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.     

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.     

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.     

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.     

片式电容Sn96.5/Ag3/Cu0.5焊点热疲劳性能比较

以0805封装片式电容器件焊点为研究对象,建立了多周期温度冲击下Sn96.5/Ag3/Cu0.5的焊点有限元分析模型,开展了多周期温度冲击条件下焊点剪切力测试工作,获得了Sn96.5/Ag3/Cu0.5和Sn63/Pb37两种焊点的周期-剪切力测试数据,并利用非线性最小二乘法得到了1 500个周期内的焊点热疲劳状态拟合曲线.结果表明,在规定试验条件下,在有限的1 500个周期内0805封装电容的Sn96.5/Ag3/Cu0.5焊点的热疲劳劣化速率略慢于Sn63/Pb37焊点.     

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.     

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.     

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.     

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.     

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...     

Dissolution and interfacial reactions of thin-film Ti/Ni/Ag metallizations in solder joints

The dissolution and interfacial reactions involving thin-film Ti/Ni/Ag metallizations on two semiconductor devices, diode and metal-oxide-semiconductor field-effect transistor (MOSFET), a Sn–3.0Ag–0.7Cu solder, and a Au-layer on the substrates are studied. To simulate the dissolution kinetics of the Ag-layer in liquid solder during the reflow process, the computational thermodynamics (Thermo-Calc) and kinetics (DICTRA: DIffusion Controlled TRAnsformations) tools are employed in conjunction with the assessed thermochemical and mobility data. The simulated results are found to be consistent with the observed as-reflowed microstructures and the measured Ag contents in the solder. In the as-reflowed joints two different intermetallic compounds (IMC) are found near the diode/solder interface. Both are in the form of particles of different morphologies, not a continuous layer, and are referred to as IMC-I and IMC-II. The former corresponds to Ni₃Sn₄ with Cu atoms residing in the Ni sublattice. It is uncertain whether IMC-II is Cu₆Sn₅ phase with Ni atoms residing in the Cu sublattice or a Cu–Ni–Sn ternary phase. Near the as-reflowed MOSFET/solder interface, both particles and a skeleton-like layer of Ni₃Sn₄ are observed. The primary microstructural dynamics during solid state aging are the coarsening of IMC particles and the reactions involving the unconsumed (after reflow) Ni- and the Ti-layer with Sn and Au. While the reaction with the Ni-layer yields only Ni₃Sn₄ intermetallic, the reaction involving the Ti-layer suggests the formation of Ti–Sn and Au–Sn–Ti intermetallics. The latter is due to the diffusion of Au from the substrate side to the die side. It is postulated that the kinetics of Au–Sn–Ti layer is primarily governed by the diffusion of Au through the Ni₃Sn₄ layer by a grain boundary mechanism.     

Local melting and shape controlling of solder joint via induction heating

A rapid yet simple methodology to form the solder joints used in electronic packaging has been demonstrated through an induction heating system in this paper. The objective is to control the shapes of the solder joints precisely and form the hourglass-shaped solder joint using local melting phenomenon. For Sn–Ag eutectic solder bullet-shaped solder bumps and hourglass-shaped solder joints can be obtained easily within 2.3 s. The reason for the formation of hourglass-shaped solder joints has been analyzed using different morphologies of Ag₃Sn intermetallic compound (IMC), and is explained by the non-uniform temperature distribution caused by a local melting phenomenon and skin effect.     

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 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.     

