SnPb Solder Research Articles

Creep Constitutive Models Suitable for Solder Alloys in Electronic Assemblies

Most solders used in electronic systems have low-melting temperature and hence experience significant amount of creep deformation throughout their life-cycle because typical operational and test conditions represent high homologous temperature. Phenomenological and mechanistic models used in the literature for predicting creep response of both bulk and grain scale specimens are reviewed in this paper. The phenomenological models reviewed in this paper are based on purely empirical observations of the creep deformation behavior or derived from qualitative interpretation of the underlying microscale mechanisms. These models have some intrinsic disadvantages since they do not have explicit mechanistic dependence on microstructural features. Therefore, the constitutive relations derived using the above models are difficult to extrapolate beyond the test conditions. This paper also reviews how some of the above limitations can be mitigated by using mechanistic or microstructurally motivated models. Mechanistic models are capable of estimating the material creep response based on the detailed physics of the underlying mechanisms and microstructure. The microstructure and constitutive response of the most popular family of lead-free solders, namely, SnAgCu (SAC) solders, are significantly different from those of previously used eutectic Sn37Pb solder. The creep deformation in Sn37Pb solder occurs primarily through diffusion-assisted grain-boundary sliding. In SAC solder joints, dislocation-based creep deformation mechanisms such as glide, climb, detachment, and cross-slip appear to be the dominant mechanisms in coarse-grained joints. Mechanistic creep models are therefore based on the deformation mechanisms listed above.

Mechanical and Physical Properties of In-Zn-Ga Lead-Free Solder Alloy for Low Energy Consumption

Due to the demand in the use of electronics devices in industry, the usage of solder connections has increased. Concerning with the toxicity of lead in Sn-37Pb solder alloy, developing lead free solder alloy with low melting temperature is one of the most important issues in electronic industry. Previously, researchers found out that the most promising candidate of lead free solder alloy is Sn-3.0Ag-0.5Cu (SAC). However, the melting temperature of this solder alloy is 217°C, 34°C higher than Sn-37Pb. This can lead to high energy consumption in electronic industry. In this paper, In-Zn-Ga solder alloy was investigated as a potential candidate replacing SAC. This study covers on the physical and mechanical properties of the solder alloy. Differential Scanning Calorimetry (DSC) testing shows that this solder alloy gave low melting temperature as low as 141.31°C. The addition of Ga in In-Zn solder alloy lowered the melting temperature compared to SAC and Sn-37Pb. From coefficient of thermal expansion (CTE) analysis, the In-Zn-Ga solder alloy gives good expansion properties and able to avoid the mismatch between the solder and copper substrates. The density of In-Zn-Ga solder alloy is 6.801g/cm3, lower than SAC and Sn-37Pb. For the strength, single lap shear testing was conducted on the In-Zn-Ga solder alloy and the results is near to the strength of SAC.

The Effects of Zinc Addition on the Microstructure, Melting Point and Microhardness of Sn-0.7Cu Lead-Free Solder Fabricated via Powder Metallurgy Method

The attempt to produce various types of lead-free solder has been actively investigated around the world in order to substitute the harmful SnPb solders. The effects of Zn addition on the microstructure, melting point and microhardness of Sn-0.7Cu lead-free solder were investigated with 1 wt% and 5 wt% of Zn additions. Powder metallurgy (PM) method was used to fabricate these Sn-0.7Cu-Zn lead-free solders. The results revealed that the addition of Zn was able to improve the solder properties. The melting point of Sn-0.7Cu-Zn lead-free solder was decreased drastically as the increasing of Zn additions. The Zn particles were distributed homogenously along the grain boundaries and produced refined dendrite β-Sn, which also lead to a superior microhardness values of solders.

