The effect of Pb contamination on the solidification behavior of Sn-Bi solders

This paper presents experimental results and theoretical calculations to evaluate the effects of Pb contamination on the solidification behavior of Sn-x Bi alloys (x=5, 10, and 58 mass %). The pasty (mushy) range, the type of solidification path, and the fraction of the ternary eutectic are described. The experimental results are obtained from thermal analysis and quantitative metallography, and the solidification calculations are performed using Lever and Scheil assumptions. The experimental results agree with the Scheil calculations. The free zing range of Pb contaminated Sn−Bi solders is greatly increased due to the formation of a ternary eutectic reaction at (95.3±0.5)°C. This in crease is a likely cause of porosity in contaminated solder joints. The results provide an example of an analysis method for use in solder alloys in general. Editor's Note: In this paper, the percentage represents mass fraction, which is the same as weight fraction.     

Tensile behavior of pb-sn solder/cu joints

Solders of nominal 95Pb-5Sn and 60Sn-40Pb were used to join Cu plates. The effect of ternary additions of In, Ag, Sb, and Bi to the near-eutectic solder were also investigated. Bulk solder and interfacial joint microstructures were characterized for each solder alloy. The solder joints were strained to failure in tension; joint strength and failure mode were determined. 95Pb-5Sn/Cu and 60Sn-40Pb/Cu specimens were tested both as-processed and after reflow. 95Pb-5Sn/Cu as-processed and reflow specimens failed in tension in a ductile mode. Voids initiated at β-Sn precipitates in the as-processed specimens and at the Cu₃Sn intermetallic in the reflow specimens. 60Sn-40Pb/Cu failed transgranularly through the Cu₆Sn₅ intermetallic in both the as-processed and reflow conditions. The joint tensile strength of the reflow specimens was approximately half that of the as-processed specimens for both the high-Pb and near-eutectic alloys. The Cu₆Sn{5} intermetallic dominated the tensile failure mode of the near-eutectic solder/Cu joints. The fracture path of the near-eutectic alloys with ternary additions depended on the presence of Cu₆Sn₅ rods in the solder within the Cu plates. Specimens with ternary additions of In and Ag contained only interfacial intermetallics and exhibited interfacial failure at the Cu₆Sn₅. Joints manufactured with ternary additions of Sb and Bi contained rods of Cu₆Sn₅ within the solder. Tensile failure of the Sb and Bi specimens occurred through the solder at the Cu₆Sn₅ rods.     

The Effect of Bi Contamination on the Solidification Behavior of Sn-Pb Solders

This paper presents experimental results and theoretical calculations that evaluate the effects of Bi contamination on the solidification behavior of Sn-Pb alloys. The pasty (mushy) range, the type of solidification path, and the microstructure of the solidified alloys are described. The experimental results are obtained from thermal analysis and metallography, and the solidification calculations are performed using the lever rule and Scheil assumptions. The experimental results show that the solidification behavior of the contaminated solder at cooling rates of 5°C/min and 23°C/min is closer to the predictions of the lever rule calculations than those of the Scheil calculations. Although the freezing range of Bi-contaminated Sn-Pb solders is increased, formation of a ternary eutectic reaction at 95°C is not observed for contamination levels below the Bi mass fraction of 6%.     

Liquidus Projection and Solidification of the Sn-In-Cu Ternary Alloys

Sn-In alloys are promising low-melting-point Pb-free solders. Knowledge of the ternary Sn-In-Cu liquidus projection is important for Sn-In solder applications. Sn-In-Cu ternary alloys were prepared and their primary solidification phases and phase-transformation temperatures during heating were determined. The liquidus projection of the Sn-In-Cu ternary system was determined based on the primary solidification phase at different compositional regimes, the phase-transformation temperatures of the ternary alloys, the phase boundaries and reaction temperatures of the constituent binary systems, and the available ternary Sn-In-Cu data in the literature. No ternary compound was found in the as-cast alloys. The Sn-In-Cu liquidus projection has 11 primary solidification phase regions and seven ternary invariant reactions with the liquid phase, and η-(Cu₆Sn₅,Cu₂In) has a very large compositional regime as the primary solidification phase. A very interesting phenomenon that was also observed is that the solidification paths of some Sn-In-Cu alloys surpass the liquidus trough after their intersections.     

