Lead-containing Solder Alloys Research Articles

Investigation of the mechanical properties of lead-free Sn-58Bi solder alloy with cobalt addition through flux doping

Purpose This study aims to investigate the effect of incorporating cobalt (Co) into Sn-58Bi alloy on its phase composition, tensile properties, hardness and thermal aging performances. The fracture morphologies of tensile-tested solders are also investigated to correlate the microstructural changes with tensile properties of the solder alloys. Then, the thermal aging performances of the solder alloys are investigated in terms of their intermetallic compound (IMC) layer morphology and thickness. Design/methodology/approach The Sn-58Bi and Sn-58Bi-xCo, where x = 1.0, 1.5 and 2.0 Wt.%, were prepared using the flux doping technique. X-ray diffraction (XRD) is used to study the phase composition of the solder alloys, whereas scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) are used to investigate the microstructure, fractography and compositions of the solders. Tensile properties such as ultimate tensile strength (UTS), Young’s modulus and elongation are tested using the tensile test, whereas the microhardness value is gained from the micro-Vickers hardness test. The morphology and thickness of the IMC layer at the solder’s joints are investigated by varying the thermally aging duration up to 56 days at 80°C. Findings XRD analysis shows the presence of Co3Sn2 phase and confirms that Co was successfully incorporated via the flux doping technique. The microstructure of all Sn-58Bi-xCo solders did not differ significantly from Sn-58Bi solders. Sn-58Bi-2.0Co solder exhibited optimum properties among all compositions, with the highest UTS (87.89 ± 2.55 MPa) at 0.01 s−1 strain rate and the lowest IMC layer thickness at the interface after being thermally aged for 56 days (3.84 ± 0.67 µm). Originality/value The originality and value of this research lie in its novel exploration of the flux doping technique to introduce minor alloying of Co into Sn-58Bi solder alloys, providing new insights into enhancing the properties and performance of these solders. This new Sn-Bi-Co alloy has the potential to replace lead-containing solder alloy in low-temperature soldering.

Mechanical Properties of the (81Sn–xCu–(19-x) In) % Alloys with x=1, 2, 3, 4, and 5

Research on Material Science has a long tradition, and lead has been used widely in many applications. Because of their low melting temperatures, low cost, and good wettability, tin -lead alloys have been preferred as solder alloys in industrial production, particularly in the electronics sector. However, due to the negative effects of lead on the environment and human health, studies on lead-free solder alloys have been increasing, and the subject is still important today. Furthermore, environmental protection agencies (RoHS and WEEE) have prohibited the use of lead-containing solder alloys in the manufacture of electronic and microelectronic products. In this work, the effect of adding Cu to alloys in five compositions of (81Sn-xCu-(19-x)In) % with x=1, 2, 3, 4, and 5 on the mechanical properties has been studied. The five samples were prepared from high purity (99%) elements (Sn –Cu –In ) using the melting technique. The creep of the ternary alloys was examined at different temperatures (25, 50, and 80 ) under three different loads (2.5, 6.3, and 11.4 MPa). It was found that the creep rate of the ternary alloys was increasing with the increases in Cu –content in all samples at different temperatures and loads. The mean values of the stress exponent were found to be in the range of (0.1-2.5) as well as the activation energies vaied in the range of (0.48-5.33) eV for all alloys under different loads (2.5, 6.3, and 11.4 MPa).

Temperature and strain-rate dependent constitutive model for prediction of thermal cycling life