Electropolishing of an Fe-Ni-Co Alloy in Acetic Acid-Perchloric Acid Mixture

Among Fe-Ni-Co alloys, Kovar (53% Fe-29% Ni-17% Co) designated as ASTM F-15[1], is a well-known glass-sealing alloy. It also is classified as low–expansion alloy [2]. Low expansion is required to avoid internal stresses for applications that involve strong ceramic to metal joining. One of the most significant problems in vacuum systems is the outgassing which is the liberation of a gas that was dissolved, trapped, frozen or absorbed in a material. To decrease the outgassing rate from the chamber walls, the material should have smooth and passivated surface finish [3]. Electropolishing, one of the finishing methods applied to decrease surface roughness, is appropriate for the systems involving parts of simple and complex geometries. However, there is very limited information on the electropolishing of Kovar alloy in the literature. Acetic acid (glacial)–perchloric acid (60%) mixture, stated as a cleaning solution before soldering Kovar [4], was used as electropolishing electrolyte in this work. A systematic study was conducted in 5 to 15% perchloric acid containing electrolytes at 25 to 35°C to find the optimum polishing conditions. A two–electrode setup employing Kovar anode and stainless steel cathode was used to perform voltammetric studies to determine low and high levels of current densities. According to linear sweep voltammetry results, shown in Figure 1, 200 to 600 mA/cm2current density range was selected to optimize electropolishing. Experimental route was designed using three parameters (current density, concentration and temperature) and three levels. The duration was kept constant as 5 minutes. Effect of electropolishing duration was determined by utilizing the condition that revealed the lowest roughness in 5 minutes. It was found that, the surface roughness of Kovar samples (having dimensions of 10×25×2 mm), was decreased from 0.21µm to 0.06 µmRa in a 15% perchloric acid containing solution. ADDIN EN.REFLIST 1. ASTMInternational, Standard Specification for Iron-Nickel-Cobalt Sealing Alloy. 2009.2. Special-Purpose Nickel Alloys, in ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys. 2000, ASM International. p. 92-101.3. Tajiri, K., Pit-free electropolishing of aluminum and its application for process chamber. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1998. 16(3): p. 1196.4. Vianco, P.T., F.M. Hosking, and J.A. Rejent, Solderability Testing of Kovar with 60Sn-40Pb Solder and Organic Fluxes, in 19th International AWS Brazing and Soldering Conference. 1988: New Orleans, LA. Figure 1

Climate specific thermomechanical fatigue of flat plate photovoltaic module solder joints

FEM simulations of PbSn solder fatigue damage are used to evaluate seven cities that represent a variety of climatic zones. It is shown that the rate of solder fatigue damage is not ranked with the cities' climate designations. For an accurate ranking, the mean maximum daily temperature, daily temperature change and a characteristic of clouding events are all required. A physics-based empirical equation is presented that accurately calculates solder fatigue damage according to these three factors. An FEM comparison of solder damage accumulated through service and thermal cycling demonstrates the number of cycles required for an equivalent exposure. For an equivalent 25-year exposure, the number of thermal cycles (−40°C to 85°C) required ranged from roughly 100 to 630 for the cities examined. It is demonstrated that increasing the maximum cycle temperature may significantly reduce the number of thermal cycles required for an equivalent exposure.

The magnetic behavior and mechanical properties of low resistance joints of GdBa2Cu3O7 − δ 2G tapes

The electrical properties of contacts of second-generation (2G) GdBCO superconducting tape conductors, which are soldered by Rose’s and PbSn solders, are investigated in an external magnetic field up to 5 T at a liquid nitrogen boiling temperature. The joint resistance at T = 77 K in the case of PbSn solder is approximately half that in the case of Rose’s alloy. The rise in the contact magnetoresistance with the field is weak and independent of the orientation of the magnetic field, and it is saturated in fields on the order of 3 T for both solders. The mechanical tensile strength of initial tape conductors and contacts is measured at room temperature. The ultimate strength of contacts produced by the PbSn solder is more than twice that for the contact made by Rose’s alloy. In the latter case, the ultimate tensile strength is lower than the critical stress of superconductor degradation.

Influence of protective atmosphere on the solderability and reliability of OSP-based solder joints

Electronic assembly technology is in the transition from traditional tin–lead to lead-free age. In comparison with Sn–Pb solders, high melting point and poor wettability of the lead-free solders lead to great challenge in many aspects. It is accepted that inert nitrogen atmosphere can improve the wettability and reliability of solder joints. But few systemic investigations have been performed on confirming the specific atmosphere requirement and optimizing the oxygen concentration in nitrogen. The focus of this study is to investigate the effect of reflow atmosphere on solder joint solderability and reliability for SAC305 lead-free solder. By in situ observations of the wetting behavior during reflow with SMT microscope, we developed a novel method to evaluate the wettability of solder joints. The spreading area and the tin climbing height of 0805 type capacitor were measured for different reflow atmospheres. Experimental results showed that nitrogen atmosphere could obviously improve the solder joint quality and the solder joint fabricated in 100 ppm O2/N2 exhibited the best wetting performance. Visual inspection through optical microscope and microstructural characterization of the solder joints after both reflow process and environmental tests were also carried out. It indicated that thermal cycle tests had serious degradation effect on the reliability of components fabricated in all of the atmospheres.

