Sn based solders with Ga additions were investigated to develop low-melting temperature solder alloys with good solder properties that can be used commercially. The objectives underlying this study were to examine the behaviour of the cooling curves of the alloys with Ga addition in terms of the growth and nucleation temperatures using thermal analysis. The as-cast alloys microstructures were also investigated via microstructure analysis using SEM and EDS to compare the phases present after adding Ga. The SEM was used again to study the formation of IMC layer around the joint interface using Cu and Ni substrates as surface finish.
The presence of Ga in the solder alloys affected both the growth and nucleation temperatures. Reduction in melting points were recorded with 2wt% Ga content due to the Ga element having low melting temperature, which caused overall growth temperatures of the alloys to decrease. The changes with 0.1wt% were insignificant as they only slightly decrease or increase except for Sn0.7Cu and SN100C alloys which had a reduction of approximately 2℃. It is suggested that repetition of thermal analysis is done to ensure higher reliability and accuracy of data.
As-cast alloys showed that there is an increase in oxidation resistance; there were less formation of oxide layer with addition of Ga. The appearance of the cast became more silvery in colour. The microstructure of the cast alloys as seen using SEM showed that little difference was recorded in the morphologies between 0wt% and 0.1wt% Ga content. The EDS analysis also failed to identify presence of Ga element in the microstructures. However, as Ga increased to 2wt%, cracks started to form on the surface of the alloys due to the Ga transforming into liquid phase during polishing and grinding while preparing the samples for analysis. EDS analysis had confirmed that the dark phases along the cracks were Ga element. It is assumed that the segregation of Ga due to the formation of low-melting point phase during solidification to be the major cause.
IMC formation analysis showed that the solders were unable to form interfacial reaction with the substrate with 2wt% Ga content due to the poor wettability and fillet geometries of the solder balls. The IMC formed with 0.1wt% Ga addition were also similar to 0wt% Ga alloys in terms of thickness and formation of phases.
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The thermal analysis done for each alloy only considered the nucleation and growth temperatures, which were analysed using Microsoft Excel and GraphPad Prism. More analyses of the cooling curve such as the recalescence temperature and undercooling properties can be done in the future to further examine the solidification behaviour of the alloys with Ga contents.
Thermal aging process can also be done following the reflow process of the solders onto the PCB. This process will be able to give further insight on the growth mechanism of the IMC at the solder/Cu and solder/Ni interfaces, using varying temperature profiles for aging and analysing the activation energy using Arrhenius plot gradient. Thus, EDS can be done to identify the elements making up the IMC layer between solders and substrates.
Since the mechanical properties of solder joints are directly associated with the IMC formation, further tests involving shear strength and mechanical testing of the joints should be carried out. This is important to determine the reliability of the solders to be implemented in the electronic packaging industry.
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Appendices
Cooling Curves of Alloys Combined
Figure 32: Cooling Curves of Sn Combined
Figure 33: Cooling Curves of SAC Combined
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Figure 34: Cooling Curves of Sn0.7Cu Combined
Figure 35: Cooling Curves of SN100C Combined
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Figure 36: Cooling Curves of SN100CV Combined
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Growth Temperatures & Nucleation Temperatures
Figure 37: Growth Temperatures of Alloys
Figure 38: Nucleation Temperatures of Alloys
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Solder Ball (Reflowed)
Figure 39: Solder Ball Orientation on PCB Top View Using BSE (Right Column) and SEI (Left Column). Sn100C-Ga (Top), Sn100C-0.1Ga (Middle), Sn100C-2Ga (Bottom)