Properties Of Lead-free Solder Joints Research Articles

Shear and Fatigue Properties of Lead-Free Solder Joints: Modeling and Microstructure Analysis

Abstract The reliability of Sn-Ag-Cu (SAC)-based solder alloys has been extensively investigated after the prohibition of lead in the electronics industry owing to their toxicity. Low-temperature solder (LTS) alloys have recently received considerable attention because of their low cost and reduced defects in complex assemblies. The shear and fatigue properties of individual solder joints were tested using an Instron micromechanical testing system in this research. Two novel solder alloys (Sn-58Bi-0.5Sb-0.15Ni and Sn-42Bi) with low melting temperatures were examined and compared with Sn-3.5Ag and Sn-3.0Ag-0.8Cu-3.0Bi. The surface finish was electroless nickel-immersion gold (ENIG) during the test. Shear testing was conducted at three strain rates, and the shear strength of each solder alloy was measured. A constant strain rate was used for the cyclic fatigue experiments. The fatigue life of each alloy was determined for various stress amplitudes. The failure mechanism in shear and fatigue tests was characterized using scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS). The results revealed that Sn-3.0Ag-0.8Cu-3.0Bi had superior shear and fatigue properties compared to other alloys but was more susceptible to brittle failure. The shear strain rate affected the failure modes of Sn-3.0Ag-0.8Cu-3.0Bi, Sn-58Bi-0.5Sb-0.15Ni, and Sn-42Bi; however, Sn-3.5Ag was found to be insensitive. Several failure modes were detected for Sn-3.5Ag in both shear strength and fatigue tests.

Effects of temperature and strain rate on tensile properties of (Ag,Cu)-Sn intermetallic compounds: A molecular dynamics study

With the advent of environment friendly microelectronic packaging technologies, the demand of lead-free solders alloy has escalated. Leadfree solder material is an alloy of Tin (Sn), Silver (Ag) and Copper (Cu) and is widely known as SAC solder. Intermetallic compounds (IMC) like Ag3Sn, Cu3Sn and Cu6Sn5 are proven to adversely affect the mechanical properties of lead-free solder joints. This study presents the effects of temperature and strain rate on mechanical performances of these IMCs found in SAC solders. In addition, weakest and most sensitive IMC among these is detected. Modified embedded atom method potential parameters are used in LAMMPS to simulate the stress-strain behavior with deformation evolution of intermetallic compounds in different temperature and strain rates. The simulation was performed at 298 K–498 K with 50 K regular increment and at the strain rates of 108 s−1, 109 s−1, and 1010 s−1. Insight from this study suggest that the tensile properties of these IMCs increased with the increased in strain rates and decrease in temperature. Furthermore, the effect of temperature on tensile strength is observed to be higher at low strain rate whereas strain rate influence is lower at low temperatures. Among all the intermetallic, Ag3Sn exhibits lowest tensile properties with lowest stability, conversely Cu3Sn exhibits highest tensile properties with highest stability.

Influence of high-power-low-frequency ultrasonic vibration time on the microstructure and mechanical properties of lead-free solder joints

Cu/SAC305/Cu solder joints were fabricated at ambient atmosphere using high-power-low-frequency ultrasonic vibration (USV) assisted hot plate reflow soldering. The influences of USV time (within 6s) on the microstructure, hardness, yield strength and shear strength of the solder joints were investigated. Refinement on the solder matrix microstructure, formation of thinner interfacial Cu6Sn5 IMC layer, and enhancement on the hardness, yield strength and shear strength were observed in all the ultrasonic-treated solder joints. Marked morphological change on the β-Sn phase was observed in the joint solder matrix when the USV time was increased from 1s to 6s. Thicker interfacial Cu6Sn5 IMC layer was observed at the top and bottom substrate/solder interfaces and the difference in thickness between these interfaces was greater at increased USV time. The solder matrix hardness increased with increasing USV time, indicating a reduction in joint ductility. The USV time has no influence on the joint shear strength, but decreased joint yield strength was observed after 1s of USV time. The influences of the USV time on the solder joint properties were contributed by the combined effects of acoustic cavitation and streaming induced by the USV in the molten solder during reflow soldering.

Microstructure and Mechanical Properties of Lead-Free Solder Joints

Microstructure and Mechanical Properties of Lead-Free Solder Joints

20306 温度サイクル負荷を受ける鉛フリーはんだ接合部の力学的特性

When we estimate the fatigue life, we have to conduct FE analysis using a constitutive model which can exhibit the mechanical properties of lead-free solder alloy. As such mechanical properties of lead-free solder joints under cyclic thermal loading, the stress strain curves, stress strain hysteresis loops and stress relaxation curves with these temperature dependencies and these rate dependencies are regarded. In addition, these solder joints under cyclic temperature load are stressed mainly in shear and the solder alloys have scale effect. Thus in this study, for FE analysis of solder joints under cyclic temperature load, we carried out tensile tests, tension-compressive tests and the stress-relaxation tests of lead-free solder alloy using the small sized shear specimen.

