Ag Lead-free Solder Research Articles

Effect of 1 wt% ZnO nanoparticles addition on the microstructure, IMC development, and mechanical properties of high Bi content Sn–57.6Bi–0.4Ag solder on Ni metalized Cu pads

In the present study, we investigate the performance of 1 wt% ZnO nanoparticles addition to Sn–57.6Bi–0.4Ag lead-free solder and thereby study the effect of nanoparticle addition on the hardness and the inter-metallic compound (IMC) growth, under different reflow conditions. The interfacial morphology of both the plain Sn–57.6Bi–0.4Ag solder and the composite Sn–57.6Bi–0.4Ag/1 % ZnO solder containing nanoparticles on Ni metalized Cu pads ball grid array substrates and the distribution of nanoparticles on the composite solder were characterized metallographically by using scanning electron microscope (SEM). Prior research efforts by others has found strong evidence that the thickness of the IMC layer increases substantially with the increase in the number of reflow cycles. At the interfaces between solder ball and substrate, scallop-shaped Sn–Ni–Cu IMC layer was found in both the plain solder and the composite solder. Meanwhile the SEM results indicated that the growth of IMC of the composite solder joint was different compared to the plain solder with same number of reflow cycles and reflow time. During the shear test, the shear strength of the composite solder samples containing nanoparticle reinforcement were consistently higher than the plain solder joints due to the fact that the IMC growth has been retarded substantially by the nanoparticle addition. The fracture surface of plain solder exhibited a brittle fracture mode with a relatively smooth surface while doped solder joints showed typical ductile failures with very rough dimpled surfaces.

Microstructure and Sn Crystal Orientation Evolution in Sn-3.5Ag Lead-Free Solders in High-Temperature Packaging Applications

Understanding the reliability of eutectic Sn-3.5Ag lead-free solders in high-temperature packaging applications is of significant interest in power electronics for the next-generation electric grid. Large-area (2.5 mm × 2.5 mm) Sn-3.5Ag solder joints between silicon dies and direct bonded copper substrates were thermally cycled between 5°C and 200°C. Sn crystal orientation and microstructure evolution during thermal cycling were characterized by electron backscatter diffraction in the scanning electron microscope. Comparisons were made between the observed initial texture and microstructure and its evolution during thermal cycling. Gradual lattice rotation and grain boundary misorientation evolution observed due to thermal cycling suggested a continuous recrystallization mechanism. Recrystallization behavior was correlated with dislocation slip activities.

A Study on the Physical Properties and Interfacial Reactions with Cu Substrate of Rapidly Solidified Sn-3.5Ag Lead-Free Solder

A rapidly solidified Sn-3.5Ag eutectic alloy produced by the melt-spinning technique was used as a sample in this research to investigate the microstructure, thermal properties, solder wettability, and inhibitory effect of Ag3Sn on Cu6Sn5 intermetallic compound (IMC). In addition, an as-cast Sn-3.5Ag solder was prepared as a reference. Rapidly solidified and as-cast Sn-3.5Ag alloys of the same size were soldered at 250°C for 1 s to observe their instant melting characteristics and for 3 s with different cooling methods to study the inhibitory effect of Ag3Sn on Cu6Sn5 IMC. Experimental techniques such as scanning electron microscopy, differential scanning calorimetry, and energy-dispersive spectrometry were used to observe and analyze the results of the study. It was found that rapidly solidified Sn-3.5Ag solder has more uniform microstructure, better wettability, and higher melting rate as compared with the as-cast material; Ag3Sn nanoparticles that formed in the rapidly solidified Sn-3.5Ag solder inhibited the growth of Cu6Sn5 IMC during aging significantly much strongly than in the as-cast material because their number in the rapidly solidified Sn-3.5Ag solder was greater than in the as-cast material with the same soldering process before aging. Among the various alternative lead-free solders, this study focused on comparison between rapidly solidified and as-cast solder alloys, with the former being observed to have better properties.

Effects of In/Ce to Sn-3.5Ag Lead-Free Solder on Microstructures and Properties

The Sn-3.5Ag based lead-free electronic solder doped with element In /Ce were prepared respectively using a smelting method with protection by molten salt (SMPMS). The effects of In addition and Ce addition on microstructure, melting point and wettability of Sn-3.5Ag solder were investigated comparatively. And the effects of addition on the intermetallic compound (IMC) layer formed at solder/Cu substrate interface and the shear strength of solder joint were also studied. The results showed that appropriate In or Ce addition can refine Ag3Sn grains and improve the shear strength of solder joint. Element In can significantly reduce the melting point, narrow the melting range of the solder alloy and improve wettability on Cu substrate. When the content of In was 3%, the spreading area was the largest and the wettability was the best. The addition of Ce can efficiently reduce the thickness of the IMC layer grown at the solder/Cu substrate interface and improve the shear strength of solder joint. When the content of Ce was 0.5%, the solder joint had the minimum average IMC thickness and the maximum shear strength.

