Influence of alloy composition on fillet-lifting phenomenon in tin binary alloys

The influence of alloy composition on the fillet-lifting phenomenon was investigated for Sn−Bi, Sn−Pb, and Sn−In binary alloys. Fillet-lifting occurs in Sn-(1–30%)Bi, Sn-(1–5%)Pb, Sn-(2–15%)In, but does not occur in pure Sn, Sn-(40–62%)Bi, Sn-(10–45%)Pb, and Sn-43 In. Fillet-lifting does not correlate with the formation of a Bi-concentrated layer at the solder/Cu land interface, previously thought to cause fillet-lifting. The solidification temperature range also does not necessarily correlate with fillet-lifting. Fillet-lifting was found to be related to the retention time (t_r) for which solidus temperature (the final solidification temperature) remains by latent heat released during eutectic solidification in the cooling curve. The released latent heat contributes to the alleviation of the temperature gradient causing fillet-fifting in the solder joint. A fillet-fifting resistant index (FRI) is proposed as a new index for fillet-lifting, and correlates well with fillet-lifting occurrence rate in each of the studied tin binary alloys.     

Comparison study on microstructure and mechanical properties of Sn-10Bi and Sn-Ag-Cu solder alloys and joints

The comparison study of Sn-10Bi and Sn-3.0Ag-0.5Cu solder alloys and joints was conducted. The results showed that the liquidus of Sn-10Bi solder alloy was lower than that of Sn-Ag-Cu slightly. The interfacial IMCs layer growth of Sn-10Bi/Cu was slower than that of Sn-Ag-Cu/Cu during liquid/solid reaction. The higher strength and lower creep strain rate of Sn-10Bi comparing with that of Sn-Ag-Cu were contributed by the solid solution strengthening effect of Bi atom in β-Sn phase. The ultimate bending load of Sn-10Bi joint was higher than that of Sn-Ag-Cu joint as the high strength of Sn-10Bi solder alloy. Moreover, the thinner and more flat IMCs layer also ensured the stable maximum bending displacement of Sn-10Bi joint at a loading speed of 1 mm/s compared with that of Sn-Ag-Cu joint.     

Effects of Trace Amounts of Rare Earth Additions on Microstructure and Properties of Sn-Bi-Based Solder Alloy

The effect of trace amounts of rare earth additions on the microstructure and properties were studied for the Sn-58Bi and Sn-58Bi-Ag solder alloys. At the same time, the intermetallic compounds (IMCs) in the solder alloys and intermetallic layer (IML) thickness at the solder/Cu substrate interface were investigated, both as-reflowed and after high-temperature aging. The results indicate that adding trace amounts of rare earth (RE) elements has little influence on the melting temperature and microhardness of the solders investigated, but adding RE elements improves the wettability and shear strength of the Sn-58Bi and Sn-58Bi-Ag solder alloys. In addition, it was found that the addition of RE elements not only refines the microstructure and size of the IMC particles, but also decreases the IML thickness and shear strength of the Sn-58Bi solder joint after high-temperature aging. Adding trace amounts of RE elements is superior to adding trace amounts of Ag for improving the properties of the Sn-58Bi solder. The reason may be related to the modification of the microstructure of the solder alloys due to the addition of trace amounts of RE elements.     

Study of Solidification Cracks in Sn-Ag-Cu Lead-Free Solder Joints

In the present work, solidification cracks in Sn-Ag-Cu solder joints were investigated. Experimental results indicate that solidification cracks existed in significant numbers in the miniature Sn-Ag-Cu solder joints. In order to create solidification cracks in the miniature solder joints during solidification and evaluate the susceptibility of Sn-Ag-Cu alloys to solidification cracking, a copper self-restraint specimen was designed, which can simulate the process of solidification crack formation. The solidification crack susceptibility of the Sn-Ag-Cu solder alloy was evaluated using the total crack length of the solder joint. In addition, the effect of trace amounts of elemental additions on solidification cracking of Sn-Ag-Cu solder joints was studied. It was found that adding trace amounts of Ni or Ce could depress the solidification cracks in Sn-3.0Ag-0.5Cu solder joints. However, P additions aggravated the formation of solidification cracks.     