Effect of bismuth on the growth kinetics of intermetallic compounds in Sn-3.5Ag solder joints: A growth kinetic model

The effect on the growth kinetics of the intermetallic compounds (IMCs) in solder/Cu joints, caused by adding Bi to eutectic Sn-3.5Ag solder alloy, was examined at the aging temperatures of 150°C and 180°C. The Cu₆Sn₅ layer growth was significantly enhanced, but the Cu₃Sn layer growth was slightly retarded by the addition of Bi, resulting in significant growth enhancement of the total (Cu₆Sn₅+Cu₃Sn) IMC layer with increasing Bi addition. The IMC layer growth in the Bi-containing solder joints was accompanied by the accumulation of Bi ahead of the Cu₆Sn₅ layer that resulted in the formation of a liquid layer at the Cu₆Sn₅/solder interface. A kinetic model was developed for the planar growth of the Cu₆Sn₅ and Cu₃Sn layers in the solder joints, accounting for the existence of interfacial reaction barriers. Predictions from the kinetic model showed that the experimental results could be well explained by the hypothesis that the formation of a Bi-rich liquid layer at the Cu₆Sn₅/solder interface reduces the interfacial reaction barrier at the interface.     

Interfacial reaction between Sn–Ag–Cu solder and Co–P films with various microstructures

We electroplated Co–P films with nanocrystalline, amorphous and nanocrystalline/amorphous mixed structures and used them as under-bump metallization (UBM) joined with Sn–Ag–Cu lead-free solder. We systematically investigated the interfacial reaction between the Sn–Ag–Cu solder and the Co–P UBM and analyzed the growth mechanisms of the intermetallic compounds formed at the interfaces of the Sn–Ag–Cu/Co–P joints through multiple reflows. Among the three kinds of Co–P films, the film with a mixed structure shows the best diffusion-barrier properties and is a good candidate for the UBM joined with the Sn–Ag–Cu solder. For the nanocrystalline Co–P UBM, Co diffuses quickly toward the solder and Sn does not diffuse into the UBM, whereas for the amorphous Co–P film not only does Co diffuse into the solder, but also Sn diffuses into the Co–P film with a large diffusion rate. In addition, the first-principles calculation shows that the exchange coupling between Co(3d⁷4s²) and Sn(5s²5p²) electrons and between Sn(5s²5p²) and P(3s²3p³) electrons results in the formation of CoSn and SnP₃, which originate from the diffusion and reaction of Co and Sn atoms, respectively; this is consistent with the experimental data of transmission electron microscopy characterization.     

Growth mechanisms of Cu3Sn on polycrystalline and single crystalline Cu substrates

A comprehensive transmission electron microscopy (TEM) study was conducted to investigate the growth mechanisms of Cu₃Sn on polycrystalline and single crystalline Cu substrates in solder joints. On single crystalline Cu the solder reflow process resulted in the growth of columnar Cu₃Sn grains aligned in a thin uniform layer perpendicular to the interface, while a thick Cu₃Sn layer formed from fine equiaxed grains on the polycrystalline substrate. In the subsequent solid state aging, columnar growth of Cu₃Sn continued on the single crystalline Cu before it was replaced by nucleation and growth of new triangular Cu₃Sn grains at the triple junction sites of the Cu/Cu₃Sn interface. On the polycrystalline Cu the solid state aging caused much more rapid growth of the Cu₃Sn layer due to nucleation and the growth of new Cu₃Sn grains at both the Cu/Cu₃Sn and Cu₆Sn₅/Cu₃Sn interfaces. These different growth behaviors of Cu₃Sn were related to the diffusive supply of reactive elements.     

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.     

Effects of residual S on Kirkendall void formation at Cu/Sn–3.5Ag solder joints

Solder joints of Cu/Sn–3.5Ag were prepared using Cu foil or electroplated Cu films with or without SPS additive. With a high level of SPS in the Cu electroplating bath, voids tended to localize at the Cu/Cu₃Sn interface during subsequent aging at 150 °C, which was highly detrimental to the drop impact resistance of the solder joints. In situ Auger electron spectroscopy of fractured joints revealed S segregation on the Cu/Cu₃Sn interface and void surfaces, suggesting that segregation of S to the Cu/Cu₃Sn interface lowered interface energy and thereby the free energy barrier for Kirkendall void nucleation. Once nucleated, voids can grow by local tensile stress, originating from residual stress in the film and/or the Kirkendall effect. Vacancy annihilation at the Cu/Cu₃Sn interface can induce tensile stress which drives the Kirkendall void growth.