Suppressing effect of 0.5 wt.% nano-TiO2 addition into Sn-3.5Ag-0.5Cu solder alloy on the intermetallic growth with Cu substrate during isothermal aging

This study investigated the effects of adding 0.5 wt.% nano-TiO₂ particles into Sn3.5Ag0.5Cu (SAC) lead-free solder alloys on the growth of intermetallic compounds (IMC) with Cu substrates during solid-state isothermal aging at temperatures of 100, 125, 150, and 175 °C for up to 7 days. The results indicate that the morphology of the Cu₆Sn₅ phase transformed from scallop-type to layer-type in both SAC solder/Cu joints and Sn3.5Ag0.5Cu-0.5 wt.% TiO₂ (SAC) composite solder/Cu joints. In the SAC solder/Cu joints, a few coarse Ag₃Sn particles were embedded in the Cu₆Sn₅ surface and grew with prolonged aging time. However, in the SAC composite solder/Cu aging, a great number of nano-Ag₃Sn particles were absorbed in the Cu₆Sn₅ surface. The morphology of adsorption of nano-Ag₃Sn particles changed dramatically from adsorption-type to moss-type, and the size of the particles increased.The apparent activation energies for the growth of overall IMC layers were calculated as 42.48 kJ/mol for SAC solder and 60.31 kJ/mol for SAC composite solder. The reduced diffusion coefficient was confirmed for the SAC composite solder/Cu joints.     

Morphology and growth kinetics of Ag3Sn during soldering reaction between liquid Sn and an Ag substrate

For the soldering of recycled Ag sputtering targets, the interfacial reaction between liquid Sn and an Ag substrate at temperatures ranging from 250 –425°C has been investigated. Experimental results show that a scallop-shaped layer of Ag₃Sn intermetallic compounds formed during the soldering reaction. Kinetics analysis indicated that the growth of such interfacial Ag₃Sn intermetallic compounds is diffusion-controlled with activation energy of 70.3kJ/mol. During the reaction, the Ag substrate dissolves into the molten Sn solder and causes the appearance of needle-shaped Ag₃Sn precipitates in the Sn matrix.     

Microstructure, kinetic analysis and hardness of Sn-Ag-Cu-1 wt% nano-ZrO2 composite solder on OSP-Cu pads

Nano-sized, nonreacting, noncoarsening ZrO₂ particle-reinforced Sn-Ag-Cu composite solders were prepared by mechanically dispersing ZrO₂ nano-particles into Sn-Ag-Cu solder and the interfacial morphology between the solder and organic solderability preservative (OSP)-Cu pads were characterized metallographically. At their interfaces, island-shaped Cu₆Sn₅ and Cu₃Sn intermetallic compound (IMC) layers were found in solder joints with and without the ZrO₂ particles and the IMC layer thickness was substantially increased with reaction time and temperature. In the solder ball region, needle-shaped Ag₃Sn and spherically-shaped Cu₆Sn₅ IMC particles were found to be uniformly distributed in the β-Sn matrix. However, after the addition of ZrO₂ nano-particles, Ag₃Sn and Cu₆Sn₅ IMC particles appeared with a fine microstructure and retarded the growth rate of the IMC layers at their interfaces. From a kinetic analysis, the calculated activation energies for the total (Cu₆Sn₅ + Cu₃Sn) IMC layers for Sn-Ag-Cu and Sn-Ag-Cu-1 wt% ZrO₂ composite solder joints on OSP-Cu pads were about 53.2 and 59.5 kJ/mol, respectively. In addition, solder joints containing ZrO₂ nano-particles displayed higher hardness due to the uniform distribution of ZrO₂ nano-particles as well as the refined IMC particles. The hardness values of the plain Sn-Ag-Cu solder joint and solder joints containing 1 wt% of ZrO₂ nano-particles after 5 min reaction at 250 °C were about 15.0 Hv and 17.1 Hv, respectively. On the other hand, their hardness values after 30 min reaction were about 13.7 Hv and 15.5 Hv, respectively.     

