Carbon Fiber Reinforced Tin-Lead Alloy as a Low Thermal Expansion Solder Preform

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 × 10^_6/°C (25–105 °C) in the direction parallel to the fibers compared to a corresponding value of 24 × 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.