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Since manufacturing is transitioning into Pb-free solder development for electronic assembly and packaging, consumer demands for more compact electronics have increased the importance of reliability issues brought about by the increased density of circuitry. Therefore, the role of alloying elements that can be added to common Pb-free solders such as Sn-Cu or Sn-Ag-Cu alloys in enhancing the properties and performance of the solder becomes important. Microstructural analysis along with direct observation in situ synchrotron radiography were used to study the effect of Ni, Zn, Au, In and Sb on the development of phases in Sn-10 wt.%Cu (Sn-10Cu). It was found that adding Ni and Zn to a Sn-10Cu alloy had the greatest impact on the microstructure with the Cu 3 Sn phase completely absent after these additions were made. Additions of Au and In also resulted in a reduction in the amount of Cu 3 Sn; however, the effect was not as pronounced. Removing the Cu 3 Sn phase from Sn-Cu Pb-free solder alloys is a possible approach for the design of more desirable microstructures that translate to better performance in modern electronic packaging.

The increased demand for microelectronic devices that function in hotter environments compels the study of Pb-free solders containing solid solution dispersoids (such as Bi and Sb), which are stable at significant concentrations in Sn at temperatures close to 200°C. In this study, the growth of Ni3Sn4 intermetallic compounds was examined at Ni/Sn-3.7Ag-0.65Cu-3.0Bi-1.43Sb-0.15Ni solder interfaces at temperatures up to 200°C, and compared to growth of Ni3Sn4 at Ni/Sn-3.5Ag interfaces, under the same conditions. The growth of Ni3Sn4 layers thicker than 5 μm was correlated with the formation of voids in the solder near the Ni3Sn4 interface, and an order of magnitude increase in the reaction constant. An almost continuous line of voids formed at the Sn/solder interface, some time after the initial formation of voids in these diffusion couples. Continued heat treatment resulted in continued growth of both the voids and the Ni3Sn4 layer, in direct proportion, consistent with a dominant Sn vacancy diffusion mechanism in the growing Ni3Sn4 layer (the ratio of average Ni3Sn4 thickness to average void thickness was one). At Ni/Sn-3.7Ag-0.65Cu-3.0Bi-1.43Sb-0.15Ni solder interfaces this occurred after only 250 h at 175°C; significant effects on the reliability of such solder joints would be expected.

The effect of a minor addition of Ti on the mechanical behavior of Zn-25Sn-xTi (x = 0 wt.%, 0.02 wt.% and 0.04 wt.%) solder alloy at high temperatures of 80°C, 100°C, and 120°C was investigated. The investigation revealed that Ti acted as nucleating agent. The grain size of the Zn-25Sn alloy was significantly refined with the addition of 0.02%Ti. The Zn-25Sn-0.02Ti exhibited the greatest elongation at all test temperatures. An excess addition of Ti (more than 0.04%) was found to cause the formation of ternary TiSn4Zn5 compounds, which is correlated with the degradation of elongation. The fractographs of the solders at high temperature revealed the presence of the TiSn4Zn5 compound in the dimple bottom, indicating that voids nucleated at the particles.

The complex reaction between liquid solder alloys and solid substrates has been studied ex-situ in a few studies, utilizing creative setups to “freeze” the reactions at different stages during the reflow soldering process. However, full understanding of the dynamics of the process is difficult due to the lack of direct observation at micro- and nano-meter resolutions. In this study, high voltage transmission electron microscopy (HV-TEM) is employed to observe the morphological changes that occur in Cu6Sn5 between a Sn-3.0 wt%Ag-0.5 wt%Cu (SAC305) solder alloy and a Cu substrate in situ at temperatures above the solidus of the alloy. This enables the continuous surveillance of rapid grain boundary movements of Cu6Sn5 during soldering and increases the fundamental understanding of reaction mechanisms in solder solid/liquid interfaces.

The high performance and downsizing technology of three-dimensional integrated circuits (3D-ICs) for mobile consumer electronic products have gained much attention in the microelectronics industry. This has been driven by the utilization of chip stacking by through-Si-via and solder microbumps. Pb-free solder microbumps are intended to replace conventional Pb-containing solder joints due to the rising awareness of environmental preservation. The use of low-volume solder microbumps has led to crucial constraints that cause several reliability issues, including excessive intermetallic compounds (IMCs) formation and solder microbump embrittlement due to IMCs growth. This article reviews technologies related to 3D-ICs, IMCs formation mechanisms and reliability issues concerning IMCs with Pb-free solder microbumps. Finally, future outlook on the potential growth of research in this area is discussed.

Subgrains and recrystallisation are common microstructural features in solder joints that have been subjected to thermal fatigue or mechanical loading. Here we study similar features in Sn-Ag and Sn-Ag-Cu solder balls after solidification. It is shown that four types of misorientation features exist to different extents in solder balls examined shortly after solidification: (i) small variations in orientation created by dendrite growth and eutectic solidification, (ii) partial polygonisation into subgrains, (iii) small grains with high angle boundaries and (iv) large anomalous grains surrounded by interlacing where the grain boundaries do not correlate with the dendrite growth pattern. The subgrains often have a boundary plane and rotation axis consistent with dislocations from a facile slip system. The recovery and misorientation features were more extensive in regions that solidified at deeper melt undercooling. The findings highlight the importance of distinguishing between the solidification and solid-state components of microstructure evolution when interpreting solder microstructures after solidification.