Microstructural evolution of intermetallic compounds in Sn–3.5Ag–X (X = 0, 0.75Ni, 1.0Zn and 1.5In)/Cu solder joints during liquid aging

In this paper, the microstructural evolution of IMCs in Sn–3.5Ag–X (X = 0, 0.75Ni, 1.0Zn, 1.5In)/Cu solder joints and their growth mechanisms during liquid aging were investigated by microstructural observations and phase analysis. The results show that two-phase (Ni₃Sn₄ and Cu₆Sn) IMC layers formed in Sn–3.5Ag–0.75Ni/Cu solder joints during their initial liquid aging stage (in the first 8 min). While after a long period of liquid aging, due to the phase transformation of the IMC layer (from Ni₃Sn₄ and Cu₆Sn phases to a (Cu, Ni)₆Sn₅ phase), the rate of growth of the IMC layer in Sn–3.5Ag–0.75Ni/Cu solder joints decreased. The two Cu₆Sn₅ and Cu₅Zn₈ phases formed in Sn–3.5Ag–1.0Zn/Cu solder joints during the initial liquid aging stage and the rate of growth of the IMC layers is close to that of the IMC layer in Sn–3.5Ag/Cu solder joints. However, the phase transformation of the two phases into a Cu–Zn–Sn phase speeded up the growth of the IMC layer. The addition of In to Sn–3.5Ag solder alloy resulted in Cu₆(Sn_x,In_1?x)₅ phase which speeded up the growth of the IMC layer in Sn–3.5Ag–1.5In/Cu solder joint.     

Evolution of Ag3Sn at Sn-3.0Ag-0.3Cu-0.05Cr/Cu joint interfaces during thermal aging

The intermetallic compounds (IMC) in the solder and at the interface of Sn-3.0Ag-0.5Cu (SAC)/Cu and Sn-3.0Ag-0.3Cu-0.05Cr (SACC)/Cu joints were investigated after isothermal aging at 150 °C for 0, 168 and 500 h. Different shaped Ag₃Sn phases were found near the IMC layer of the latter joint. Interestingly, fine rod-shaped and branch-like Ag₃Sn were detected near the interface after soldering and long Ag₃Sn changed into shorter rods and small particles during aging. It is investigated that the Cr addition and thermal aging have effect on the evolution of Ag₃Sn morphologies and it is controlled by interfacial diffusion. Energy minimization theory and the redistribution of elements are used to explain the morphological evolution of Ag₃Sn. Small Ag₃Sn particles were also found on the IMC layer after aging, unlike the large Ag₃Sn at that of SAC/Cu joints. In conclusion, a favorable morphology of the joint interface leads to better bonding properties for SACC/Cu joints against thermal aging than that for SAC/Cu.     

Minor Ga addition to effectively inhibit PdSn4 growth between Sn solder and Pd substrate

PdSn₄ is the major reaction phase in the Sn-based or Sn–Ag–Cu solder joints with Pd substrate and it exhibits an extremely high growth rate. Ga is considered as a candidate alloying element in the Sn–Ag–Cu solders. This paper investigates the effects of Ga addition on the interfacial reactions between Pd and Sn–Ga (0.1wt.%–1wt.%Ga) solders by solid-state aging at 160, 180, and 200 °C and liquid-state aging at 250 °C. The most important finding is that minor Ga addition can effectively inhibit the fast PdSn₄ growth. In the solid-state reaction, with only 0.1wt.%Ga addition, the PdSn₄ growth was suppressed by ∼50%, compared with the pure Sn/Pd reaction. When the Ga content increased to 0.5wt.%, the PdSn₄ growth was further reduced by over 90%. In addition, a thin PdGa phase layer was formed at the interface between PdSn₄ and Pd, which was the main cause for the inhibition of PdSn₄ growth. The growth kinetics was systematically explored. The PdSn₄ growth had a higher activation energy for the higher Ga addition (>0.5wt.%). Furthermore, a similar reduction in the PdSn₄ growth was observed in the liquid-state reaction, but it was not as strong as in the solid-state reactions. The PdGa phase was not formed in the liquid-state reaction and the growth inhibition could be attributed to the Ga doping in the PdSn₄ lattice to retard the Sn diffusion.     

