Liquid-state interfacial reactions in Sn–Zn/Pd couples and phase equilibria of the Sn–Zn–Pd system at 260 °C

This study investigated the effects of Zn contents on the reaction products and microstructural evolution in the liquid/solid Sn–Zn/Pd interfacial reactions at 260 °C. A uniform Pd₂Zn₉ layer was formed at the Sn–9 wt.%Zn/Pd interface. The reaction phase transited from Pd₂Zn₉ to PdSn₄ when the Zn content decreased from 2 wt.% to 1 wt.%. The most striking feature is that the PdSn₄ growth was greatly suppressed with only 0.5 wt.% Zn addition in solders. Additionally, a drastic microstructural evolution was observed in the Sn–1.5 wt.%Zn/Pd reaction. The Pd₂Zn₉ layer was initially formed and then it was detached from the interface due to the decrease in the Zn content. Subsequently, the dominant phase changed to the PdSn₄ phase. Furthermore, a partial isothermal section in the Sn–Zn–Pd ternary system (less than 20 at.%Pd) at 260 °C was experimentally determined. The liquid apex of the liquid + PdSn₄ + Pd₂Zn₉ tie-triangle was located at Sn–2.7 at.%Zn–1.0 at.%Pd. The phase transition from Pd₂Zn₉ to PdSn₄ in the interfacial reactions was in good agreement with the phase equilibria relationship.     

Inhibiting CoSn3 growth at the Sn/Co system by minor Zn addition

CoSn₃ is the major reaction product in Sn/Co interfacial reactions. Compared with other substrate materials, e.g., Cu and Ni, the CoSn₃ growth is much faster during reflow soldering and thermal aging. However, excessive formation of intermetallic compound (IMC) layer might deteriorate the reliability of solder joints. This study demonstrated that the CoSn₃ growth was effectively slowed down by adding minor amounts of Zn in Sn solder. The solder reactions between Co and Sn with addition of various Zn contents (0.05 wt% ∼ 2 wt%) were systematically examined in liquid-state and solid-state reactions. For liquid-state reactions at 240 °C–260 °C, the CoSn₃ growth rate was decreased by 50% with 0.1 wt%Zn addition, and further decreased by 75% with 0.5 wt%Zn. Moreover, the growth kinetics of CoSn₃ was investigated with various Zn additions. The CoSn₃ growth was found to be proportional with reaction time, and their apparent activation energies were very close (between 105 kJ/mol ∼ 110 kJ/mol). For solid-state reactions, the effect of Zn addition on the inhibition of CoSn₃ growth was more significant. The growth rate was greatly reduced by 75% with only 0.05 wt%Zn addition. The possible inhibition mechanism was discussed.     

Interfacial reactions between high-Pb solders and Ag

Interfacial reactions between high-Pb solders (Pb-10Sn, Pb-5Sn, and Pb-3Sn, in wt.%) and immersion Ag layer at 350 °C are investigated. Upon decreasing the Sn concentration from 10 wt.% to 5 wt.%, the reaction product formed at the solder/Ag interface changes from the Ag₃Sn phase to the Ag₄Sn phase. When the Sn concentration reduces to only 3 wt.%, the reaction product is the Ag₄Sn phase at the initial stage of reaction but transforms to the (Ag) phase dissolved with Sn at the later stage of reaction. Pb penetrates across the (Ag) phase via grain boundary and forms a continuous Pb-rich layer between the (Ag) phase and the bottom Cu layer. The correlation between the phase transformation and the solder composition is discussed based on the calculated Sn-Pb-Ag isothermal section.     

Effect of Ga on the Oxidation Properties of Sn–8.5Zn–0.5Ag–0.1Al–xGa Solders

The oxidation properties of Sn–8.5Zn–0.5Ag–0.1Al–xGa lead-free solders in the liquid state (250?°C) under O₂ atmosphere were investigated using a thermal gravimetric analyzer. The Ga content of the investigated solders was 0.05–2?wt%. The results indicate that Ga enhances the oxidation resistance of Sn–Zn–Ag–Al solder. Cross-sections of the solder surfaces were examined using focus ion beam milling. The thickness of the oxidation layer, which was about 30–100?nm, increased with increasing Ga content. The oxidation layer was found to be nonuniform at low Ga content. The oxide layers on the surface of solders were investigated using Auger electron spectroscopy and thin-film XRD. The results showed that the oxide layer formed was ZnO. Al and Ga tended to segregate on the surface of the solder.     

