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SEM and EBSD analysis of the solder joint aged at 175 °C for 64 h. a SEM backscattered image, b EBSD inverse pole figure map, c magnified image at A location, and d magnified image at B location
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In this paper, in situ tensile tests under various amounts of deformation were performed on Sn3.0Ag0.5 Cu lead-free solder joints subjected to multi-reflow and isothermal aging processes by using a scanning electron microscope. Microstructure evolution and deformation behavior of the solder joints were observed. Effects of the intermetallic compoun...
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... 6 Sn 5 IMC forms the different morphologies by two mechanisms: (1) the dissolution of large amounts of Cu into the solder leads to the precipitation of Cu 6 Sn 5 in the form of long rods during solidification and (2) the Cu 6 Sn 5 at the interface may be broken into segments and then directly migrate into the solder joint. Figure 4 shows the microstructure and EBSD image of a solder joint aged at 175 °C for 64 h. The isothermal aging treatment does not change the number and size of the grains. ...
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... segments and then directly migrate into the solder joint. Figure 4 shows the microstructure and EBSD image of a solder joint aged at 175 °C for 64 h. The isothermal aging treatment does not change the number and size of the grains. However, both the Ag 3 Sn and the Cu 6 Sn 5 IMCs inside the solder matrix were aggregated and coarsened, as shown in Fig. 4c. The classical thermodynamics theory assumes that the atoms diffuse rapidly at high temperature, resulting in the second phase coarsening by volume dif- fusion mechanism. The coarsening speed is related to the surface energy. Since the b-Sn has the lattice body-cen- tered cubic (b.c.c), and Ag 3 Sn and Cu 6 Sn 5 have ortho- rhombic and ...
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... the surface energy. Since the b-Sn has the lattice body-cen- tered cubic (b.c.c), and Ag 3 Sn and Cu 6 Sn 5 have ortho- rhombic and hexagonal lattices, respectively, incoherent grain boundaries will accelerate the coarsening process of the second phases. The size of the Cu 6 Sn 5 particles is larger than that of the Ag 3 Sn particles, as shown in Fig. 4b. This may be attributed to Cu diffusion from the substrate into the bulk solder during the aging process, aggregating around the Cu 6 Sn 5 particles, and subsquently reacting with Sn to form additional Cu 6 Sn 5 . As shown in Fig. 4c, it was found that Cu 6 Sn 5 is prone to nucleate and grow at the grain and phase boundaries, where ...
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... the second phases. The size of the Cu 6 Sn 5 particles is larger than that of the Ag 3 Sn particles, as shown in Fig. 4b. This may be attributed to Cu diffusion from the substrate into the bulk solder during the aging process, aggregating around the Cu 6 Sn 5 particles, and subsquently reacting with Sn to form additional Cu 6 Sn 5 . As shown in Fig. 4c, it was found that Cu 6 Sn 5 is prone to nucleate and grow at the grain and phase boundaries, where these boundaries pos- sess many defects with high energy. Similarly, the high energy of the grain and phase boundaries leads to the formation of the Ag 3 Sn particles. The thickness of the IMC layers formed at the interface between the ...
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... the Ag 3 Sn particles. The thickness of the IMC layers formed at the interface between the solder and the Cu substrate is significantly increased after 64 h aging. The layer adjacent to the Cu substrate is Cu 3 Sn and the layer adjacent to the solder is Cu 6 Sn 5 . Compared to the joint formed at the reflow condition, the scalloped Cu 6 Sn 5 in Fig. 4c is planarized. and EBSD analysis of the as-reflowed solder joint. a SEM backscattered image, b EBSD inverse pole figure map, and c magnified image at location A From the above analysis, it can be found that the solder joints only contain a few Sn grains. Kinyanjui et al. [19] reported that the supercooling degree increases with the ...
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... (Fig. 6). As shown in Fig. 3d, some of the Cu 6 Sn 5 IMCs present short hollow shaped morphology, which were precipitated during the solidification of solder joints or migrate from the interfacial IMCs. After aging, Cu 6 Sn 5 in the joint coarsens and becomes ball-shaped or rod-shaped, and then aggregates at the grain boundaries, as shown in Fig. 4c. The different morphologies and distribution of Cu 6 Sn 5 IMCs in the solder joints subjected to the multi-reflow process and isothermal aging significantly affect the fracture behavior of solder joints during in situ tensile testing. Figure 8 presents the facture behavior of short hollow Cu 6 Sn 5 IMCs in the multi-reflowed solder ...
