Nickel-alloyed tin-lead eutectic solder for surface mount technology
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Thomas G. Ference IBM Technology Products, East Fishkill, Route 52, Hopewell Junction, New York 12533 (Received 17 September 1992; accepted 30 December 1992)
A new ternary solder alloy of tin-lead-nickel was developed for solder joints. It has an optimum composition range of (60-65)% Sn, (35-40)% Pb, (0.5-1.0)% Ni by weight. The alloy exhibits a higher strength and longer fatigue life than the pure 63% Sn-37% Pb eutectic. Melting point, wettability, and fatigue life were the key properties used to determine this composition. In particular, the melting point of the alloyed eutectic solder did not change for rapidly solidified samples when up to 2 wt. % Ni was added. Instead of modifying the solution melting temperature, the majority of the Ni precipitated out as a fine dispersion of Ni3Sn4. These Ni3Sn4 particles help to strengthen the solder matrix. Reflow experiments on rapidly solidified material showed minimal coarsening and agglomeration of the Ni3Sn4 particles for up to 5 min of reflow cycles. The wettability of the solder, however, decreased significantly for contents of Ni above 1.5%. Low cycle fatigue tests show that for a solder with 0.5% Ni, the fatigue life was about twice as long as that of standard eutectic solder. Suggested processing for the alloy is rapid solidification to form the alloy powder which is incorporated into a reflowable paste.
I. INTRODUCTION Surface mount technology (SMT) has recently emerged as an alternate method of attaching electronic components to the surface of printed circuit boards.1 With SMT, as with all microelectronics packaging, one must consider the important issue of joint reliability. Of the many reliability concerns, a major one is solder fatigue resulting from the cyclic application of thermal strains to solder joints. 2 Some methods of controlling thermal strains are choosing new substrate materials that better match the thermal coefficient of expansion (TCE) of the package materials,3 improving thermal management to reduce the temperature differential between active devices and their environments, or producing "taller" solder joints. 4 An alternate approach is to develop a new solder alloy, which has better thermal fatigue resistance than conventional solders. Because SMT solder joints serve not only as electrical connections, but also as mechanical connections, high joint strength is demanded as well. Also, since most solder alloys exhibit creep deformation even at room temperature,5'6 the new solder should possess creep resistant characteristics. In the present investigation, a methodology practiced for the development of high temperature materials is adopted for developing a stronger and more fatigue resistant Sn-Pb based solder. Here we proceed with the basic ideas that the ductility and fatigue properties J. Mater. Res., Vol. 8, No. 5, May 1993
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rely on the matrix material, while the strength and creep resistance properties are derived from fine, hard particles in the matrix, so-called "precipitation or
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