Compound formation for electroplated Ni and electroless Ni in the under-bump metallurgy with Sn-58Bi solder during aging

The Ni-based under-bump metallurgies (UBMs) are of interest because they have a slower reaction rate with Sn-rich solders compared to Cu-based UBMs. In this study, several UBM schemes using Ni as the diffusion barrier are investigated. Joints of Sn-58Bi/Au/electroless nickel (EN)/Cu/Al₂O₃ and Sn-58Bi/Au/electroplated nickel/Cu/Al₂O₃ were aged at 110°C and 130°C for 1–25 days to study the interfacial reaction and microstructural evolution. The Sn-Bi solder reacts with the Ni-based multimetallization and forms the ternary Sn-Ni-Bi intermetallic compound (IMC) during aging at 110°C. Compositions of ternary IMC were (78–80)at.%Sn-(12–16)at.%Ni-(5–8)at.%Bi in joints of Sn-58Bi/Au/Ni-5.5wt.%P/Cu, Sn-58Bi/Au/Ni-12wt.%P/Cu, and Sn-58Bi/Au/Ni/Cu. Elevated aging at 130°C accelerates the IMC growth rate and results in the formation of (Ni,Cu)₃Sn₄ and (Cu,Ni)₆Sn₅ adjacent to the ternary Sn-Ni-Bi IMC for the Sn-58Bi/Au/Ni-12wt.%P/Cu and Sn-58Bi/Au/Ni/Cu joints, respectively. The Cu content in the (Cu,Ni)₆Sn₅ IMC is six times that in (Ni,Cu)₃Sn₄. Electroplated Ni fails to prevent Cu diffusion toward the Ni/solder interface as compared to EN-based joints. Cracks are observed in the Sn-58Bi/Au/Ni-5.5wt.%P/Cu/Al₂O₃ joint aged at 130°C for 25 days. It is more favorable to employ Ni-12wt.%P for the Sn-58Bi/Au/EN/Cu joint. Electroless nickel, with the higher P content of 12 wt.%, is a more effective diffusion barrier during aging. In addition, P enrichment occurs near the interface of the EN/solder, and the degree of P enrichment is enhanced with aging time. The Au(Sn,Bi)₄, with pyramidal and cubic shape, is observed in the Sn-58Bi/Au/Ni/Cu/Al₂O₃ joint.     

Sn-0.3Ag-0.7Cu-xBi低银无铅钎料的润湿性

以Bi为添加剂对低银型Sn-0.3Ag-0.7Cu无铅钎料进行改性,应用SAT—5100型润湿平衡仪对Sn-0.3Ag-0.7Cu-xBi(x=0,1,3和4.5)钎料的润湿性能作了对比分析。结果表明:适量Bi元素的加入可以改善Sn-0.3Ag-0.7Cu钎料合金的润湿性能,且在240℃下Sn-0.3Ag-0.7Cu-3.0Bi无铅钎料具有最佳的润湿性能,在250℃其润湿力达到最大值3.2×10–3N/cm。     

The thermal property of lead-free Sn-8.55Zn-1Ag-XAl solder alloys and their wetting interaction with Cu

The wetting behaviors between the quaternary Sn-8.55Zn-1Ag-XAl solder alloys and Cu have been investigated with the wetting balance method. The Al contents, x, of the quaternary solder alloys investigated were 0.01–0.45 wt.%. The results of differential scanning calorimeter (DSC) analysis indicate that the solders exhibit a solid-liquid coexisting range of about 7–10°C. The solidus temperature of the quaternary Sn-8.55Zn-1Ag-XAl solder alloys is about 198.2°C, while the liquidus temperatures are 205–207°C. The experimental results showed that the wettability of the Sn-8.55Zn-1Ag-XAl solder alloys is improved by the addition of Al. The mean maximum wetting force of the solders with Cu is within 0.75–1.18 mN and the mean wetting time is around 1.0–1.1 sec, better than the ∼1.3 sec of eutectic Sn-9Zn and Sn-8.55Zn-1Ag solder alloys. The addition of Al also depresses the formation of ε-Ag-Zn compounds at the interface between Sn-8.55Zn-1Ag-XAl solders and copper.     