Investigation of small Sn–3.5Ag–0.5Cu additions on the microstructure and properties of Sn–8Zn–3Bi solder on Au/Ni/Cu pads

The formation of intermetallic compounds and the shear strength of Sn–Zn–Bi solder alloys with various (0, 1, 3, 5 and 7 wt%) weight percentages of Sn–Ag–Cu were investigated on Au/Ni metallized Cu pads depending on the number of reflow cycles. In Sn–Zn–Bi solder joints, scallop-shaped AuZn₃ intermetallic compound (IMC) particles were found at the interfaces and in the solder ball regions, fine Bi- and needle-shaped Zn-rich phase were observed in the Sn matrix. After Sn–Ag–Cu additions, an additional Ag–Zn intermetallic compound layer was adhered to the top surface of the AuZn₃ layer at the interface and fine spherical-shaped AgZn₃ intermetallic compound particles were detected in the solder ball regions together with Bi- and Zn-rich phase volumes. After the addition of Sn–Ag–Cu, the shear strength of Sn–Zn–Bi solder joints increased due to the formation of the fine AgZn₃ intermetallic compound particles. The shear strengths of Sn–Zn–Bi and Sn–Zn–Bi/7 wt% Sn–Ag–Cu solder joints after one reflow cycle were about 44.5 and 53.1 MPa, respectively and their shear strengths after eight reflow cycles were about 43.4 and 51.6 MPa, respectively.     

62Sn36Pb2Ag钎料中Ag元素对AgCu/SnPbAg/CuBe焊缝性能的影响

分别采用62Sn36Pb2Ag钎料和63Sn37Pb共晶钎料焊接AgCu合金块和CuBe合金片进行试验,对比分析两种钎料形成的焊缝性能和显微组织结构,阐述了62Sn36Pb2Ag钎料中Ag元素的存在对AgCu/SnPbAg/CuBe焊缝性能的影响机制.结果表明,62Sn36Pb2Ag钎料中的Ag元素对于润湿铺展状态的改...     

Effects of Pd concentration on the interfacial reaction and mechanical reliability of the Sn-Pd/Ni system

The liquid-solid reaction between Sn-xPd alloy and Ni (x = 0.05-1 wt.%) and the resulting mechanical reliability of the system were examined in this study. The reactions strongly depended on the Pd concentration and the reaction time. When the Pd concentration was low (i.e., x = 0.05 wt.%), the reaction product was only Ni₃Sn₄. In contrast, when the Pd concentration was high (i.e., x ≥ 0.2 wt.%), the reaction product became a dual-layer structure of (Pd,Ni)Sn₄-Ni₃Sn₄. Between 0.05 wt.% and 0.2 wt.% (e.g., x = 0.1 wt.%), discontinuous (Pd,Ni)Sn₄ grains scattered over the Ni₃Sn₄ layer developed. Interestingly, the (Pd,Ni)Sn₄ grains were gradually dispersed in the molten Sn-Pd alloy, leaving the Ni₃Sn₄ at the interface, as the reaction time increased. These Pd-dependent reactions were dictated by thermodynamics and can be rationalized using the Pd-Ni-Sn isotherm. Furthermore, the results of the high-speed-ball-shear (HSBS) test indicated that the mechanical strength of the Sn-Pd/Ni joints dramatically degraded by over one third due to the formation of (Pd,Ni)Sn₄ at the interface. The implication is that the Pd concentration in Sn-Pd solder joints should be reduced to a level below 0.2 wt.% to prevent the creation of an undesired microstructure.     