A comparative study of the kinetics of interfacial reaction between eutectic solders and Cu/Ni/Pd metallization

A comparative study of the kinetics of interfacial reaction between the eutectic solders (Sn-3.5Ag, Sn-57Bi, and Sn-38Pb) and electroplated Ni/Pd on Cu substrate (Cu/Ni/NiPd/Ni/Pd) was performed. The interfacial microstructure was characterized by imaging and energy dispersive x-ray analysis in scanning electron microscope (SEM). For a Pd-layer thickness of less than 75 nm, the presence or the absence of Pd-bearing intermetallic was found to be dependent on the reaction temperature. In the case of Sn-3.5Ag solder, we did not observe any Pd-bearing intermetallic after reaction even at 230°C. In the case of Sn-57Bi solder the PdSn₄ intermetallic was observed after reaction at 150°C and 180°C, while in the case of Sn-38Pb solder the PdSn₄ intermetallic was observed after reaction only at 200°C. The PdSn₄ grains were always dispersed in the bulk solder within about 10 μm from the solder/substrate interface. At higher reaction temperatures, there was no Pd-bearing intermetallic due to increased solubility in the liquid solder. The presence or absence of Pd-bearing intermetallic was correlated with the diffusion path in the calculated Pd-Sn-X (X=Ag, Bi, Pb) isothermal sections. In the presence of unconsumed Ni, only Ni₃Sn₄ intermetallic was observed at the solder-substrate interface by SEM. The presence of Ni₃Sn₄ intermetallic was consistent with the expected diffusion path based on the calculated Ni-Sn-X (X=Ag, Bi, Pb) isothermal sections. Selective etching of solders revealed that Ni₃Sn₄ had a faceted scallop morphology. Both the radial growth and the thickening kinetics of Ni₃Sn₄ intermetallic were studied. In the thickness regime of 0.14 μm to 1.2 μm, the growth kinetics always yielded a time exponent n >3 for liquid-state reaction. The temporal law for coarsening also yielded time exponent m >3. The apparent activation energies for thickening were: 16936J/mol for the Sn-3.5Ag solder, 17804 J/mol for the Sn-57Bi solder, and 25749 J/mol for the Sn-38Pb solder during liquid-state reaction. The corresponding activation energies for coarsening were very similar. However, an apparent activation energy of 37599 J/mol was obtained for the growth of Ni₃Sn₄ intermetallic layer during solid-state aging of the Sn-57Bi/substrate diffusion couples. The kinetic parameters associated with thickening and radial growth were discussed in terms of current theories.     

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.     

Influence of alloy composition on fillet-lifting phenomenon in Sn-Ag-Bi alloys

The influence of the Bi content on the fillet-lifting phenomenon in Sn-3Ag-Bi alloys was investigated. The peculiar composition dependence on the fillet-lifting exists in Sn-3Ag-xBi alloys. With fillet-lifting, they are 3–20%Bi. Without fillet-lifting, they are 0–1%Bi, 30%Bi, and above. This is related to the alloy solidification characteristics, in particular, the amount of latent heat release near the final solidification temperature (solidus temperature) during solidification. Fillet-lifting is likely to occur in the alloy with a latent heat released which is insufficient for alleviation of the temperature gradient in the solder joint. A new predictable index for the occurrence of fillet-lifting is proposed as the fillet-lifting resistant index (FRI) that correlates well with the fillet-lifting occurrence rate. The fillet-lifting does not occur when a solder alloy has a FRI of 0.75 and above. The magnitude of the solidification temperature range (T) does not necessarily correlate with the fillet-lifting, because there are some alloys without fillet-lifting in spite of their having a wide T. There is also little correlation between the fillet-lifting and the formation of a Bi concentration at the solder/Cu land interface, previously thought to cause fillet-lifting. The Bi concentration at the solder/Cu land interface may occur as a consequence of the Sn consumption by the intermetallic compound formation with the Cu dissolving into the molten solder and existing at the interface rather than the solidification segregation.     