Mechanical properties of Sn–Ag lead-free solder alloys based on the dendritic array and Ag3Sn morphology

The aim of this experimental investigation is to evaluate the tensile properties of as-cast Sn–Ag alloys as a function of both the resulting secondary dendritic arm spacings and the morphology of the Ag3Sn IMC (intermetallic compound). This comparative experimental investigation was carried out with a view to assess the application of Sn–Ag alloys as alternative solder materials. A directional water-cooled solidification apparatus was used to obtain the as-cast samples. The resulting microstructures, ultimate and yield tensile strengths and elongation of Sn–2wt.% Ag and Sn–3.5wt.% Ag alloys were experimentally determined and compared with the corresponding results of the traditional Sn–40wt.% Pb solder alloy. It was found that the Sn–Ag alloys examined comply with the compromise between compatible mechanical strength and environmental protection.

Shear Deformation Behaviors of Sn3.5Ag Lead-free Solder Samples

In this study, shear tests have been performed on the as-reflowed Sn3.5Ag solder bumps and joints to investigate the deformation behavior of Sn3.5Ag lead-free solder samples. Scanning electron microscopy (SEM) was employed to characterize the microstructures of the samples and orientation imaging microscopy (OIM) with electron backscattered diffraction (EBSD) in SEM was used to obtain crystallographic orientation of grains to provide a detailed characterization of the deformation behavior in Sn3.5Ag solder samples after shear tests. The deformation behavior in solder samples under shear stress was discussed. The experimental results suggest that the dynamic recrystallization could occur under shear stress at room temperature and recrystallized grains should evolve from subgrains by rotation. Compared with that of non-recrystallized and as-reflowed microstructures, the microhardness of the recrystallized microstructure decreased after shear tests.

Indentation creep of lead-free Sn–3.5Ag solder alloy: Effects of cooling rate and Zn/Sb addition

The effect of cooling rate and separate additions of 1.5wt% Zn and 1.5wt% Sb on the creep behavior of Sn–3.5wt% Ag lead-free solder alloy was investigated by indentation creep testing at 298 and 370K. All three alloys were prepared at two different cooling rates of 0.01Ks−1 and 8Ks−1, in order to examine the influence of cooling rate on their creep resistances. This affected the microstructure and thus, the creep behavior of the materials. Fast cooling enhanced the creep resistance by refining the coarse plate-like Ag3Sn particles. Both the ternary alloys showed creep resistances higher than that of the eutectic binary Sn–3.5Ag alloy. The higher creep resistance of the Zn-containing alloy is attributed to the refinement of the Ag3Sn precipitates, while solid solution hardening of Sb in the Sn matrix is responsible for the superior creep behavior of the Sb-containing alloy. The higher creep resistance of the ternary alloys is more pronounced at higher temperatures, where the binary base alloy weakens more readily. The stress exponents were found to be in the range 6.6–9.8 and 7.1–10.2 for the slowly cooled (SC) and the fast cooled (FC) conditions, respectively. These are in agreement with those determined by room temperature conventional creep and other creep testing methods of the same materials reported in the literature. The high stress exponents together with the activation energy of about 80kJ/mol may suggest that the dominant creep mechanism is dislocation creep.

Effect of indium addition on the microstructural formation and soldered interfaces of Sn-2.5Bi-1Zn-0.3Ag lead-free solder

The microstructural formation and properties of Sn-2.5Bi−xIn-1Zn-0.3Ag (in wt%) alloys and the evolution of soldered interfaces on a Cu substrate were investigated. Apart from the relatively low melting point (about 195°C), which is close to that of conventional eutectic Sn-Pb solder, the investigated solder presents superior wettability, solderability, and ductility. The refined equiaxial grains enhance the mechanical properties, and the embedded bulk intermetallic compounds (IMCs) (Cu6Sn5 and Cu5Zn8) and granular Bi particles improve the joint reliability. The addition of In reduces the solubility of Zn in the β-Sn matrix and strongly influences the separation and growth behaviors of the IMCs. The soldered interface of Sn-2.5Bi-xIn-1Zn-0.3Ag/Cu consists of Cu-Zn and Cu-Sn IMC layers.