Electrochemical Properties of Joints Formed Between Sn-9Zn-1.5Ag-1Bi Alloys and Cu Substrates in a 3.5 wt.% NaCl Solution

The electrochemical properties of the joints formed between Sn-9Zn-1.5Ag-1Bi alloys and Cu substrates in a 3.5 wt.% NaCl solution have been investigated by potentiodynamic polarization, X-ray diffraction, and scanning electron microscopy. For the Sn-9Zn-1.5Ag-1Bi/Cu joints in a 3.5 wt.% NaCl solution, corrosion current (I _corr), corrosion potential (E _corr) and corrosion resistance (R _p) are 2.46 × 10⁻⁶ A/cm², −1.18 V, and 7.54 × 10³ Ωcm², respectively. Cu₆Sn₅, Cu₅Zn₈, and Ag₃Sn are formed at the interface between the Sn-9Zn-1.5Ag-xBi solder alloy and Cu substrate. The corrosion products of ZnCl₂, SnCl₂ and ZnO are formed at the Sn-9Zn-1.5Ag-xBi/Cu joints after polarization in a 3.5 wt.% NaCl solution. Pits are also formed on the surface of the solder alloys.     

Effects of Ce Addition on the Microstructure and Mechanical Properties of Sn-58Bi Solder Joints

The effects of a rare-earth element on the microstructure, mechanical properties, and whisker growth of Sn-58Bi alloys and solder joints in ball grid array (BGA) packages with Ag/Cu pads have been investigated. Mechanical testing indicated that the elongation of Sn-58Bi alloys doped with Ce increased significantly, and the tensile strength decreased slightly, in compar- ison with undoped Sn-58Bi. In addition, the growth of both fiber- and hillock-shaped tin whiskers on the surface of Sn-58Bi-0.5Ce was retarded in the case of Sn-3Ag-0.5Cu-0.5Ce alloys. The growth of interfacial intermetallic compounds (IMC) in Sn-58Bi-0.5Ce solder joints was slower than that in Sn-58Bi because the activity of Ce atoms at the interface of the Cu₆Sn₅ IMC/solder was reduced. The reflowed Sn-58Bi and Sn-58Bi-0.5Ce BGA packages with Ag/Cu pads had a ball shear strength of 7.91 N and 7.64 N, which decreased to about 7.13 N and 6.87 N after aging at 100°C for 1000 h, respectively. The reflowed and aged solder joints fractured across the solder balls with ductile characteristics after ball shear tests.     

Low-cycle fatigue characteristics of Sn-based solder joints

Low-cycle, lap-shear fatigue behavior of Sn-based, Pb-free solder alloys, Sn-3.5Ag, Sn-3.5Ag-Cu, Sn-3.5Ag-Bi, and Sn-0.7Cu, were studied at room temperature using specimens with printed circuit board (PCB)/solder/PCB structure under total displacement of ±10 μm, 12 μm, 15 μm, and 20 μm. The fatigue lives of various solder joint materials, defined as 50% load drop, were correlated with the fracture paths and analyzed using the Coffin-Manson relation, Morrow’s plastic-energy dissipation model, and Solomon’s load-drop parameter. The Sn-3.5Ag, Sn-0.7Cu eutectics, and Sn-3.5Ag-Cu ternary alloys showed the same level of fatigue resistance, while Bi-containing alloys showed substantially worse fatigue properties. Cross-sectional fractography revealed cracks initiated at the solder wedge near the solder mask and subsequently propagated into the solder matrix in the former group of alloys, in contrast with the crack propagation along the solder/under bump metallurgy (UBM) interfaces in the Sn-3.5Ag-Bi alloys. Inferior fatigue resistance of Bi-containing alloys was ascribed to high matrix hardness, high stiffness, possible Bi segregation to the interface, and high residual stress in the interfacial area.     