Solder wetting behavior enhancement via laser-textured surface microcosmic topography

In order to reduce or even replace the use of Sn-Pb solder in electronics industry, the laser-textured surface microstructures were used to enhance the wetting behavior of lead free solder during soldering. According to wetting theory and Sn-Ag-Cu lead free solder performance, we calculated and designed four microcosmic structures with the similar shape and different sizes to control the wetting behavior of lead free solder. The micro-structured surfaces with different dimensions were processed on copper plates by fiber femtosecond laser, and the effect of microstructures on wetting behavior was verified experimentally. The results showed that the wetting angle of Sn-Ag-Cu solder on the copper plate with microstructures decreased effectively compared with that on the smooth copper plate. The wetting angles had a sound fit with the theoretical values calculated by wetting model. The novel method provided a feasible route for adjusting the wetting behavior of solders and optimizing solders system.

Improvement in Intermetallic Thickness and Joint Strength in Carbon Nanotube Composite Sn-3.5Ag Lead-free Solder

The toxicity in the Sn-Pb solder has promoted the development of Pb-free solder in the electronics industries. Among the Pb-solders, the Sn-3.5Ag solder is considered a potential replacement and being studied by many researchers. Lately, nano material addition into the solder plays a significant role in the solder material in order to improve the joint reliability. In this study the influence of carbon nanotube (CNT) on the intermetallic formation and joint strength of Sn-3.5Ag solder under liquid state aging were investigated. The Sn-3.5Ag and Sn-3.5Ag-0.1CNT were prepared using the powder metallurgy method. The solders were soldered with Cu substrate at 250°C at different soldering time. The Cu6Sn5 and Cu3Sn intermetallics were observed under the scanning electron microscope with the Sn-3.5Ag-0.1CNT solder has a lower intermetallic thickness. The addition of CNT retarded the growth of Cu6Sn5 intermetallic from 1.69 x 10-10 to 1.02 x 10-10 cm2/s. Furthermore, CNT retarded the growth of Cu3Sn intermetallic from 4.9 x 10-11 to 2.5 x 10-11cm2/s. At the same time the Sn-3.5Ag-0.1CNT solder shows higher joint strength and lower ductility.

Isothermal Aging of Low-Ag SAC with Al Addition

Zinc sulfide (ZnS) thin films have been successfully deposited via spray pyrolysis using an aqueous solution of thiourea and zinc Amongst the candidates of lead-free solders, eutectic or near eutectic Sn-Ag-Cu or SAC has been found to be the most suitable candidate to replace the traditional tin-lead (Sn-Pb) solder. However, due to the higher cost of SAC (expensive Ag), research have now been directed to find a lower cost lead-free solder with comparable or better performance to the Sn-Pb or SAC, i.e low Ag SAC or Sn-Cu solders. The solder alloys developed in this work were SAC 0305 (0.3 wt% Ag, 0.5 wt% Cu and bal. Sn) and SAC0305 with 1 wt% Al and 2 wt% Al. The addition of Al is expected to refine the β-Sn grains and contribute to higher solder strength. Characterization of the solder alloys focused on the bulk solder microstructure, intermetallic compound (IMC) evaluation and wettability of solder alloys in reflowed and aged conditions. Reflow temperature was 270°C while aging was done isothermally in normal atmosphere for 100, 200 and 500hours at 100°C and 150°C. Microstructure of bulk solder and the IMC formed at interface between solder and Cu substrate were observed using SEM equipped with EDX. Wettability of solder was evaluated via wetting force and time, and the wetting angle between solder and the Cu substrate. A low Ag content resulted in fine Ag3Sn IMC as compared to much larger needle Ag3Sn observed in the commercial SAC 305 (Ag 3.0 wt%). With Al addition, SEM result showed finer β-Sn dendrites with fine Ag3Sn and Cu6Sn5 distributed within the eutectic colony of the bulk solder, and also thinner IMC layer in reflowed samples. Isothermally aged samples on the other hand, showed thicker IMC layer for Al added solder samples. This could be attributed to the finer β-Sn dendrites in the bulk solder providing more diffusion path and increased the thickness of IMC layer. The IMC formed at the interface between solder and the copper substrate was identified as Cu6Sn5 in reflowed samples, and both Cu6Sn5 and Cu3Sn when aged. Wettability of the solder alloys decreased with the addition of Al.