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.     

Interfacial microstructure and the kinetics of interfacial reaction in diffusion couples between Sn–Pb solder and Cu/Ni/Pd metallization

The interfacial microstructure and the kinetics of interfacial reaction between eutectic Sn–Pb solder and electroplated Ni/Pd on a Cu substrate have been studied by scanning, transmission and analytical electron microscopies. Besides PdSn₄ and Ni₃Sn₄, small grains of Ni₃Sn₂ with a hexagonal structure are also observed after long-time aging of the diffusion couples at 125°C. The presence of intermetallic phases is correlated with the diffusion paths in the calculated Pd–Pb–Sn and Ni–Pb–Sn isothermal sections. The growth kinetics of the Ni₃Sn₄ scallops in the submicrometer length scale was analyzed with an Arrhenius type of equation. The thickening kinetics yields a time exponent n=3.1 and an apparent activation energy (Q_h) of 25,750 J/mol, while the radial growth kinetics data yield a time exponent m≈6.6 and an apparent activation energy (Q_d) of 15,300 J/mol. The radial size distributions (RSDs) of Ni₃Sn₄ scallops were also quantified. The parameters describing RSDs are consistent with the theories of coarsening in two-phase systems containing a very high volume fraction of the second phase. Selective etching of solder revealed the three-dimensional morphology of PdSn₄ and Ni₃Sn₄, and also the dynamical phenomena, such as faceting, competitive growth and coalescence of Ni₃Sn₄ scallops during interfacial reaction. Non-parabolic growth kinetics is discussed in terms of the existing theories and characteristics of the evolving microstructure.     

Influence of copper addition on adhesive strength of Sn–3Ag–1·5Sb solder joints

Abstract

The present work investigates the effects of adding a small amount of Cu to Sn–3Ag–1·5Sb solders. The present results indicate that adding 0·5 and 1·0 wt-%Cu to Sn–3Ag–1·5Sb solders causes the liquidus temperature to decrease from its original value of 233·4°C to 231·6°C and to 231·4°C, respectively. Furthermore, it is noted that the addition of 1·0 wt-%Cu reduces the difference between the liquidus and solidus temperatures. It is shown that the added Cu reacts with the Sn content of the solder to form Cu₆Sn₅ particles in the β-Sn matrix, which are distributed non-uniformly since the Cu content is low. The experimental results also reveal that the growth rate of the solder joint interfacial intermetallic compound layers increases at higher levels of Cu addition. Finally, it is established that adding Cu to the Sn–3Ag–1·5Sb solder not only improves the adhesive strength of the solder joints, but also reduces the rate of degradation of the adhesive strength of the joints during thermal storage.     

Effect of rare earth metal Ce addition to Sn-Ag solder on interfacial reactions with Cu substrate

The effect of adding a small amount of rare earth cerium (Ce) element to low Ag containing Sn-1wt%Ag Pb-free solder on its interfacial reactions with Cu substrate was investigated. The growth of intermetallic compounds (IMCs) between three Sn-1Ag-xCe solders with different Ce contents and a Cu substrate was studied and the results were compared to those obtained for the Ce-free Sn-1Ag/Cu systems. In the solid-state reactions of the Sn-1Ag(-xCe)/Cu solder joints, the two IMC layers, Cu₆Sn₅ and Cu₃Sn, grew as aging time increased. Compared to the Sn-1Ag/Cu joint, the growth of the Cu₆Sn₅ and Cu₃Sn layers was depressed for the Ce-containing Sn-1Ag-xCe/Cu joint. The addition of Ce to the Sn-Ag solder reduced the growth of the interfacial Cu-Sn IMCs and prevented the IMCs from spalling from the interface. The evenly-distributed Ce elements in the solder region blocked the diffusion of Sn atoms to the interface and retarded the growth of the interfacial IMC layer.     

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.     

