb)-free solders and their processes, properties, and reliability must be studied and data must be collected and compared to those of the Pb-bearing, especially the SnPb alloys, widely used today. At present, the low melting point eutectic SnPb solder is commonly used in the second-level packaging, for example SMT (surface mount technology) and PTH (Pin Through Hole) technologies in the circuit-board assembly. In chip joint bonding (the first level packaging), the high melting point Sn5 Pb95 and Sn10 Pb90 alloys are chosen. Therefore, there exists a two-level soldering operation in device packaging, and it is obvious that during the second level soldering the first level solder joints should not melt. For the low melting point solder, current studies on Pb-free solders such SnAg and SnInAg based alloys are active. On the other hand, very little is known about the high melting point (about 300 ±C) Pb-free solders. To replace those Pb-bearing solders by Pb-free solders, we must understand the role of Pb and the effect of J. Mater. Res., Vol. 13, No. 1, Jan 1998
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the composition of SnPb alloys in the soldering reaction on Cu. For this purpose, we have carried out a systematic study of the wetting angle, side band fo
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