Solderability and intermetallic compounds formation of Sn-9Zn-xAg lead-free solders wetted on Cu substrate

The eutectic Sn-9Zn alloy was doped with Ag (0 wt.%-1 wt.%) to form Sn-9Zn-xAg lead-free solder alloys. The effect of the addition of Ag on the microstructure and solderability of this alloy was investigated and intermetallic compounds (IMCs) formed at the solder/Cu interface were also examined in this study. The results show that, due to the addition of Ag, the microstructure of the solder changes. When the quantity of Ag is lower than 0.3 wt.%, the needle-like Zn-rich phase decreases gradually. However, when the quantity of Ag is 0.5 wt.%-1 wt.%, Ag-Zn intermetallic compounds appear in the solder. In particular, adding 0.3 wt.% Ag improves the wetting behavior due to the better oxidation resistance of the Sn-9Zn solder. The addition of an excessive amount of Ag will deteriorate the wetting property because the glutinosity and fluidity of Sn-9Zn-(0.5, 1)Ag solder decrease. The results also indicate that the addition of Ag to the Sn-Zn solder leads to the precipitation of ε-AgZn₃ from the liquid solder on preformed interfacial intermetallics (Cu₅Zn₈). The peripheral AgZn₃, nodular on the Cu₅Zn₈ IMCs layer, is likely to be generated by a peritectic reaction L + γ-Ag₅Zn₈ → ɛ-AgZn₃ and the following crystallization of AgZn₃.     

In situ investigation of SnAgCu solder alloy microstructure

In situ X-ray diffraction experiments, using synchrotron radiation, were employed to analyze microstructure evolution of the 96.5Sn3Ag0.5Cu (wt.%)—SAC305 lead-free solder alloy during heating (30-240 °C), isothermal dwell (240 °C) and cooling (240-30 °C). The special emphasis was placed on the study of the melting and solidification processes, explaining formation, distribution and the order of crystallization of the crystal phases (β-Sn, intermetallic compounds) in the solder alloy. Furthermore, thermal expansion behaviour of the main constituent phase β-Sn was analyzed prior to melting and after the consequent solidification.     

Scaling effect on Ag3Sn growth behaviours in micro-joints for flip chip assemblies

The scaling effect on Ag₃Sn growth behaviours in Sn–3.0Ag–0.5Cu (SAC305) micro-joints of flip chip assemblies was investigated using thermal shock (TS) tests. After assembly reflow, the large plate-like Ag₃Sn compounds only emerged from the Cu interface of small joints, which was strongly related with a higher local Ag concentration in the remaining solder. During TS cycling, the growth of Ag₃Sn grains exhibited comparatively more pronounced growth in the solder matrix of small joints, due to the stronger strain-enhanced coarsening induced by more cycle stress and strain. Coarsening kinetic models based on TS experiments were employed to predict Ag₃Sn growth, the kinetic constants N were determined to clarify the correlation of the joints scaling and Ag₃Sn coarsening in depth.     

Kinetics of Ag3Sn growth in Ag–Sn–Ag system during transient liquid phase soldering process

The kinetics of the interfacial reaction of a thin layer of Sn sandwiched between two pieces of Ag foil has been investigated at temperatures of 260 °C, 300 °C and 340 °C. A time dependence of the form t^1/n with n = 3 was obtained for the kinetics of both the consumption of the Sn remaining and the thickening growth of the Ag₃Sn scallops formed between Sn and Ag. Such a result can be explained well using the model of grain boundary/molten channel-controlled growth of intermetallic compounds. In this case, the diffusion of Ag atoms through the molten channels existing between the previously formed Ag₃Sn scallops is the controlling mechanism for the kinetics. We also report here the derived kinetic constants including reaction constants and the associated activation energy for guiding the practical transient liquid phase soldering of the Ag–Sn–Ag system.     