Growth mechanism of Ni3Sn4 in a Sn/Ni liquid/solid interfacial reaction

The chemical interfacial reaction of Ni plates with eutectic Sn–3.5Ag lead-free solder was studied by microstructural observations and mathematical calculations. Compared with the Sn–3.5Ag–0.75Ni/Ni interfacial reaction, based on a simple model of the growth of the liquid/solid chemical compound layer, the growth mechanism of Ni₃Sn₄ in the Sn–3.5Ag/Ni interfacial reaction is discussed and presented. The growth process of Ni₃Sn₄ in the Sn/Ni liquid/solid reaction interface involves the net effect of several interrelated phenomena, such as volume diffusion, grain boundary diffusion, grain boundary grooving, grain coarsening, and dissolution into the molten solder. The growth time exponent n and morphology of Ni₃Sn₄ were found to be dependent on these factors.     

Ag,Al,Ga对Sn-9Zn无铅钎料润湿性能的影响

采用润湿平衡法测试了Sn-9Zn-X(X为Ag,Al,Ga)无铅钎料分别配合ZnCl2-NH4Cl钎剂和免清洗助焊剂,在空气和氮气保护的两种条件下的润湿性能,分析研究了合金元素Ag,Al,Ga的添加量对Sn-9Zn-X无铅钎料润湿性的影响规律.结果表明,合金元素Ag,Al,Ga在Sn-9Zn中的最佳添加量(质量分数)分别为0.3%,0.005%～0.02%,0.5%.采用氮气保护可以显著改善Sn-9Zn-X无铅钎料的润湿性,而Sn-9Zn-X无铅钎料配合ZnCl2-NH4Cl钎剂时具有较好的润湿性,甚至优于Sn-3.5Ag-0.5Cu在相同条件下的润湿性.这一研究结果表明,通过研发适合于Sn-Zn系无铅钎料的高性能助焊剂,从而改善Sn-Zn系钎料的润湿性能是完全可行的.     

Effect of fine dispersoids and anisotropic nature of β-Sn on thermal fatigue properties of flip chips connected by Sn–xAg–0·5Cu (x: 1, 3 and 4 mass-%) lead free solders

Abstract

A significance of two factors, fine dispersoids in a solder and the anisotropic nature of β-Sn, on thermal fatigue endurance is discussed using flip chips connected by Sn–xAg–0·5Cu (x: 1, 3 and 4 mass-%) lead free solders, together with Sn and Sn–1·2Ag–0·5Cu–0·05Ni, for comparison. Both 3Ag and 4Ag showed better thermal fatigue properties than Sn and 1Ag, and a thermal fatigue life of 1·2Ag with Ni was close to that of 3Ag despite of its low silver content. Microstructures of the solders before thermal fatigue tests can be classified into a single crystal-like and a fine grain type. However, this classification, which affects the amount of thermal strain by the anisotropic nature of β-Sn, cannot accurately describe thermal fatigue lives observed. On the other hand, Vickers microhardness of the solders, which was resulted from fine dispersoids, showed good relationship with observed thermal fatigue endurance.     

Ga和Ce复合添加对低银无镉钎料组织及性能的影响

通过向Ag17CuZnSn钎料中复合添加微量的Ga元素和稀土元素Ce,研究了Ga和Ce元素的复合添加对低银无镉钎料组织及焊接性能的影响。采用火焰钎焊方法得到黄铜与不锈钢异种金属钎焊接头。试验表明,随着Ga和Ce元素的添加低银钎料的固液相线温度不断下降。当Ga和Ce含量分别为2%和0.15%时,钎料在母材表面的铺展面达到最大值。Ga和Ce元素的复合添加对低银钎料中的CuZn化合物相有明显的细化作用,黄铜与不锈钢钎焊接头的抗剪强度最大值为378.6 MPa。在钎焊过程中,钎缝与不锈钢母材之间发生了明显的元素扩散,Ga元素的扩散率大于其他元素。同时当低银钎料中的Ce元素含量大于0.3%时,钎料组织中出现了新生成的块状的稀土相。     

In-situ alloying of Sn–3.5Ag solder during reflow through Zn nanoparticle addition and its effects on interfacial intermetallic layers

Driven by the necessity to improve the reliability of lead free electronic products and by the trend towards miniaturization, researchers are putting intense efforts to improve the properties of Sn based solders. The present work investigates the effects of Zn nanoparticle addition to Sn-3.5Ag (SA) alloy through paste mixing on the interfacial structure between solder and copper substrate during reflow. Results show that the addition of Zn nanoparticles does not alter the morphology of the interfacial intermetallic compounds although they substantially suppress their growth. Zn nanoparticles are seen to be most efficient compared with Co and Ni nanoparticles in suppressing the growth of Cu₃Sn layers. It is suggested that Zn nanoparticles exert their influence through an in-situ dissolution and alloying effect.     