Effect of Purging Gas on Wetting Behavior of Sn-3.5Ag Lead-Free Solder on Nickel-Coated Aluminum Substrate

The wetting characteristics of Sn-3.5Ag lead-free solder alloy on nickel-coated aluminum substrates in air (ambient), nitrogen, and argon atmospheres were investigated. The contact angles for the solder alloy obtained under air and argon atmospheres were in the range of 36°-38°. With nitrogen atmosphere the contact angle was found to be significantly lower at about 26°. Solder solidifying in air exhibited needle-shaped tin-rich dendrites surrounded by a eutectic matrix. The amount of tin dendrites decreased in argon atmosphere. However, the morphology of tin dendrites transformed from needle-shaped to nearly non-dendritic shape as the soldering atmosphere was changed from air to nitrogen. The interfacial microstructures revealed the presence of Ni3Sn and Ni3Sn4 IMCs at the interface. The enhanced wettability observed under nitrogen atmosphere is attributed to the higher thermal conductivity of nitrogen gas and the formation of higher amount of Ni3Sn IMCs at the interface compared to air and argon atmospheres.

Electrical resistivity of Sn–3.5Ag–xBi solder melts

This article reported the temperature dependence of the electrical resistivity (ρ) of liquid Sn–3.5Ag lead-free solder alloy in continuous heating and cooling processes with varying Bi content in the range of 0, 2, 3.5, 5, and 7 wt.% Abnormal transitions can be observed on ρ–T curves, which indicate liquid–liquid structure transition (LLST) occurs in Sn–3.5Ag–xBi melts. Interestingly, unlike the pattern at first heating cycle, the LLST is reversible during subsequent cooling and heating cycles. Sn may play an important role on the reversibility, and Bi has a noticeable influence on the turning temperatures and characteristics during first heating cycle. The transition mechanism is analyzed from the viewpoint of short-range orders.

Tensile properties and wettability of SAC0307 and SAC105 low Ag lead-free solder alloys

In this article, the tensile properties of low Ag lead-free solder alloys, SAC0307 and SAC105, are examined under various strain rates and temperatures. The wettability of these solders on Cu pad is also characterized by using different fluxes. The SAC305 and Sn37Pb solder alloys are also studied for comparison. Our results show that the properties of all solder alloys are dependent on the strain rate and temperature. The ultimate tensile strength increases monotonously with the increment of strain rate. Both SAC0307 and SAC105 alloys possess lower strength and higher elongation ratio than SAC305 and Sn37Pb alloys. For all the fluxes used in this study, the SAC0307 and SAC105 alloys show the similar wettability to SAC305, whereas worse than that of Sn37Pb alloy. Increasing the activity of the flux does not improve the wettability of the SAC solder alloys on Cu pad effectively.

Reactions of Sn-3.5Ag-Based Solders Containing Zn and Al Additions on Cu and Ni(P) Substrates

In this study we consider the effect of separately adding 0.5 wt.% to 1.5 wt.% Zn or 0.5 wt.% to 2 wt.% Al to the eutectic Sn-3.5Ag lead-free solder alloy to limit intermetallic compound (IMC) growth between a limited volume of solder and the contact metallization. The resultant solder joint microstructure after reflow and high-temperature storage at 150°C for up to 1000 h was investigated. Experimental results confirmed that the addition of 1.0 wt.% to 1.5 wt.% Zn leads to the formation of Cu-Zn on the Cu substrate, followed by massive spalling of the Cu-Zn IMC from the Cu substrate. Growth of the Cu 6 Sn 5 IMC layer is significantly suppressed. The addition of 0.5 wt.% Zn does not result in the formation of a Cu-Zn layer. On Ni substrates, the Zn segregates to the Ni 3 Sn 4 IMC layer and suppresses its growth. The addition of Al to Sn-3.5Ag solder results in the formation of Al-Cu IMC particles in the solder matrix when reflowed on the Cu substrate, while on Ni substrates Al-Ni IMCs spall into the solder matrix. The formation of a continuous barrier layer in the presence of Al and Zn, as reported when using solder baths, is not observed because of the limited solder volumes used, which are more typical of reflow soldering.

The microstructure and properties of the Sn-xBi-0.9Zn-0.3Ag lead-free solders

Low Ag content, Sn-Bi-Zn-Ag lead-free solders, have been studied. The microstructures of the Sn-xBi-0.9Zn-0.3Ag (x = 0, 1 and 3, in weight percent, hereafter) specimens are composed of β-Sn phase, AgZn3 intermetallic compounds (IMCs) and Zn-rich phase. Bi precipitates appear when the Bi content amounts to 3 wt%. The Sn-Bi-Zn-Ag alloys exhibit high performance on tensile properties, microhardness and melting temperature, owing to the existence of Bi precipitates. Moreover, the reactions upon heating and cooling of the investigated Sn-Bi-Zn-Ag lead-free solders are also systematically studied.