Modified Norris–Landzberg Model and Optimum Design of Temperature Cycling Alt

Accelerated life testing (ALT) is an effective way to assess the lifetime of a product. Due to the complex nature of its testing profile, it is difficult to carry out temperature cycling ALT. This paper establishes a modified Norris–Landzberg model as acceleration model, and proposes the optimum design method of temperature cycling ALT. First, the FEA method is used to study the influence of temperature cycling profile parameters on the thermal fatigue life of 63Sn–37Pb solder joints. Then, a modified Norris–Landzberg model is proposed by introducing ramp time and dwell time with an added weight value. Finally, the temperature cycling ALT is regarded as a special multi-stress ALT to study its optimum design method. The uniform design theory is used to determine the combined mode. The optimum model is established with the objective of minimizing the asymptotic variance of the estimation of median lifetime under normal use conditions, and the simulation example shows the workability of the proposed method.

Properties and Microstructures of Sn-Bi-X Lead-Free Solders

The Sn-Bi base lead-free solders are proposed as one of the most popular alloys due to the low melting temperature (eutectic point: 139°C) and low cost. However, they are not widely used because of the lower wettability, fatigue resistance, and elongation compared to traditional Sn-Pb solders. So the alloying is considered as an effective way to improve the properties of Sn-Bi solders with the addition of elements (Al, Cu, Zn, Ga, Ag, In, Sb, and rare earth) and nanoparticles. In this paper, the development of Sn-Bi lead-free solders bearing elements and nanoparticles was reviewed. The variation of wettability, melting characteristic, electromigration, mechanical properties, microstructures, intermetallic compounds reaction, and creep behaviors was analyzed systematically, which can provide a reference for investigation of Sn-Bi base solders.

Comparison of hermetic sealing using SAC and SnPb solder for a MEMS pressure sensor

In order to minimize the influence of packaging stress on the signal of MEMS pressure sensors, the pressure inlet can be reduced in footprint and mechanically decoupled from the mechanically moving parts. Moreover, the formation of a hermetic seal between the sensor inlet and an external inlet becomes more challenging as the footprint is reduced. Soldering is a preferred solution as a hermetic seal is achievable despite some surface roughness at the surfaces to be joined. However, the metallization on the MEMS, the solder and the metallization on the external inlet must be carefully matched to assure long term stability; the solder will react quickly with the metal layers deposited on the surfaces during the reflow process and later at a reduced rate during storage and application. The formation of intermetallic compounds (IMC) can catastrophically degrade the integrity of a joint if large amounts of voids are formed or the mechanical compliance significantly reduces as a result of the IMC formation. The metallization alternatives for the MEMS in this case were sputtered TiW/Au and NiCr/Au. The TiW and NiCr are adhesion layers whereas the Au is applied as a wetting layer which is normally fully consumed during the soldering process. A thick layer of plated Au, or a thick layer of plated Ni with a thin surface finish layer of Au, were possible metallization alternatives for the external inlet. Dewetting of solder from TiW is frequently mentioned in literature, but less conclusive work is published about soldering to NiCr/Au [1–3]. In particular, limited work has been published on long term effects of soldering to NiCr/Au surfaces using a SAC solder. In this work TiW and NiCr were compared as adhesion layers. In addition, SAC and SnPb were compared as solder, and Au and Ni/Au were compared as metallization on the external inlet. A total of 10–20 assemblies were prepared for each of 12 tested combinations. Half of the assemblies were exposed to high temperature storage (HTS) for ~300 hours at 130–150 °C. Shear testing and inspection of fracture surfaces and cross sections using light microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were performed for samples

Consistency of ZT-Scanner for Thermoelectric Measurements from 300 K to 700 K: A Comparative Analysis Using Si80Ge20 Polycrystalline Alloys

A Harman-based instrument for the characterization of thermoelectric (TE) materials in a wide temperature range (the ZT-Scanner) was introduced in an earlier publication, with a focus on a two-sample system calibration (2SSC) procedure used for the precise evaluation of thermal losses during the measurements. This technique offers an option to accurately measure the main TE parameters from 300 K to 700 K. We now report the results of ZT-Scanner measurements of p-type Si80Ge20 polycrystalline samples, including the TE figure of merit ZT, Seebeck coefficient, and thermal and electrical conductivities. These samples proved to be extremely stable up to the maximum temperature of measurement, and could eventually serve as a standard for thermoelectric characterization. The measurements were performed using both PbSn solder and conductive silver paste contacts. In all cases, Ni plating was used as a protective barrier between the TE alloys and the contact material. The experimental data has been compared to the typical data measured by the Jet Propulsion Laboratory on similar samples, providing a quantitative estimation of the accuracy of the measurement system, which has been found to be better than 0.015, or 5%, up to 700 K for ZT. The consistency of the TE measurements is evaluated by means of a statistical analysis of repetitive tests on the same and on different samples of identical nature. We also analyze the influence of thermal and electrical contact resistance on the measured properties.