IMC and creep behavior in Lead-free solder joints of Sn-Ag and Sn-Ag-Cu alloy system by SP method

The creep properties of Lead-free solder joints were evaluated by Shear Punch (SP) method. Lead-free solder joints with Sn-4Ag, Sn-4Ag-0.5Cu and Sn-4Ag-1.0Cu solder tested and compared to the lead solder joint with Sn-37Pb. SP creep tests were performed at temperature of 30, 50 and 80°C. For the SP method, constant shear stresses from 3 to 22 MPa were applied into the SP specimen(10 × 10 × 0.5 mm). From the SEM, IMC layer of Cu6Sn5 were observed on the interface between solder and Cu. From the SEM and EDX, the Lead-free solder joints have much thicker interfacial IMC layer than the Sn-37Pb solder joint. From the power law relationships, the Lead-free solder joints show superior creep resistance than the Sn-37Pb solder joint. And among Lead-free solder joints, at 30 and 50°C, the ternary SnAgCu solder joints have high creep resistance than the binary SnAg solder joint. Sn-4Ag-1.0Cu solder joint shows the best creep resistance at 30°C, and Sn-4Ag-0.5Cu solder joint shows the best creep resistance at 50°C. However, at high temperature 80°C, all of the Lead-free solder joints have similar creep properties. The activation energy were expressed with creep strain rate and temperature. The ternary SnAgCu solder joints have the higher activation energy than the binary SnAg solder joint at 17.68 and 18.72 MPa. Average activation energy of Lead-free solder joints were lower than that of Sn-37Pb 75.11 kJ/mol. The average activation energy of Sn-4Ag-1.0Cu, Sn-4Ag-0.5Cu and Sn-4Ag solder joint at high stress is 102.68, 62.52 and 58.61 kJ/mol, respectively.

Mechanical Properties of Lead-Free Solder Joints Under High-Speed Shear Impact Loading

In this study we expanded on recently reported research by using a modified miniature Charpy impact-testing system to investigate the shear deformation behavior of Sn–3.0Ag–0.5Cu lead-free solder joints at high strain rates ranging from 1.1 × 103 s−1 to 5.5 × 103 s−1. The experimental results revealed that the maximum shear strength of the solder joint decreased with increasing load speed in the ranges tested in this study. For solder joints tested at a shear speed exceeding 1 m/s, corresponding to an approximate strain rate that exceeds 1950 s−1, the brittle fracture mode is the main failure mode, whereas lower strain rates result in a ductile-to-brittle transition in the fracture surfaces of solder joints. In addition, the mode II stress intensity factor (KII) used to evaluate the fracture toughness (KC) of an interfacial intermetallic compound layer between Sn–3.0Ag–0.5Cu solder and the toughness of copper substrate was found to decrease from 1.63 MPa m0.5 to 0.97 MPa m0.5 in the speed range tested here.

Investigation of the Mechanical Properties of Lead-Free Solder Materials

Reliability and life-time of electric/electronic products is still a highly actual and discussed topic. Every electric/electronic product is heterogeneous system which consists not only from components and constructional parts, but also from many interconnections. Basic segments of interconnection are solder joints, recently created by lead-free materials. The passing from well-proven lead solders to lead free compositions brought many new aspects in reliability and life-time of electronic systems. Some of the negative factors are their poor electro-mechanical properties, which are based on complicated microstructure of interlayer interface. This fact causes defects in solder joints and consequential failure of the function. This article is focused on an investigation of mechanical properties of lead-free solder joints. There are investigated various properties of common lead-free solder paste depending on various concentrations of O2 by reflow process and on various components size. Next, there are shown microstructures by micro sections for various concentrations of O2. Mechanical properties (mechanical strength) are tested by shear test equipment DAGE 2400.

Comparison of isothermal mechanical fatigue properties of lead-free solder joints and bulk solders

Low-cycle isothermal mechanical fatigue testing of bulk solders and solder joints of eutectic Sn–37 wt% Pb, Sn–3.5 wt% Ag and Sn–4.0 wt% Ag–0.5 wt% Cu were carried out at room temperature over a wide range of strains (1–10%). The fatigue results of both bulk and solder joints were compared; the eutectic Sn–37 wt% Pb was used as reference. Concerning bulk solders and solder joints, the two lead-free solders showed better fatigue properties than Sn–37Pb. For all three solder alloys tested, bulk solders showed better fatigue performance than solder joints when subjected to higher strains. The situation was the opposite at lower strains, resulting in bulk solders depicting larger values for the Coffin–Manson fatigue exponent and ductility coefficient compared to solder joints.

Studies of the mechanical and electrical properties of lead-free solder joints

The mechanical and electrical properties of several Pb-free solder joints have been investigated including the interfacial reactions, namely, the thickness and morphology of the intermetallic layers, which are correlated with the shear strength of the solder joint as well as its electrical resistance. A model joint was made by joining two “L-shaped” copper coupons with three Pb-free solders, Sn-3.5Ag (SA), Sn-3.8Ag-0.7Cu (SAC), and Sn-3.5Ag-3Bi (SAB) (all in wt.%), and combined with two surface finishes, Cu and Ni(P)/Au. The thickness and morphology of the intermetallic compounds (IMCs) formed at the interface were affected by solder composition, solder volume, and surface finish. The mechanical and electrical properties of Pb-free solder joints were evaluated and correlated with their interfacial reactions. The microstructure of the solder joints was also investigated to understand the electrical and mechanical characteristics of the Pb-free solder joints.

The creep properties of lead-free solder joints

This paper describes the creep behavior of three tin-rich solders that have become candidates for use in lead-free solder joints: Sn-3.5Ag, Sn-3Ag-0.5Cu, and Sn-0.7Cu. The three solders show the same general behavior when tested in thin joints between copper and Ni/Au metallized pads at temperatures between 60‡C and 130°C. Their steady-state creep rates are separated into two regimes with different stress exponents. The low-stress exponents range from ∼3–6, while the high-stress exponents are anomalously high (7–12). Strikingly, the high-stress exponent has a strong temperature dependence near room temperature, increasing significantly as the temperature drops from 95°C to 60°C. The anomalous creep behavior of the solders appears to be due to the dominant tin constituent. Research on creep in bulk samples of pure tin suggests that the anomalous temperature dependence of the stress exponent may show a change in the dominant mechanism of creep. Whatever its source, it has the consequence that conventional constitutive relations for steady-state creep must be used with caution in treating tin-rich solder joints, and qualification tests that are intended to verify performance should be carefully designed.