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Citations
... In the solder matrix, the diffused Cu atoms will adhere to Cu6Sn5 particles to nucleate and react with Sn around the Cu6Sn5 particles. After several reflow soldering, Cu6Sn5 phases were also found by Tian[13], and two mechanisms of Cu6Sn5 growth were proposed: (1) the dissolution of large amounts of Cu into the solder leads to the precipitation of Cu6Sn5 in the form of long rods during solidification; and (2) the Cu6Sn5 at the interface may be broken into segments and then directly migrate into the solder joints; long Cu6Sn5 whiskers are easily broken into many small segments during in situ tensile test, and the crack can propagate and induce the failure of solder joints. The Cu6Sn5 can grow out as a hexagonal rod along a screw dislocation using the ledge mechanism[14]; this proposed mechanism of intermetallic formation incorporates the theory whereand (2) the Cu 6 Sn 5 at the interface may be broken into segments and then directly migrate into the solder joints; long Cu 6 Sn 5 whiskers are easily broken into many small segments during in situ tensile test, and the crack can propagate and induce the failure of solder joints. ...
Cu6Sn5 whiskers precipitated in Sn3.0Ag0.5Cu/Cu interconnection in concentrator silicon solar cells solder layer were found and investigated after reflow soldering and during aging. Ag3Sn fibers can be observed around Cu6Sn5 whiskers in the matrix microstructure, which can play an active effect on the reliability of interconnection. Different morphologies of Cu6Sn5 whiskers can be observed, and hexagonal rod structure is the main morphology of Cu6Sn5 whiskers. A hollow structure can be observed in hexagonal Cu6Sn5 whiskers, and a screw dislocation mechanism was used to represent the Cu6Sn5 growth. Based on mechanical property testing and finite element simulation, Cu6Sn5 whiskers were regarded as having a negative effect on the durability of Sn3.0Ag0.5Cu/Cu interconnection in concentrator silicon solar cells solder layer.
... Grain coarsening generally leads to a reduction in bond strength of solder joints and is a major reliability problem in electronics. Especially when intermetallics are still thin, embrittlement due to grain coarsening can be the dominant failure mode [24, 25, 26]. Investigations of field aged PV modules reveal that fatigue cracking within coarsened solder joints is commonly observed after prolonged outdoor exposure [27, 28, 29, 30]. ...
Intermetallic compounds (IMC) in soldered interconnections influence the reliability of PV modules. Thus, the microstructure of solar cell interconnections and the growth of IMCs are to be investigated in this paper. Sn60Pb40 and Sn41Bi57Ag2 are chosen as alloy coatings of copper interconnectors and semi-automatically soldered to screen-printed front Ag-busbars of industrial mono-crystalline solar cells. The microstructure of the solder bonds is characterized with metallographic cross sections and confocal laser microscopy, as well as scanning electron microscopy and electron dispersive x-ray spectroscopy. The cross section samples are isothermally aged between 85°C to 150°C and for 15 hour to 155 hour to obtain the kinetic parameters of a diffusion-based growth model of the IMCs. The model is used to estimate the IMC thickness after 3000h at 85°C, and after 600 thermal cycles as well as after 25 years in the outdoor location Freiburg, Germany. It is found that extensive microstructural changes take place within the solder bonds during thermal aging. Grain coarsening within the solder matrix, in particular for Sn41Bi57Ag2 solders, is observed, which can lead to an entire Sn depletion of the solder matrix. Moreover, non-uniform Sn penetration and IMC growth at cavities and lead-glass particles of the busbar are observed for both solders, which is discussed in terms of its effect on metallization adhesion. Eventually, simulating the IMC growth for 3000h at 85°C forecasts a 3.7μm thick Ag3Sn IMC at the busbar for the Sn41Bi57Ag2 solder compared to 2.6μm for Sn60Pb40. The prognosis of the IMC thickness after 25 years in Freiburg yields an Ag3Sn thickness of 1.3μm for Sn41Bi57Ag2.
Multiple reflow processes are utilized in complex electronic devices with multi-layered PCBs, stacked components, or 3D configurations to ensure reliable connections and minimize defects. A systematic analysis was performed to examine the influence of multiple reflow cycles on the microstructure and interfacial reaction of In–35Sn microalloyed with 0.05 wt% Ni on a Cu substrate. The size of the primary IMC particles in the In–35Sn solder was observed during multiple reflow process on Cu using in-situ real-time synchrotron radiography imaging. Multiple reflow soldering caused the primary Cu(In,Sn)2 and interfacial Cu3(Sn,In) layer to coarsen. The Ni was present in the primary (Cu,Ni)1(In,Sn)2 particles and interfacial (Cu,Ni)3(Sn,In) layer and caused grain size refinement of the IMCs after multiple reflow soldering. The shear test showed that there was a minor increase in shear force after multiple reflows. However, the shear energy decreased, indicating a more notable decrease in displacement to fracture rather than shear force which suggests a higher tendency towards ductile fracture.