With temperature cycling as the characteristic working environment of aerospace electronic chips, various constitutive models have been proposed to predict the failure problems of solder joints. However, most researchers adopted only a set of constant parameters to describe the solder joint material properties throughout the temperature cycles, which is obviously unreasonable concerning the possible wide range of working temperature. In fact, with the changing temperature and strain rate, some material parameters will correspondingly evolve as observed in the experiments. In this paper, the framework of the Anand constitutive model is adopted to verify the effect of material parameters of different temperatures and strain rates on the mechanical properties of materials under the scenario of temperature cycling. The lead-containing solder alloy 63Sn37Pb material that is most widely used in aerospace is selected as the solder material for the interconnection structure. In addition, a typical plastic ball grid array (PBGA) packaging structure is used to analyze the influence of the constitutive model parameters on the PBGA thermal fatigue life. Based on experimental data, seven sets of constitutive model parameters with different temperatures (-55°C~125°C) under a region of low strain rate (1×10-4/s) were employed to compare the mechanical properties of the material under temperature cycling. The sensitivity analysis of material parameters is performed and the underlying mechanism are also explained so that the present study can promote the optimization of the constitutive model in numerical simulations in practice.

Assessing microstructure and mechanical behavior changes in a Sn-Sb solder alloy induced by cooling rate

In the present investigation a directional solidification experiment was performed in order to examine distinct microstructures related to different slices of the solidified Sn-2 wt.%Sb alloy casting. Such alloy is an alternative of interest with the target of replacing lead-containing solder alloys (containing 85 to 97 wt% of Pb) given that lead (Pb) is considered an important environmental complaint and has devastating effects on the human body. The imposed conditions in the present experiment may lead to solutal and thermal stability of the melt throughout solid growth towards the liquid. It was found that Sn-rich cells may prevail for cooling rates higher than 1.0 K/s whereas only Sn-rich dendrites appear for specimens solidified at rates lower than 0.3 K/s. The growth of dendrites is delayed when compared to previous results in the literature. In the presence of convective flow originated either thermally or solutally, β-Sn dendrites were reported to grow for samples solidified at rates as high as 1.5 K/s (i.e., 5 times higher). It appears that convection currents induce instabilities to happen at the solidification front and the growth of dendrites is benefited over such conditions. Tensile tests were also performed for Sn-Sb samples having distinct cellular and dendritic dimensions. It was found that unstable plastic flow happened during all tensile tests. The formation of bands along a specimen gauge was recognized as being a manifestation of the Portevin – Le Chatelier (PLC) effect. A homogeneous deformation stage preceded the start of serrations of stresses in the samples of the investigated alloy. The amplitudes of the serrations were found to be lower in the samples having cells as compared to those associated with dendritic microstructures.

Experiments and simulations of uniaxial ratchetting deformation of Sn–3Ag–0.5Cu and Sn–37Pb solder alloys

This paper discusses uniaxial ratchetting deformation of lead-free solder alloy Sn–3Ag–0.5Cu and lead-containing solder alloy Sn–37Pb, which were subjected to tension–compression loading with several stress amplitudes and stress ratios, minimum stress over maximum stress. First the uniaxial ratchetting tests were conducted with three maximum stresses and four stress ratios. All tests were conducted using cylindrical bulk specimens of the solder alloys at 313 K. The test results show that there are differences in the ratchetting deformation behavior of the two solder alloys; the larger ratchetting strain occurs in the lead-containing solder alloy than in the lead-free solder alloy. The ratchetting deformation was simulated by the dislocation based constitutive model proposed by Estrin et al. (J Eng Mater Technol 118:441, 1996). The evolution equation of the back stress employed in the constitutive model was modified considering a dynamic recovery term. The effect of the modification of the back stress evolution is discussed by comparing the simulations with the corresponding experimental results. The simulations suggest that the recovery term in the kinematic hardening rule plays an important role in fitting the simulation to the experimental results of the ratchetting deformation of the solder alloys.

Experimental and Theoretical Investigation of Viscoplastic Deformation of Solder Alloys for Electronic Packaging

This paper discusses ratchetting deformation of lead-free solder Sn/3Ag/0.5Cu and lead-containing solder alloy Sn/37Pb with several stress amplitudes and stress ratios of the maximum stress to the minimum stress. First the uniaxial ratchetting testsare conducted with three maximum stresses and five stress ratios. The all tests are conducted using cylindrical bulk specimens of the solder alloys at 313 K. The test results show that there is the difference in the viscoplastic deformation behavior between two solder alloys. The relationship between ratchetting strain and time is estimated by Biley-Norton law to explain that the uniaxial ratchetting deformation is strongly dominated by the viscous deformation. Finally, the ratchetting deformation is simulated by the dislocation based constitutive model proposed by Estrin [1]. The simulations show that there is a possibility to simulate the uniaxial ratchetting by clarifying the dislocation mechanism of the solder alloys.