Effect of Bi content on spalling behavior of Sn–Bi–Zn–Ag/Cu interface

The effect of the Bi content on the formation of intermetallic compounds (IMCs) layers between the Sn-xBi-0.9Zn-0.3Ag lead-free solder (with x = 1, 2, 3 and 4, in weight percent, hereafter) and Cu substrate was investigated. The structure of the IMC layer in the soldered interface varies apparently with increasing the Bi content. When the Bi content is 1 wt%, the interface soldered is consisted of CuZn and Cu6Sn5 IMC layers, which are separated by an intermediate solder layer. As the Bi content increases, the spalling phenomenon tends to disappear. Moreover, the layer between the Sn-2Bi-0.9Zn-0.3Ag solder and Cu substrate is thicker than others. The evolution of the soldered interfacial structure could be attributed to the existence of Bi.

Sn-3.5Ag系とSn-5Sb系鉛フリーはんだ合金の高温クリープ挙動

Sn-3.5Ag系とSn-5Sb系鉛フリーはんだ合金の高温クリープ挙動

Synthesis and DSC study on Sn3.5Ag alloy nanoparticles used for lower melting temperature solder

The traditional Sn–Pb eutectic solder alloys are being phased out from the electronics industry due to the toxicity of lead (Pb), leading to the development and implementation of lead-free solders. Sn3.5Ag lead-free solder alloy, considered to be one of the promising alternatives to replace the traditionally used Sn–Pb solder, however, still has some weaknesses, such as its higher melting temperature than that of the Sn–Pb solder alloy. A possible way to decrease the melting temperature of a solder alloy is to decrease the alloy particle size to the nanometer range. Sn3.5Ag nanoparticles with different size distribution were synthesized using chemical reduction method by applying NaBH4 as reduction agent. The melting properties of these synthesized nanoparticles were studied by differential scanning calorimetry (DSC), and size-dependent melting temperature depression of these nanoparticles has been observed. Gibbs–Thomson equation was used to analyze the size-dependent melting temperature property, giving a good prediction of the melting temperature depression for the Sn-based lead-free solder alloy nanoparticles.

Growth mechanism of Ni 3Sn 4 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 Ni 3Sn 4 in the Sn–3.5Ag/Ni interfacial reaction is discussed and presented. The growth process of Ni 3Sn 4 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 Ni 3Sn 4 were found to be dependent on these factors.

Using Microwave-Assisted Powder Metallurgy Route and Nano-size Reinforcements to Develop High-Strength Solder Composites

In the present study, Sn-0.7Cu and Sn-3.5Ag lead-free solders used in the electronics packaging industry were reinforced with different volume percentages of nano-size alumina and tin oxide particulates, respectively, to synthesize two new sets of nanocomposites. These composites were developed using microwave-assisted powder metallurgy route followed by extrusion. The effects of addition of particulates on the physical, microstructural, and mechanical properties of the nanocomposites were investigated. Mechanical properties (microhardness, 0.2% YS, and UTS) for both composite systems increase with the presence of particulates. The best tensile strength was realized for composite solders reinforced with 1.5 vol.% alumina and 0.7 vol.% tin oxide particulates, which far exceeds the strength of eutectic Sn-Pb solder. The morphology of pores was observed to be one of the most dominating factors affecting the strength of materials.

Development of Thermal Mechanical Fatigue Testing Machine for Solders

This paper describes development of a thermal mechanical fatigue testing machine for solders. Several researches studying on creep, low cycle fatigue and creep-fatigue for solders at constant temperature have been reported recently, but no or small number of studies on thermal mechanical fatigue of solders have been reported. It might be resulted from that suitable thermal mechanical fatigue testing machines, which can perform the test at temperatures applicable to solders, have not been developed. In this study, the thermal mechanical fatigue testing machine which can cover the test temperatures between 233K (–40°C) and 423K (150°C) is developed. Using the developed testing machine, thermal mechanical fatigue test for Sn-3.5Ag lead-free solder was carried out and an applicability of the testing machine is investigated. This study also discusses the obtained thermal mechanical fatigue life in comparing with data in low cycle fatigue test at constant temperature.

Rate-dependent deformation of Sn–3.5Ag lead-free solder

Compression experiments on bulk Sn-3.5Ag lead-free solder specimens have been carried out to help formulate the material constitutive behaviour of this alloy using the concept of an evolving internal stress. Tests covered the temperature range 0–125 °C and fixed strain rates between 3 × 10−7–3 × 10−3 s−1. Flow behaviour was found to be compatible with that for a deforming a tin-rich matrix (stress exponent n = 7, activation energy Q = 46.7 kJ/mol) in which the external applied stress is reduced by an internal back stress due to the presence of precipitate phase particles. Stress–strain curves have been satisfactorily modelled using rate equations incorporating linear hardening and diffusion-controlled recovery. Comparison with supplementary tension and creep experiments, and with data from other researchers, indicates that inconsistencies in reported flow behaviour is most likely to be due to variations in initial microstructure rather than the nature of the applied loading.