Heterogeneous integration is leading to unprecedented miniaturization of solder joints, often with thousands of joints within a single package. The thermomechanical behavior of such SAC solder joints is critically important to assembly performance and reliability, but can be difficult to predict due to the significant joint-to-joint variability caused by the stochastic variability of the arrangement of a few highly-anisotropic grains in each joint. This study relies on grain-scale testing to characterize the mechanical behavior of such oligocrystalline solder joints, while a grain-scale modeling approach has been developed to assess the effect of microstructure that lacks statistical homogeneity. The contribution of the grain boundaries is modeled with isotropic cohesive elements and identified by an inverse iterative method that extracts material properties by comparing simulation with experimental measurements. The properties are extracted from the results of one test and validated by verifying reasonable agreement with test results from a different specimen. Equivalent creep strain heterogeneity within the same specimen and between different specimens are compared to assess typical variability due to the variability of microstructure.
The trend of electrification and miniaturization brings the reliability of interconnects to the forefront. The solder joints are subjected to thermo-mechanical mismatch due to the Joule heating and electromigration (EM)-induced degradation under high current stressing conditions. However, most of the studies are limited to lab test samples or lab test conditions, which lack transferability in engineering applications. In this paper, automotive electronic components are studied subjected to field-like current pulse conditions. The work aims to understand the field-relevant failure modes of solder joints exposed to high current stressing via experimental and numerical investigations. Shunt components with and without Ni plating are analyzed in power cycling tests. A polarity effect is observed in the growth of intermetallic compounds (IMCs). The thickness of IMC at the anode exhibits an approximately linear growth with current stressing time, whereas the thickness of IMC at the cathode remains almost unchanged or slightly decreased. Ni, as diffusion barrier, serves to restrain Cu consumption from the terminals of shunts and also reduces Cu mass transport into and within the solder. The net Cu flux at the interface and in the solder is discussed to analyze the IMC growth kinetics. The shunts with Ni plating undergo less EM of Cu and yield longer lifetime. Voids and crack formation are detected in computer tomography (CT) scans and cross-sectional analysis by scanning electron microscopy (SEM). Finite element method (FEM) is implemented to simulate thermo-electro-mechanical fields of the assembly under power cycling test conditions. The distribution of divergence of material flux density corresponds well with the location of voids formation and provides good estimation for the lifetime.
This paper details an investigation into the microstructure, thermal behaviors and joint strength of Sn-0.7Cu-1.5Bi solder alloy on electroless nickel immersion gold (ENIG) surface finish. Besides conventional techniques, the real-time synchrotron imaging was used to analyze the microstructure evolution in Sn-0.7Cu-1.5Bi/ENIG. This research investigated the growth behavior of the primary (Cu,Ni)6Sn5 intermetallic compounds (IMCs) in the solder joint with the Bi alloying. The elemental distribution analysis showed the Ni diffused from the ENIG surface finish and dissolved into the bulk solder during solidification and that the size of the primary (Cu,Ni)6Sn5 IMCs decreased due to the addition of 1.5 wt% Bi. The average kinetic growth rate of the primary (Cu,Ni)6Sn5 IMCs in Sn-0.7Cu-1.5Bi/ENIG was lower than that of the Sn-0.7Cu/ENIG. The thermal analysis revealed that the pasty range slightly increased and the undercooling degree decreased due to the addition of 1.5 wt% Bi for free-standing solder and soldering on ENIG surface finish. The shear strength of the Sn-0.7Cu-1.5Bi/ENIG was determined using a high-speed bond tester, and it increased by ∼12 % at bulk solder fracture of ∼15 % within the solder joint interfacial fracture due to the addition of 1.5 wt% Bi into the Sn-0.7Cu. These occurrences can be attributed to the solid solution strengthening effect at the bulk solder and the formation of finer interfacial (Cu,Ni)6Sn5 IMCs within the solder joints. The results indicated that the microstructural changes, especially the size reduction of IMCs, in Sn-0.7Cu-1.5Bi/ENIG impacted the joint strength.