Effect of adding 1 wt% Bi into the Sn–2.8Ag–0.5Cu solder alloy on the intermetallic formations with Cu-substrate during soldering and isothermal aging

Intermetallic compound (IMC) formations of Sn–2.8Ag–0.5Cu solder with additional 1 wt% Bi were studied for Cu-substrate during soldering at 255 °C and isothermal aging at 150 °C. It was found that addition of 1 wt% Bi into the Sn–2.8Ag–0.5Cu solder inhibits the excessive formation of intermetallic compounds during the soldering reaction and thereafter in aging condition. Though the intermetallic compound layer was Cu₆Sn₅, after 14 days of aging a thin Cu₃Sn layer was also observed for both solders. A significant increase of intermetallic layer thickness was observed for both solders where the increasing tendency was lower for Bi-containing solder. After various days of aging, Sn–2.8Ag–0.5Cu–1.0Bi solder gives comparatively planar intermetallic layer at the solder–substrate interface than that of the Sn–2.8Ag–0.5Cu solder. The formation of intermetallic compounds during aging for both solders follows the diffusion control mechanism and the diffusion of Cu is more pronounced for Sn–2.8Ag–0.5Cu solder. Intermetallic growth rate constants for Sn–2.8Ag–0.5Cu and Sn–2.8Ag–0.5Cu–1.0Bi solders were calculated as 2.21 × 10⁻¹⁷ and 1.91 × 10⁻¹⁷ m²/s, respectively, which had significant effect on the growth behavior of intermetallic compounds during aging.     

Growth Behavior of Intermetallic Compounds at SnAgCu/Ni and Cu Interfaces

The growth behavior of reaction-formed intermetallic compounds (IMCs) at Sn3.5Ag0.5Cu/Ni and Cu interfaces under thermal-shear cycling conditions was investigated. The results show that the morphology of (Cu _x Ni_1–x )₆Sn₅ and Cu₆Sn₅ IMCs formed both at Sn3.5Ag0.5Cu/Ni and Cu interfaces gradually changed from scallop-like to chunk-like, and different IMC thicknesses developed with increasing thermal-shear cycling time. Furthermore, Cu₆Sn₅ IMC growth rate at the Sn3.5Ag0.5Cu/Cu interface was higher than that of (Cu _x Ni_1–x )₆Sn₅ IMC under thermal-shear cycling. Compared to isothermal aging, thermal-shear cycling led to only one Cu₆Sn₅ layer at the interface between SnAgCu solder and Cu substrate after 720 cycles. Moreover, Ag₃Sn IMC was dispersed uniformly in the solder after reflow. The planar Ag₃Sn formed near the interface changed remarkably and merged together to large platelets with increasing cycles. The mechanism of formation of Cu₆Sn₅, (Cu _x Ni_1–x )₆Sn₅ and Ag₃Sn IMCs during thermal-shear cycling process was investigated.     

Effect of antimony on vigorous interfacial reaction of Sn-Sb/Te couples

A vigorous reaction between molten Sn-Sb solder and Te substrate leads to a thick SnTe-Sn mixture layer at the soldered junction. The effect of Sb addition in Sn on the kinetics of Sn-Sb/Te interfacial reaction and formation of SnTe-Sn reaction layer is reported. Sb element was found to expedite the growth of SnTe-Sn reaction layer in the Sn-Sb/Te couples dramatically. With increasing Sb content in Sn-Sb solder, the growth rate of SnTe-Sn reaction layer increases while both the size of SnTe grains and the fraction of Sn in SnTe-Sn decrease. The thickness of SnTe-Sn layer follows a parabolic law with reaction time for Sn-Sb/Te couples, unlike the linear dependence with time for Sn/Te couples. An apparent diffusivity of 2 × 10⁻⁷ cm²/s was determined for Sn transport through the SnTe-Sn layer in the Sn-Sb/Te couples reacting at 250 °C.     