Effect of Ag Content on Solidification Cracking Susceptibility of Sn-Ag-Cu Solder Joints

In the present work, the effect of Ag content on solidification cracking susceptibility of Sn-Ag-Cu solder joints has been investigated. Solders containing 1.0 wt.% to 3.8 wt.% Ag were used in the experiment. Solidification cracks were created using a copper self-restraint specimen, which could simulate the process of solidification cracking. Meanwhile, solidification cracking susceptibility was evaluated by comparing the total crack length of the solder joint. The results indicate that solidification cracks exist in solder joints with 1.0 wt.% to 3.0 wt.% Ag content, whereas there are no cracks in Sn-3.8Ag-0.7Cu solder joints. When the Ag content increases from 1.0 wt.% to 3.0 wt.%, the total crack length of Sn-Ag-Cu solder joints increases to a maximum and then drops to zero when the Ag content reaches 3.8 wt.%. In addition, the susceptibility to solidification cracking is observed as follows: SAC207 > SAC305 > SAC107 > SAC387.     

Comparative study of wetting behavior and mechanical properties (microhardness) of Sn-Zn and Sn-Pb solders

In this paper the solder balling, wetting, spreading, slumping and microhardness testing of the Sn-Zn based solders have been compared with the Sn-Pb solder. Two types of solders (Sn-9Zn and Sn-8Zn-3Bi) have been investigated along with Sn-37Pb solder for reference. The variation of these tests has been done as a function of reflow temperature from 220-250 °C. Solder balls of these three solder pastes after 15 min heating at 230 °C show no ball formation surrounding the central ball. Spread test shows that above 240 °C Sn-9Zn is very good and can be comparable to Sn-37Pb. The wetting angle of Sn-9Zn (39°) at 250 °C is even lower than the Sn-37Pb solder (41°). In case of Sn-8Zn-3Bi, the wetting angle is very high (77°) at 220 °C, which is unacceptable but it drops down to 48° at 250 °C. Line profiles of slump test show that after preheating at 160 °C, Sn-9Zn behaves similar to Sn-37Pb with better distinction in the finer pitch (120 μm). Microhardness shows two different characteristics for eutectic and non-eutectic solder pastes. Hardness of Sn-37Pb and Sn-9Zn (eutectic) decreases with increasing reflow temperature while the microhardness of Sn-8Zn-3Bi (non-eutectic) increases with increasing reflow temperature. Microstructural characterization at 220 and 250 °C shows grain coarsening in Sn-37Pb and Sn-9Zn solders, which cause the hardness to drop a little. For Sn-8Zn-3Bi, with increasing temperature the amount of hard Bi segregation increases which is the main cause of the rise in hardness. SEM images show the formation of Pb rich islands in Sn-37Pb, formation of Zn rod from spheroids in Sn-9Zn and precipitation of Bi-rich phase in Sn-8Zn-3Bi are the important features that contribute to different hardness nature.     