This study examines factor(s) behind the formation of primary Cu6Sn5 (in the bulk, rather than at the interface) in solder joints, even though solder alloys are hypoeutectic. To understand the contribution from copper (Cu) dissolution from the substrate a Cu-free alloy, tin-3.5 silver (Sn-3.5Ag), was used as a soldered-on copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish substrates. Microstructure observations including in situ synchrotron were used to observe microstructure development real-time and confirm the time and location for nucleation of primary Cu6Sn5. High-speed shear tests were performed to determine the solder joint’s strengths. The results confirm that Cu dissolution during soldering is responsible for the formation of primary Cu6Sn5. The ENIG finish prevented Cu dissolution and the formation of Cu6Sn5 resulting in higher solder joint strength for the Sn-3.5Ag/ENIG solder joints. The findings can be used to understand the evolution of primary Cu6Sn5 and how it can be suppressed to improve joint strength.
The creep behaviour and microstructural evolution of a Sn-3Ag-0.5Cu wt.% sample with a columnar microstructure have been investigated through in-situ creep testing under constant stress of 30 MPa at ∼298 K. This is important, as 298 K is high temperature within the solder system and in-situ observations of microstructure evolutions confirm the mechanisms involved in deformation and ultimately failure of the material. The sample has been observed in-situ using repeat and automatic forescatter diode and auto electron backscatter diffraction imaging. During deformation, polygonisation and recrystallisation are observed heterogeneously with increasing strain, and these correlate with local lattice rotations near matrix-intermetallic compound interfaces. Recrystallised grains have either twin or special boundary relationships to their parent grains. The combination of these two imaging methods reveal one grain (loading direction, LD, 10.4 ° from [100]) deforms less than the neighbour grain 2 (LD 18.8° from [110]), with slip traces in the strain localised regions. In grain 1, (11¯0)[001] slip system are observed and in grain 2 (11¯0)[1¯1¯1]/2 and (110)[1¯11]/2 slip systems are observed. Lattice orientation gradients build up with increasing plastic strain and near fracture recrystallisation is observed concurrent with fracture.
This paper elucidated the effects of the Bi element (0 wt%, 0.5 wt% and 1.5 wt%) on the microstructure, electrical, wettability and mechanical properties of the Sn-0.7Cu-0.05Ni as a high strength solder. Besides using the conventional cross-sectioned microstructure image, the real-time synchrotron radiation imaging and synchrotron micro-X-ray fluorescence (XRF) technique was also used to investigate the microstructure, focusing on the in-situ growth behaviour of the primary (Cu,Ni)6Sn5 intermetallic and elemental distribution that had occurred in the Sn-0.7Cu-0.05Ni-1.5Bi. Other essential properties of solder material, such as wettability, electrical resistance, and shear strength, were also determined. The results showed that the addition of 1.5 wt% Bi refined the primary (Cu,Ni)6Sn5 intermetallics formation in the solder joint, where it grew earlier and faster relative to that in the Sn-0.7Cu-0.05Ni/Cu joint. Additionally, the addition of 1.5 wt% Bi resulted with a 3% reduction of its electrical resistance while increasing the wettability of the solder alloy. 1.5 wt% addition of the Bi element also found to have contributed to a significant increment of shear strength relative to that of the Sn-0.7Cu-0.05Ni. The results confirmed that the developed material is applicable as a potential high strength solder material in the context of advanced interconnecting applications. Keywords: Soldering, Solder, Interconnects, Intermetallics, Solid solution, Microstructure, Solder properties
Environmental-friendly Sn-Ag-Cu-based composite material was prepared through evenly mixing of similar density nonreacting samarium oxide (Sm 2 O 3 ) nanoparticles into Sn-3Ag-0.5Cu (wt%) powders. Further, the interfacial structural morphology and strength on various surface-finished e.g., Cu, Au/Ni-plated and Ag-plated Cu pads were studied on aging period with different temperatures. At low-temperature aging, a layer-type Cu 6 Sn 5 intermetallic compound (IMC) phase was detected on Cu and Ag-plated Cu pad surface. Further, a thin film-type Cu 3 Sn IMC phase was grown during the aging treatment. Moreover, in Au/Ni-plated Cu substrate, a (Cu, Ni) 6 Sn 5 IMC phase was grown at the pad surface. However, these IMC layers become thicker with the aging temperature and time. Additionally, in the solder matrix, a spherical-shape Cu 6 Sn 5 and needle-shape Ag 3 Sn IMC phases were evenly dispersed in β-Sn matrix. However, it was worthy noted that these IMC particles and layers growth nature of interconnection made by composite material was slower than that of reference solder systems. This phenomenon was occurred due to set the surface-active Sm 2 O 3 nanoparticles on IMC phase. Moreover, nanoparticles doped composite material ensured the higher shear strength due to reduce the growth rate of IMC thickness, control the IMC particles and evenly distributed of Sm 2 O 3 nanoparticles in the solder matrix.