For reliable electronics, it is important to have an understanding of solder joint failure mechanisms. However, because of difficulties in real-time atomistic scale analysis during deformation, we still do not fully understand these mechanisms. Here, we report on the development of an innovative in situ method of observing the response of the microstructure to tensile strain at room temperature using high-voltage transmission electron microscopy (HV-TEM). This technique was used to observe events including dislocation formation and movement, grain boundary formation and separation, and crack initiation and propagation in a Sn-3 wt.%Ag-0.5 wt.%Cu (SAC305) alloy joint formed between copper substrates.

Ag sinter joining technology has gained increasing attention for its excellent thermal and mechanical properties, especially for high-temperature applications. This study focuses on the lifetime prediction of a SiC power module by Ag sinter joining based on mechanical properties including tensile, fatigue, and creep properties from room temperature to 200°C, as well as thermal properties including thermal conduction and the coefficient of thermal expansion. These mechanical properties and thermal properties of sintered Ag paste were evaluated in the study and the results show that mechanical properties of sintered Ag largely depend on the test temperature. The sintered Ag paste tends to soften at high temperature, and the fracture changed from nearly brittle to totally ductile with the testing temperature increase. From the S–N curve, the fatigue is close to the Morrow equation but not the Coffin–Manson law at room temperature. The finite element simulation of the lifetime based on Morrow’s equation for the sintered Ag layer shows that there has a crack occurrence with one fourth the side length after 10,000 cycles from − 40°C to 200°C but the crack extension area is less than one tenth of the sintered Ag layer. This study will add to the understanding of the high temperature properties and high temperature reliability as well as the lifetime of Ag sinter joining in high-temperature applications.

Ga and Ga-based alloys have received significant attention due to their potential application in the liquid state for low-temperature bonding in microelectronics. This study investigated the interfacial reactions between liquid Ga and pure Cu substrates at room temperature. The directional thermal expansion behaviour of the resulting CuGa2 was analysed by synchrotron x-ray powder diffraction with supporting observations of single crystal foils in high-voltage transmission electron microscopy. The mechanical properties of CuGa2 were evaluated by nano-indentation. CuGa2 was found to have advantages over other intermetallics that are present in assemblies made with current generation lead-free solders, including Ag3Sn, Cu6Sn5 and Cu3Sn. In addition to enabling lower process temperatures, solder alloys that form CuGa2 at the interface with Cu offer the possibility of providing more reliable connections in the very small joints that play an increasingly important role as the trend to miniaturisation of electronics continues.

The diminutive additions of nickel (Ni) element have been fused to Sn-1.5Ag-0.5 mass% Cu (SAC155) lead-free solder alloy. The study was examined experimentally and computationally (using JMatPro software program) the microstructural features, thermal behavior, density, thermal diffusivity, and conductivity as well as tensile stress strain of the Sn-1.5Ag-0.5Cu-x mass %Ni (x = 0.00, 0.05, 0.10, 0.20, and 0.50) solder alloys (SAC155-xNi). The fusing additions of Ni have a little impact on the melting point of SAC155 alloy which increasing from 502 to 504.2 K . The microstructure of SAC155 solder alloy included coarse grains of β-Sn besides, large eutectic regions, and embedded IMCs of Ag 3 Sn and Cu 6 Sn 5 . The computed values of Gibbs free energy (G) during solidification of the β-Sn phase, Ag 3 Sn, and Sn 3 Sn 4 IMCs show the stability at - 130.7 × 10 3 J .kg - 1 , - 157.7 × 10 3 J .kg - 1 , and - 377.13 × 10 3 J .kg - 1 , respectively. The SAC155-xNi alloys involved finer β-Sn grains, large fibrous eutectic regions, (Cu,Ni) 6 Sn 5 and (Cu,Ni) 3 Sn 4 IMCs. The G of Cu 6 Sn 5 decreased from - 233.1 × 10 3 J .kg - 1 to - 317.9 × 10 3 J .kg - 1 when Ni content increased up to 0.25 mass %, then stable and steady at - 318.4 × 10 3 J .kg - 1 with more addition of Ni element. The formation of the (Cu, Ni) 6 Sn 5 and (Ni, Cu) 3 Sn 4 IMCs is motivated by the lowest Gibbs free energy, especially when Ni mass% is added to a sufficient level. The measured and computed values of specific heat at constant pressure ( C p ) of SAC155-xNi alloys show a good matching especially at lower temperatures and a little mismatch at high temperatures which decreases with increasing Ni content. The activation energy of atomic arrangement (Q) increased from 11.36 to 13.41 kJ .mole - 1 with increasing Ni content in SAC155-xNi alloys. Thermal diffusivity (α) and conductivity (κ) of SAC155-xNi gradually decreased with increasing temperature in the range from 303 to 423 K and/or Ni content in alloys. The measured values of (κ) are slightly lower than the computed value at similar temperatures of the SAC155-xNi alloys. The decrease in (κ) may be assigned to scattering the electrons or reducing the phonon contributions due to the presence of various solute atoms and different IMCs. The results of stress-strain graphs reveal the enhancement of YS and UTS for all Ni-containing alloys. The improvement of the yield stress (YS) and ultimate tensile strength (UTS) of Ni-containing alloys is attributed to the uniform distribution of the IMCs, the reduction of β-Sn grain size, and the smoothed enlargement of the eutectic region.