Effects of sulfide-forming element additions on the Kirkendall void formation and drop impact reliability of Cu/Sn–3.5Ag solder joints

Ternary Pb-free solders, Sn–3.5Ag–X, containing 0.5 wt.% of Zn, Mn and Cr, were reacted with Cu UBM, which was electroplated using SPS additive. Characteristics of Cu–Sn IMCs and Kirkendall void formation at the Cu/Sn–3.5Ag solder joints were significantly affected by the third element, and the potency to suppress Kirkendall voids at the solder joint increased in the order of Cr, Mn, Zn, which was indeed the order of the drop reliability improvement. From the AES analyses, it was suggested that the sulfide-forming elements in the solder diffused into the Cu UBM and reduced the segregation of S atoms to the Cu/Cu₃Sn interface by scavenging S, which led to the suppression of Kirkendall void nucleation at the Cu/Cu₃Sn interface and the drop reliability improvement. In the case of the Zn-containing solder joint, Cu₃Sn phase, known to be a host of Kirkendall voids, did not form at all even after extended aging treatments. The magnitude of the tensile stress at the Cu₃Sn/Cu interface which drove the Kirkendall void growth was estimated to be 10–100 MPa.     

Effect of the addition of In on the microstructural formation of Sn–Ag–Zn lead-free solder

The effect of addition of In, up to 1 wt.%, on the formation of intermetallic compounds (IMCs) in the solidified Sn–3.7%Ag–0.9%Zn lead-free solder was investigated. As observed by microstructural analysis, the typical structure of Sn–Ag–Zn solder is composed of β-Sn phase and mixed granules of Ag₃Sn and AgZn IMCs. After alloying with In, it evolves into a mixture of randomly distributed rods and granules of Ag₃Sn and AgZn. Clearly, the addition of In into the explored Sn–Ag–Zn solder promotes the formation of rod-like IMCs for the reason that the growth competition of the Ag₃Sn and AgZn IMCs was destroyed by the selective adsorption of In atoms on a certain preferable crystalline planes of the separated IMCs. The change in the morphology of the formed IMCs leads to a great difference in the mechanical performances, for example, the measured microhardness of the investigated solders evolves from 16.95 HV to 21.35 HV with the increase of In content.     

Kinetics of intermetallic phase formation at the interface of Sn-Ag-Cu-X (X = Bi, In) solders with Cu substrate

The effects of Bi and In additions on intermetallic phase formation in lead-free solder joints of Sn-3.7Ag-0.7Cu; Sn-1.0Ag-0.5Cu-1.0Bi and Sn-1.5Ag-0.7Cu-9.5In (composition given in weight %) with copper substrate are studied. Soldering of copper plate was conducted at 250 °C for 5 s. The joints were subsequently aged at temperatures of 130-170 °C for 2-16 days in a convection oven. The aged interfaces were analyzed by optical microscopy and energy dispersive X-ray spectroscopy (EDX) microanalysis. Two intermetallic layers are observed at the interface - Cu₃Sn and Cu₆Sn₅. Cu₆Sn₅ is formed during soldering. Cu₃Sn is formed during solid state ageing. Bi and In decrease the growth rate of Cu₃Sn since they appear to inhibit tin diffusion through the grain boundaries. Furthermore, indium was found to produce a new phase - Cu₆(Sn,In)₅ instead of Cu₆Sn₅, with a higher rate constant. The mechanism of the Cu₆(Sn,In)₅ layer growth is discussed and the conclusions for the optimal solder chemical composition are presented.     

Sequential interfacial intermetallic compound formation of Cu6Sn5 and Ni3Sn4 between Sn-Ag-Cu solder and ENEPIG substrate during a reflow process

The interfacial reactions between Sn-3.0 wt.% Ag-0.5 wt.% Cu solder and an electroless nickel-electroless palladium-immersion gold (ENEPIG) substrate were investigated. After initial reflowing, discontinuous polygonal-shape (Cu,Ni)₆Sn₅ intermetallic compounds (IMCs) formed at the interface. During reflowing for up to 60 min, the interfacial IMCs were sequentially changed in the following order: discontinuous (Cu,Ni)₆Sn₅, (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄, and embedded (Cu,Ni)₆Sn₅ in (Ni,Cu)₃Sn₄. The interfacial product variation resulted from the preferential consumption of Cu atoms within the solder and continuous Ni diffusion from the Ni(P) layer.