Wetting and reaction of Sn-2.8Ag-0.5Cu-1.0Bi solder with Cu and Ni substrates

The wettability of newly developed Sn-2.8Ag-0.5Cu-1.0Bi lead-free solder on Cu and Ni substrates was assessed through the wetting balance tests. The wettability assessment parameters such as contact angle (ϑ_c) and maximum wetting force (F_w) were documented for three solder bath temperatures with three commercial fluxes, namely, no-clean (NC), nonactivated (R), and water-soluble organic acid flux (WS). It was found that the lead-free Sn-2.8Ag-0.5Cu-1.0Bi solder exhibited less wetting force, i.e., poorer wettability, than the conventional Sn-37Pb solder for all flux types and solder bath temperatures. The wettability of Sn-2.8Ag-0.5Cu-1.0Bi lead-free solder on Cu substrate was much higher than that on Ni substrate. Nonwetting for Sn-2.8Ag-0.5Cu-1.0Bi and Sn-Pb solders on Ni substrate occurred when R-type flux was used. A model was built and simulations were performed for the wetting balance test. The simulation results were found very close to the experimental results. It was also observed that larger values of immersion depth resulted in a decrease of the wetting force and corresponding meniscus height, whereas the increase in substrate perimeter enhanced the wettability. The wetting reactions between the solder and Cu/Ni substrates were also investigated, and it was found that Cu atoms diffused into the solder through the intermetallic compounds (IMCs) much faster than did the Ni atoms. Rapid formation of IMCs inhibited the wettability of Sn-2.8Ag-0.5Cu-1.0Bi solder compared to the Sn-Pb solder.     

Microstructure, Thermal and Wetting Properties of Sn-Bi-Zn Lead-Free Solder

The microstructures, phase transformations, and wettability of Sn-Bi-Zn solder alloys were investigated by scanning electron microscopy, x-ray diffraction, and differential scanning calorimetry (DSC). The results show that the alloys are composed of primary Sn-rich phase or Zn-rich phase, (Sn + Zn) eutectic structure, and (Sn + Bi + Zn) ternary eutectic structure. The microstructural characterization of Sn-xBi-Zn alloys indicates that, with increasing Bi content, more of the eutectic (Sn-Bi-Zn) structures is formed. DSC profiles reveal that the eutectic peak of the samples did not differ very much, but the reaction temperature of the alloys decreases with increased Bi content. The spreading rates of solders increased with the addition of Zn, which affects the interfacial reactions between the solders and copper.     

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.     

Development of a low-cycle fatigue life curve for 80In15Pb5Ag

The purpose of this study is to develop a methodology to predict the low-cycle (large strain—from 0.1 to 0.35 strain) fatigue life of solders subject to thermal cycling. Solders are commonly used in electronic assemblies. Using thermal fatigue data measured for 80In15Pb5Ag, a low-cycle fatigue curve for 80In15Pb5Ag solder subject to thermal cycling was developed. Specifically a Coffin-Manson relationship was derived for the solder, with a high degree of correlation (see Table I), for four different failure criteria, defined in the body of the paper. This relationship, together with calculated strains in the solder joint, allows the low-cycle fatigue life of the solder joint to be predicted.     

Properties of quad flat package joints using Sn-Zn-Bi solder with varying lead-plating materials

The effects of plating materials (Sn-10Pb, Sn-3.5Ag, Sn-3Bi, Sn-0.7Cu, and Au/Pd/Ni) on Cu leads on quad flat package (QFP) joints using a Sn-8Zn-3Bi solder were investigated. The joints with Sn-3.5Ag plating and Sn-8Zn-3Bi solder had the slowest growth rate of interfacial reaction layers and the highest strength. The Ag dissolution into the interfacial reaction layers causes this increased strength. The Sn-Ag plating is the best plating material for Cu leads among the five kinds of plating using Sn-8Zn-3Bi solder.     

Formation of Pb/63Sn solder bumps using a solder droplet jetting method

Formation processes of Pb/63Sn solder droplets using a solder droplet jetting have not been sufficiently reported. Solving problems such as satellite droplets and position errors are very important for a uniform bump size and reliable flip-chip solder bump formation process. First, this paper presents the optimization of jet conditions of Pb/63Sn solder droplets and the formation process of Pb/63Sn solder bumps using a solder droplet jetting method. Second, interfacial reactions and mechanical strength of jetted Pb/63Sn solder bumps and electroless Ni-P/Au UBM joints have been investigated. Interfacial reactions have been investigated after the second solder reflow and aging, and results were compared with those of solder bumps formed by a solder screen-printing method. Third, jetted solder bumps with variable bump sizes have been demonstrated by a multiple jetting method and the control of waveform induced to a jet nozzle. Multiple droplets jetting method can control various height and size of solder bumps. Finally, real applications of jetted Pb/63Sn solder bumps have been successfully demonstrated on conventional DRAM chips and integrated passive devices (IPDs).     