无铅电子钎料合金蠕变性能研究

设计制作了一种简单可靠的弯折蠕变测量装置,比较了两种无铅电子钎料合金Sn-9Zn和Sn-3.5Cu-0.7Ag与传统电子钎料合金Sn-40Pb的常温蠕变性能,以及冷却条件对其蠕变强度的影响。结果表明:两种无铅钎料的抗蠕变性能大大优于传统锡铅钎料;Sn-3.5Ag-0.7Cu合金的抗蠕变性能优于Sn-9Zn合金;冷却速率对Sn-9Zn合金和Sn-3.5Ag-0.7Cu合金组织的影响类似,然而对蠕变强度的影响却相反:水冷使两种合金的组织相对于空冷都明显细化,Sn-9Zn合金的蠕变强度因之降低,而Sn-3.5Ag-0.7Cu合金的蠕变强度却因之提高。对可能产生的原因进行了讨论。     

Three-dimensional analysis of the interface between an Sn-8wt.%Zn-3wt.%Bi solder and a substrate by using an angle-lapping method

The validity of application of the angle-lapping method to investigate the microstructure between an Sn-8wt.%Zn-3wt.%Bi Pb-free solder and an Auimmersed Ni-7.0%P plate or a Cu substrate was studied using transmission electron microscopy. The method enabled a three-dimensional analysis of the interface layers. The morphology of the Au layer was clarified as an intermetallic compound with Zn with numerous voids that looked like cracks spreading in all directions. Sn and Bi could diffuse into the interface layer only when the Ni-P plate was used, but Ni did not diffuse actively and formed a narrow interface layer. When a Cu substrate was used, Cu diffused, rather than Sn or Bi, and formed a relatively wide interface layer. These results were obtained with the angle-lapping method.     

Effect of Ag Content and the Minor Alloying Element Fe on the Mechanical Properties and Microstructural Stability of Sn-Ag-Cu Solder Alloy Under High-Temperature Annealing

This study compares the high-Ag-content Sn-3Ag-0.5Cu with the low- Ag-content Sn-1Ag-0.5Cu solder alloy and the three quaternary solder alloys Sn-1Ag-0.5Cu-0.1Fe, Sn-1Ag-0.5Cu-0.3Fe, and Sn-1Ag-0.5Cu-0.5Fe to understand the beneficial effects of Fe on the microstructural stability, mechanical properties, and thermal behavior of the low-Ag-content Sn-1Ag-0.5Cu solder alloy. The results indicate that the Sn-3Ag-0.5Cu solder alloy possesses small primary β-Sn dendrites and wide interdendritic regions consisting of a large number of fine Ag₃Sn intermetallic compound (IMC) particles. However, the Sn-1Ag-0.5Cu solder alloy possesses large primary β-Sn dendrites and narrow interdendritic regions of sparsely distributed Ag₃Sn IMC particles. The Fe-bearing SAC105 solder alloys possess large primary β-Sn dendrites and narrow interdendritic regions of sparsely distributed Ag₃Sn IMC particles containing a small amount of Fe. Moreover, the addition of Fe leads to the formation of large circular FeSn₂ IMC particles located in the interdendritic regions. On the one hand, tensile tests indicate that the elastic modulus, yield strength, and ultimate tensile strength (UTS) increase with increasing Ag content. On the other hand, increasing the Ag content reduces the total elongation. The addition of Fe decreases the elastic modulus, yield strength, and UTS, while the total elongation is still maintained at the Sn-1Ag-0.5Cu level. The effect of aging on the mechanical behavior was studied. After 720 h and 24 h of aging at 100°C and 180°C, respectively, the Sn-1Ag-0.5Cu solder alloy experienced a large degradation in its mechanical properties after both of the aging conditions, whereas the mechanical properties of the Sn-3Ag-0.5Cu solder alloy degraded more dramatically after 24 h of aging at 180°C. However, the Fe-bearing SAC105 solder alloys exhibited only slight changes in their mechanical properties after both aging procedures. The inclusion of Fe in the Ag₃Sn IMC particles suppresses their IMC coarsening, which stabilizes the mechanical properties of the Fe-bearing SAC105 solder alloys after aging. The results from differential scanning calorimetry (DSC) tests indicate that the addition of Fe has a negligible effect on the melting behavior. However, the addition of Fe significantly reduces the solidification onset temperature and consequently increases the degree of undercooling. In addition, fracture surface analysis indicates that the addition of Fe to the Sn-1Ag-0.5Cu alloy does not affect the mode of fracture, and all tested alloys exhibited large ductile dimples on the fracture surface.     