New lead-containing solder alloy with improved properties

Structure, hardness, mechanical and electrical transport properties of the rapidly solidified Pb–25Sn alloy with Cd additions have been investigated. The results show that, a continuous increase in Cd content causes more precipitations of Cd as a third phase, which in turn causes continuous enhancement of the wetting, electrical and mechanical properties of this alloy. In spite of the higher value of TCR for the alloy Pb–25Sn–20Cd, it has the most enhanced properties when compared with the others. It has a lower electrical resistivity, lower wetting angle, higher Vickers microhardness values and yield strength when compared with the eutectic Pb–Sn solder alloy.

Experimental Observation of Correlation Between Creep and Uniaxial Ratchetting of Sn∕37Pb and Sn∕3Ag∕0.5Cu Solder Alloys

Time-dependent deformations such as creep and ratchetting of solder alloys are significant deformation phenomena that need to be understood to ensure the safety and reliability of solder joints in electronic packaging. There is much research on creep deformation of solder alloys, but ratchetting deformation, especially the correlation between creep and ratchetting deformation of solder alloys has not been investigated. This paper discusses the correlation between creep and uniaxial ratchetting deformation to establish the differences in the time-dependent deformation of lead-free and lead-containing solder alloys. Uniaxial ratchetting tests were conducted by cyclic tension–compression tests or cyclic tension–unloading tests at several ratios of the maximum to minimum stresses. Additional creep tests following the uniaxial ratchetting were also conducted to observe the effect of the uniaxial ratchetting on creep deformation. An empirical method to select an optimal lead-free solder alloy is discussed by defining a uniaxial ratchetting strain rate. The additional creep tests also show that the uniaxial ratchetting deformation has a strong correlation to the creep deformation and that the correlation is different for lead-free and lead-containing solder alloys.

3559 はんだ材のラチェット・クリープ変形特性とシミュレーション(S23-2 弾性・塑性・粘塑性特性,S23 電子実装用はんだの強度特性評価)

This paper conducts both experiments and numerical simulations of viscoplastic deformation such as creep and uniaxial ratcheting deformation of solder alloys. The correlation between creep and uniaxial ratcheting deformation is firstly discussed to establish the differences in the time-dependent deformation of lead-free and lead-containing solder alloys. The experiments show that the ratcheting deformation of lead-free solder alloy is much smaller than the lead-containing solder alloy. Especially, there is much difference in the effect of the uniaxial ratcheting on the creep deformation between the two solder alloys. Then numerical simulations are also carried out using the dislocation based constitutive model. There is amoderate agreement between experiment and simulation.

A quantitative evaluation of time-independent and time-dependent deformations of lead-free and lead-containing solder alloys

A method to separate plasticity and creep is discussed for a quantitative evaluation of the plastic, transient creep, and steady-state creep deformations of solder alloys. The method of separation employs an elasto-plastic-creep constitutive model comprised of the sum of the plastic, transient creep, and steady-state creep deformations. The plastic deformation is expressed by the Ramberg-Osgood law, the steady-state creep deformation by Garofalo’s creep law, and the transient creep deformation by a model proposed here. A method to estimate the material constants in the elasto-plastic-creep constitutive model is also proposed. The method of separation of the various deformations is applied to the deformation of the lead-free solder alloy Sn/3Ag/0.5Cu and the lead-containing solder alloy Sn/37Pb to compare the differences in the plastic, transient creep, and steady-state creep deformations. The method of separation provides a powerful tool to select the optimum lead-free solder alloys for solder joints of electronic devices.