Interconnection materials between components and circuitries represent an interesting microcosm of the varied concerns. They play a key role in fixing devices on substrates and dissipating the heat produced from electronic devices to the substrate. For nearly all the interconnection in electronics packages, solders and soldering have been mostly used. As the electronic devices and semiconductor chips become to be faster, more dense and smaller, the volume of the solder interconnection region has to be smaller and the reliability in the solder interconnection becomes more important.
The interface phenomena between the Pb-free solder and the substrate were studied. Sn-3.5Ag solder alloy was selected in this work. The phases and the morphologies of the intermetallic compounds over Cu and Ni plates were observed. $η-Cu_6Sn_5$ phase was first formed on Cu plate and its shape looks like a scallop type. $Ni_3Sn_4$ and $Ni_3Sn$ phases were first formed on Ni plate and the morphology of $Ni_3Sn_4$ phase was layer type.
It was found that the interfacial phenomena during the soldering depended on the solubility of the substrate element in the molten solder. And the growth behavior of the intermetallic compound was closely related with the initial morphology just after soldering due to the high interfacial energy between the
solid phases.
It could be predicted well that the phase formed first during the soldering was determined by the lowest value of $\frac{ΔH^3_m} {ΔG^2_d}$. It was confirmed by other`s experimental results. And it was agreed well between the prediction using the value of $\frac{ΔH^3_m} {ΔG^2_d}$ and the experimental observation. The morphology of intermetallic compounds during the soldering was dependent on the Jackson`s $α^*$ parameter. As the value of $α^*$ parameter decreased, the morphologies of the intermetallic compounds were rougher. The value of $α^*$ parameter of the $η-Cu_6Sn_5$ phase was lower value than that of the Ni_3Sn_4. Thus the former looked rougher than the latter. The morphology prediction trough the calculation of the value of $α^*$ parameter could be applied to the other intermetallic compounds.
Sn-3.5Ag-X(X=In, Cu, Ni) ternary systems were studied for solder alloys. To lower the melting temperature of Sn-3.5Ag solder, Indium was alloyed. Through the thermodynamic calculation of Sn-3.5Ag-xIn system, the addition of 9wt% In goes down the liquidus temperature below 210℃. The mechanical property of this system was deteriorated by the In addition. However, by the fast cooling process, the mechanical property of Sn-2.5Ag-xIn system could be improved as good as that of Sn-3.5Ag. The wetting property of Sn-3.5Ag-xIn system over the Cu plate was improved, not by the surface energy between the liquid solder and the vapor, but by the surface energy between the liquid solder and the solid substrate, because it may be lowered by an increase in the driving force for the intermetallic compound.
To improve the interfacial characteristics by restricting the dissolution of the substrate element into the molten solder, Cu and Ni elements were alloyed to Sn-3.5Ag system. Through the thermodynamic calculation, the amounts of Cu and Ni additions have to be below about 1.0 wt%, because the small amount of addition raises the liquidus temperature abruptly. The solder system which Cu was added to the solubility limit of Cu in Sn-3.5Ag at 250℃ has the smallest intermetallic compound thickness over the Cu plate after soldering. In the solder system that Cu was added over than Cu solubility limit, η-Cu_6Sn_5 phases were found near the interface between the solder and the substrate, and they affected the growth behavior during aging.
Sn-3.5Ag-6.5In, Sn-3.5Ag-0.5Cu, Sn-3.5Ag-1.0Cu, Sn-3.5Ag-1.5Cu and Sn-3.5Ag-0.5Ni solder alloys were jointed on printed circuit boards(PCBs). The substrate consisted of Cu pad/Ni layer(4㎛)/Au layer(0.6㎛). The shear strength was examined using the ball shear tests. The value of the shear strength in the Sn-3.5Ag-1.0Cu solder joint was highest after the soldering. However, the values of the shear strengths in the Sn-3.5Ag-0.5Cu and Sn-3.5Ag-0.5Ni joints were highest after the thermal cycling fatigue (T/C) test repeated 1000 times in the temperature range from -65℃ to 150℃. It may be resulted from the smallest intermetallic compound thickness among them. According to the results by the FEM analysis, the region on which the stress was concentrated after the T/C test could be found. And the path of the crack propagation could be obtained. It was found that the intermetallic compound thickness effected on the stress concentration mainly. From these results, because the intermetallic compounds in Sn-3.5Ag-0.5Cu and Sn-3.5Ag-0.5Ni systems grow slowly, these systems were promising for the replacement of the Sn-37Pb solder alloy.