Sn-Zn-Bi无铅焊料表面张力及润湿性研究

采用气泡最大压力法对Sn-9Zn-XBi焊料进行了表面张力测试,用平衡法测试焊料的润湿性。结果表明:Bi的添加大大降低了Sn-Zn系焊料熔体的表面张力;然而焊料暴露在空气环境下1min后,表面形成ZnO导致其表面张力增大;Bi的增加提高了焊料在铜片上的润湿力,缩短了润湿时间;Sn-9Zn-XBi焊料润湿力仍低于Sn-40Pb,其原因是焊料–铜界面能偏高。     

Fatigue crack-growth behavior of Sn-Ag-Cu and Sn-Ag-Cu-Bi lead-free solders

Fatigue crack-growth behavior and mechanical properties of Sn-3Ag-0.5Cu, Sn-3Ag-0.5Cu-1Bi, and Sn-3Ag-0.5Cu-3Bi solders have been investigated at room temperature (20°C). The tensile strength and hardness of the solders increased with increasing Bi content. However, the yield strengths of Sn-3Ag-0.5Cu-1Bi and Sn-3Ag-0.5Cu-3Bi solders were nearly similar, but the 3Bi solder exhibited the lowest ductility. Fatigue crack-growth behavior of the solders was dominantly cycle dependent in the range of stress ratios from 0.1–0.7 at a frequency of 10 Hz, except for the Sn-3Ag-0.5Cu solder tested at a stress ratio of 0.7. Mixed intergranular/transgranular crack propagation was observed for the Sn-3Ag-0.5Cu solder tested at the stress ratio of 0.7, indicating the importance of creep in crack growth. The Sn-3Ag-0.5Cu-1Bi and Sn-3Ag-0.5Cu-3Bi solders had higher resistance to time-dependent crack growth, resulting from the strengthening effect of the Bi constituent. It appears that the addition of Bi above a certain concentration is harmful to the mechanical properties of Sn-3Ag-0.5Cu.     

Creep and rupture behavior of Cu wire/lead-free solder-alloy joint specimen

Creep and rupture behavior of Cu wire/lead-free solder-alloy joint specimens have been investigated using Sn-3.5% Ag and Sn-0.5% Cu alloys. A Sn-37% Pb solder alloy is also used as a reference material. The present authors have fabricated a creep-rupture testing machine for Cu wire/solder-alloy joint specimens, performed creep and rupture tests at 303 K and 403 K, analyzed the characteristics of the creep and rupture behavior, and compared these to test specimens cut from the same alloy ingots. It is also found that the rupture strength of the joint specimens is related to the rupture strength of the alloys.     

Interfacial reaction and microstructural evolution for electroplated Ni and electroless Ni in the under bump metallurgy with 42Sn58Bi solder during annealing

