| Summary: | ABSTRACT
Lead-free solders have been used as replacement materials for eutectic tin-lead (Sn-Pb) solder for interconnection in microelectronic devices. Among the available candidates, low melting temperature (Tm¬) lead-free solders have demonstrated their potential as a suitable choice to replace the Sn-Pb solder due to the low thermal stress built up at the solder joint and less thermal impact on the surrounding IC components owning to the low reflow temperature used for the assembling process. In the low Tm solder family, tin-bismuth (Sn-Bi) solder has attracted much attention because of its superior yield strength and comparable fracture resistance as Sn-Pb solder. The abundance of Bi and thus the relatively low cost of the Sn-Bi alloy make this type of solder even more attractive from economic point of view. However, the appliance of Sn-Bi solder in the industry is not unruffled as the intrinsic poor creep resistance shortens the service life of the electronic devices which overshadows the advantage of the excellent ductility and thermal properties of this material. To address the challenge faced by the low Tm solder alloy, the present study is dedicated to develop new composite Sn-Bi based solder alloys with improved creep resistance suitable for microelectronic applications. Nanoindentation analysis is employed to investigate the creep behavior in this study owing to its exclusive convenience in probing the mechanical properties of submicron sized features.
In this thesis work, existing indentation creep testing methods used by the research community have been critically reviewed and compared by experiment using polycrystalline Sn and aluminum samples. Constant strain rate (CSR) test is recommended as the evaluation method for the following studies on Sn-Bi solder alloys. The creep mechanisms of the Sn-Bi and its composite alloys were suggested by correlating the creep parameters, i.e., stress exponent and activation energy, with the materials microstructural characteristics obtained through prior- and post-indentation analysis. The deformation mechanism of the two-phase Sn-Bi solder is found to be stress-sensitive. At higher strain rate regime (2 × 10-3 to 0.1 s-1), the creep rate of this alloy is controlled by dislocation climb in either Sn-rich phase or both constituent phases through core diffusion; while at lower strain rate range (5 × 10-4 to 2 × 10-3 s-1), grain / phase boundary sliding is the rate controlling mechanism for this alloy.
Addition of reactive and non-reactive fillers was implemented to enhance the creep resistance of the Sn-Bi alloy. At low filler concentration (≤ 4wt%), the creep mechanism of the solder alloy does not vary with filler addition regardless of the filler type and the particle size. However, the creep rate is found to reduce significantly by the filler addition. Without chemical bonding between filler particles and solder matrix, non-reactive filler demonstrates superiority in limiting the creep rate of the Sn-Bi alloy due to the microstructural change, i.e., disruption of the discontinuous lamellar structure, as well as the homogeneous distribution of filler particles across the whole solder matrix. It is thus recommended as a suitable filler type when the composite alloy is deformed in the strain rate range of 5 × 10-4 to 0.1 s-1 and temperature range of 25 to 75 °C. Due to the inhomogeneous distribution of the reactive fillers in the solder matrix, the creep rate reduction by addition of this filler type is limited. However, the chemical bonding formed in the intermetallic compound (IMC) effectively enhances the solder stiffness as well as hardness. It is thus recommended as a choice when both stiffness and creep resistance are required for the solder performance.
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