Properties of ternary Sn-Ag-Bi solder alloys: Part II—Wettability and mechanical properties analyses

Bismuth additions of 1% to 10% were made to the 96.5Sn-3.5Ag (wt.%) alloy in a study to develop a Sn-Ag-Bi ternary composition. Thermal properties and microstructural analyses of selected alloy compositions were reported in Part I. Wettability and mechanical properties are described in this paper. Contact angle measurements demonstrated that Bi additions improved wetting/spreading performance on Cu; a minimum contact angle of 31±4° was observed with 4.83 wt.% Bi addition. Increasing the Bi content of the ternary alloy raised the Cu/solder/Cu solder joint shear strength to 81 MPa as determined by the ring-and-plug tests. TEM analysis of the 91.84Sn-3.33Ag-4.83Bi composition presented in Part I indicated that the strength improvement was attributed to solid-solution and precipitation strengthening effects by the Bi addition residing in the Sn-rich phase. Microhardness measurements of the Sn-Ag-Bi alloy, as a function of Bi content, reached maximum values of 30 (Knoop, 50 g) and 110 (Knoop, 5 g) for Bi contents greater than approximately 4–5 wt.%.     

Primary solidification phases of the Sn-rich Sn-Ag-Cu-Ni quaternary system

The eutectic and near-eutectic Sn-Ag-Cu solders are the most promising lead-free solders, and nickel is frequently used as the barrier layer material. Nickel dissolves into the molten Sn-Ag-Ni alloy during the soldering process, and the ternary solder becomes a Sn-Ag-Cu-Ni quaternary melt near the nickel substrate. Liquidus projection is the projection of the liquidus trough and it delineates the boundaries of various primary solidification phases. Information of liquidus projection is helpful for understanding the alloys’ solidification behavior. This study prepared the Sn-Ag-Cu-Ni alloys of various compositions at the Sn-rich corner. The alloys were melted at higher temperatures and solidified in air. The solidified alloys were metallographically examined to determine the phases formed, especially the primary solidification phases. No ternary or quaternary compounds were found. The knowledge of the primary solidification phases, phase formation sequences, and reaction temperatures determined in this study were put together with all of the available liquidus projections of the constituent ternary systems to determine the primary solidification phases of the quaternary Sn-Ag-Cu-Ni system at the Sn-rich corner.     

Superplastic creep of low melting point solder joints

In prior work, we showed that eutectic Sn-Pb solder joints exhibit superplastic behavior after rapid solidification. Further examples of superplasticity in nominally air-cooled solder joints are reported in this study of three low-melting point alloys: 40In-40Sn-20Pb (wt. %), eutectic 52In-48Sn, and 43Sn-43Pb-14Bi, which were creep-tested in shear at 20°, 65°, and 90° C. The test results indicate that above 65° C, the indium-containing solders have stress exponents between 2.4 to 2.9, a possible overall shear strains of 500%, and an absence of primary creep; at 90° C, 43Sn-43Pb-14Bi solder has a stress exponent close to 2.3. Optical microstructures of the three solders are presented; they help to explain the superplastic behavior.     

微量元素对焊料特性的影响

在Sn-20Bi焊料中添加微量Ag、In、Ga和Sb,观察焊料的熔化特性和显微组织的变化.结果表明当Ag的质量分数为0.7%、In为0.1%、Ga为0.5%时焊料的熔点都分别有所降低,Bi的偏析明显减小;Sb不宜作为单独的第三组元加入,但与Ag、In、Ga一起加入时Sb能起到很好的固溶强化且有抑制Sn的相变的作用.通过最佳配比制成的Sn-Bi-X焊料,其熔点与Sn-37Pb接近,而且已不存在Bi的偏析,接近于实用.