In the present study, several under bump metallization (UBM) schemes using either electroplated Ni or electroless Ni (EN) as the solderable layer are investigated. The EN and electroplated Ni are first deposited on Cu/Al₂O₃ substrates, followed by electroplating of thin gold coatings. Joints of 42Sn-58Bi/Au/EN/Cu/Al₂O₃ and 42Sn-58Bi/Au/Ni/Cu/Al₂O₃ are annealed at 145 C and 185CC for 30–180 minutes to investigate the interfacial reaction between the solder and metallized substrates. For 42Sn-58Bi/Au/Ni-5.5wt.%P/Cu/Al₂O₃, 42Sn-58Bi/Au/Ni-12.1wt.%P/Cu/Al₂O₃, and 42Sn-58Bi/Au/Ni/CU/Al₂O₃ joints annealed at 145 C, only Ni₃Sn₄ intermetallic compound (IMC) formed at the solder/EN interace. When annealed at an elevated temperature of 185 C, plate-like Ni₃Sn₄ IMC forms at the solder/Ni-5.5wt.%P interface, while a trace of (Ni, Cu)₃Sn₄ IMC is observed at the solder/Ni-12.1wt.%P interface and within the solder region. For the electroplated Ni-based multi-metallization substrate, the Ni₃Sn₄ IMC is present at the solder/Ni interface during annealing at 185 C for a short period of time. In the 42Sn-58Bi/Au/EN/Cu/Al₂O₃ joint, the EN spalls off the EN layer and migrates into the solder region when annealed at 185 C. The interface of the solder/electroplating Ni becomes saw-toothed as the annealing temperature is raised to 185 C. In addition, an enrichment of phosphorus is observed at the interface of the Ni-Sn IMC and EN.     

Characterization of eutectic Sn-Bi solder joints

This report presents experimental results on 58Bi-42Sn solder joints, optical and SEM microstructures of their matrix and of their interface with copper, solidification behavior studied by differential scanning calorimetry, wettability to copper, creep, and low cycle fatigue. These results are discussed in comparison with 60Sn-40Pb solder, and with three low temperature solders, 52In-48Sn, 43Sn-43Pb-14Bi, and 40In-40Sn-20Pb. The 58Bi-42Sn solder paste with RMA flux wets Cu matrix with a wetting angle of 35° and had a 15° C undercooling during solidification. The constitutive equation of the steady state shear strain rate, and the Coffin-Manson relation constants for the low cycle shear fatigue life at 65° C have been determined. The test results show that this solder has the best creep resistance but the poorest fatigue strength compared with the other four solders.     

Spreading Behavior and Evolution of IMCs During Reactive Wetting of SAC Solders on Smooth and Rough Copper Substrates

The effect of surface roughness of copper substrate on the reactive wetting of Sn-Ag-Cu solder alloys and morphology of intermetallic compounds (IMCs) was investigated. The spreading behavior of solder alloys on smooth and rough Cu substrates was categorized into capillary, diffusion/reaction, and contact angle stabilization zones. The increase in substrate surface roughness improved the wetting of solder alloys, being attributed to the presence of thick Cu₃Sn IMC at the interface. The morphology of IMCs transformed from long needle shaped to short protruded type with an increase in the substrate surface roughness for the Sn-0.3Ag-0.7Cu and Sn-3Ag-0.5Cu solder alloys. However, for the Sn-2.5Ag-0.5Cu solder alloy the needle-shaped IMCs transformed to the completely scallop type with increase in the substrate surface roughness. The effect of Ag content on wetting behavior was not significant.     

Creep Behavior of Lead-Free Sn-Ag-Cu + Ni-Ge Solder Alloys

We developed a new lead-free solder alloy, an Sn-Ag-Cu base to which a small amount of Ni and Ge is added, to improve the mechanical properties of solder alloys. We examined creep deformation in bulk and through-hole (TH)␣form for two lead-free solder alloys, Sn-3.5Ag-0.5Cu-Ni-Ge and Sn-3.0Ag-0.5Cu, at elevated temperatures, finding that the creep rupture life of the Sn-3.5Ag-0.5Cu-Ni-Ge solder alloy was over three times better than that of the Sn-3.0Ag-0.5Cu solder at 398 K. Adding Ni to the solder appears to make microstructural development finer and more uniform. The Ni added to the solder readily combined with Cu to form stable intermetallic compounds of (Cu, Ni)₆Sn₅ capable of improving the creep behavior of solder alloys. Moreover, microstructural characterization based on transmission electron microscopy analyses observing creep behavior in detail showed that such particles in the Sn-3.5Ag-0.5Cu-Ni-Ge solder alloy prevent dislocation and movement.