Effects of Ga,Al, Ag,and Ce multi-additions on the wetting characteristics of Sn-9Zn lead-free solder

An orthogonal method was used to evaluate the effects of Ga, Al, Ag, and Ce multi-additions on the wetting characteristics of Sn-9Zn lead-free solders by wetting balance method. The results show that the optimal loading of Ga, Al, Ag, and Ce was 0.2 wt.%, 0.002 wt.%, 0.25 wt.%, and 0.15 wt.%, respectively. Intermetallic compounds (IMCs) formed at the interface between Sn-9Zn-0.2Ga-0.002Al-0.25Ag-0.15Ce solder and Cu substrate were investigated by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analysis. The SEM images illustrate that the IMCs can be divided into two portions from the substrate side to the solder side: a planar Cu₅Zn₈ layer and an additional continuous scallop-like AgZn₃ layer. The EDS analysis also shows that Ga segregates in the solder abutting upon the interface. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) of the surface components of Sn-9Zn-0.2Ga-0.002Al-0.25Ag-0.15Ce solder indicate that Al aggregates at the surface in the form of Al₂O₃ protective film, which prevents the further oxidation of the solder surface. On the other hand, Ce aggregates at the subsurface, which may reduce the surface tension of the solder and improve the wettability in consequence.     

Growth behavior of Cu6Sn5 in Sn–6.5 Cu solders under DC considering trace Al: In situ observation

High resolution time-resolved X-ray imaging with synchrotron radiation has been used to in situ observe the growth behavior of Cu₆Sn₅ intermetallic compounds (IMCs) during solidification in Sn–6.5 Cu and Sn–6.5 Cu–0.2 Al (wt. %) solders under applied direct current (DC) field. The morphological evolution of Cu₆Sn₅ with I-like, X-like, Y-like and bird-like shapes is directly observed. It is shown that trace levels of Al have a marked effect on the solder microstructures and refining the size of the primary Cu₆Sn₅. The solidification pathway leading to the refinement is observed in real time using synchrotron microradiography. After adding the trace Al, I-like shapes bifurcate into X-like shapes. Furthermore, when DC field with 10 A/cm² is applied, both the growth rate and the mean size of Cu₆Sn₅ are increased but decreased when 100 A/cm² is applied. Meanwhile, the effect of thermodynamic potential barrier caused by DC field on the growth behavior of Cu₆Sn₅ is discussed.     

合金元素Ga和In对Mg阳极组织和电化学性能的影响

采用动电位极化和恒电流曲线测试合金元素Ga和In对Mg阳极材料电化学性能的影响。采用扫描电镜法分析Mg?In?Ga合金的显微组织和腐蚀表面,并用X射线衍射法检测Mg?0.8%In合金和Mg?0.8%Ga?0.3%In 合金的腐蚀产物。结果表明：Mg?xIn (x=0?0.8%)合金中没有第二相出现,Mg?0.8%In?xGa (x=0?0.8%)合金中存在富含Ga和In元素的晶间化合物。合金元素In和0.05%?0.5%Ga的添加提高了镁阳极的耐腐蚀性能,Ga元素的添加更促进了Mg?In合金的电化学活性。Mg?0.8%In?0.8%Ga合金的平均电位最负,为?1.682 V,此电位比AZ91D合金的?1.406 V更负。Mg?In?Ga合金的腐蚀类型是全面腐蚀,其腐蚀产物是Mg(OH)2。     

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.     

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.     

Electrochemical corrosion behaviour of Bi-11Ag alloy for electronic packaging applications

This study investigated the electrochemical corrosion properties of the solders for die-attach applications in 3.5% NaCl solution. Compared with Pb-5Sn and Zn-40Sn, Bi-11Ag exhibited higher corrosion potential and relatively low corrosion current density. The ductile Ag-rich phase which dispersed in the Bi matrix was able to accommodate the stress arising from the formation of a passive layer and contributed to the two-stage passivation. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) results confirm that the corrosion products comprised BiOCl, Bi₂O₃ and AgCl.