Carbon fiber reinforced tin-lead alloy as a low thermal expansion solder preform
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Tin-lead (40 wt. % Pb) solder-matrix composites containing 8-54 vol. % continuous unidirectional copper plated carbon fibers were fabricated by squeeze casting for use as low thermal expansion solder preforms. The low thermal expansion greatly increased the thermal fatigue life for solder joints between materials with low thermal expansion coefficients. For example, for 29 vol. % fibers, the thermal expansion coefficient was 8 x 10"6/°C (25-105 °C) in the direction parallel to the fibers compared to a corresponding value of 24 x 10~6/°C for plain solder. The thermal fatigue life for cycling 2 cm long alumina-to-alumina solder joints between 25 and 100 °C was increased from 98 to 183 cycles by using 29 vol. % carbon fibers in the composite solder. The fibers also increased the tensile modulus and tensile strength of the solder, but the ductility was decreased. The copper coating on the carbon fibers increased the tensile strength and ductility of the composite.
I. INTRODUCTION
Solder preforms are used to join various parts of an electronic package. For example, they are used to join a ceramic cap and a multilayer ceramic substrate in a multichip module. Because the thermal expansion coefficient of a solder is in general much higher than that of a ceramic, the solder joint suffers from a poor resistance to thermal fatigue. Thus, there is a need for a solder with a low coefficient of thermal expansion. Composite materials can be tailored to exhibit a chosen thermal expansion coefficient, as the filler species and the filler volume fraction can be judiciously chosen. For a low thermal expansion composite, the filler must have a low thermal expansion coefficient. Moreover, the filler should preferably be a good electrical conductor, because the soldered joint may serve as an electrical connection as well as a mechanical connection. It should also preferably be a good thermal conductor for heat dissipation from the electronic package. In addition, the filler should be wetted well by liquid solder in order to facilitate composite fabrication. A further requirement is that, upon remelting and solidification of the solder in the solder-matrix composite, the filler distribution in the composite remains uniform. Molybdenum particles have been used as a filler in solder-matrix composites.1 However, their distribution in the solder (Sn-40% Pb alloy) became nonuniform after remelting and solidification of the solder in the composite.2 Thus, molybdenum is not a suitable filler in spite of its low coefficient of thermal expansion. Carbon fibers have nearly zero thermal expansion coefficient. In contrast to ceramic fibers (which also 1266
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J. Mater. Res., Vol. 5, No. 6, Jun 1990
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have low thermal expansion coefficients), carbon fibers are a good electrical conductor and a good thermal conductor. Furthermore, carbon fibers are widely available in a continuous form, which makes them even more effective for lowering the thermal expansion coefficient of the composite and which p
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