The Spontaneous Growth of Pb Whisker from the Pb<sub>60</sub>Sn<sub>40</sub> Solder Layer on the Au Coating

In a recent failure analysis case of relay, the pure Pb whisker growth is found on the Pb60Sn40 solder layer on the Au coating of the spring leaf. The content analysis by the EDX shows that the Pb whisker grows from the Pb grain, which is surrounded by the AuSn intermetallic compound. It is thought that the elastic force of the spring leaf and the compressive stress from the intermetallic compound contributes to the Pb whisker growth, and the later factor is confirmed by the FIB (focused ion beam) result. Besides, the statistics result shows that the longer Pb whisker corresponds to the thicker Au coating, which could be explained by the effect of Au: forming AuSn intermetallic compound and Pb rich area. This work enriches our understandings on the metal whisker growth and consequently potential failure mode of SnPb solder.

Application of transient liquid phase sintering (TLPS) interconnect material for high temperature Pb-free RoHS compliant MLCC lead attachment

Transient Liquid Phase Sintering is a process that provides high temperature Pb-free RoHS compliant interconnect solutions that exceed the high temperature capabilities of Pb-Sn solders. KEMET, working in collaboration with Ormet Circuits Inc. has successfully applied Ormet's TLPS technology to a line of Leaded Multi-layer Ceramic Capacitors (MLCC) components for high temperature applications. The material is Pb-free, RoHS compliant and able to withstand process and operating temperatures > 400°C while having initial processing temperatures of less than 300°C. Potential applications for TLPS are in the automotive, aerospace, oil, gas, and geothermal exploration industries where electronics are being exposed to higher operating temperatures and require robust interconnects capable of withstanding harsh environments.

Thermally stable Cu3Sn/Cu composite joint for high-temperature power device

This paper describes a novel transient liquid phase sinter (TLPS) bonding that utilizes Sn-coated micro-sized Cu particles for high-temperature power device packaging. When used as die attach materials for Cu–Cu bonding, a thermally stable joint comprising a Cu3Sn intermetallic compounds (IMCs) with a dispersion of ductile Cu particles was obtained after bonding at 300°C for 30s, and a shear strength equivalent to that of conventional Pb–5Sn solder could be achieved. After isothermal aging at 300°C for 200h, the shear strength and microstructure of the bonding joints were almost unchanged.

Microstructural discovery of Al addition on Sn–0.5Cu-based Pb-free solder design

It is important to develop Pb-free solder alloys suitable for automotive use instead of traditional Sn–Pb solder due to environmental regulations (e.g., Restriction of Hazardous Substances (RoHS)). Al addition has been spotlighted to enhance solder properties. In this study, we investigated the microstructural change of Sn–0.5Cu wt.% based Pb-free solder alloys with Al addition (0.01–0.05 wt.%). The small amount of Al addition caused a remarkable microstructural change. The Al was favored to form Cu–Al intermetallic compounds inside the solder matrix. We identified the Cu–Al intermetallic compound as Cu33Al17, which has a rhombohedral structure, using EPMA and TEM analyses. This resulted in refined Cu6Sn5 networks in the Sn–0.5Cu based solder alloy. In addition, we conducted thermal analysis to confirm its stability at a high temperature of approximately 230 °C, which is the necessary temperature range for automotive applications. The solidification results were substantiated thermodynamically using the Scheil solidification model. We can provide criteria for the minimum aluminum content to modify the microstructure of Pb-free solder alloys.

Leaching behaviour and environmental risk assessment of heavy metals from electronic solder in acidified soil

The leaching behaviour of Sn and Pb elements from eutectic SnPb solder of electronic waste in acidic soil was investigated through acidification with HCl-H2SO4 solution and compared with saline solution. The amounts of Sn and Pb elements leached, when subjected to acidic soil, are higher than those with saline soil. Evidence for the significantly preferential release of Sn into the leachate is provided; the galvanic couple accelerated such preferential release. Surface product analysis reveals the slight damage of SnPb in saline soil. Serious dissolution due to electrochemical reaction and a thick, porous PbSO4 surface layer are observed in acidified soil, suggesting more severe toxicity potential of Pb in soil rather than in water.

Thermal property, wettability and interfacial characterization of novel Sn–Zn–Bi–In alloys as low-temperature lead-free solders

Abstract The effect of indium (In) addition on thermal property, microstructure, wettability and interfacial reactions of Sn–8Zn–3Bi lead-free solder alloys has been investigated. Results showed that addition of In could lower both solidus and liquidus temperatures of the solder alloys with wettabilty significantly improved. The spreading area of Sn–8Zn–3Bi–1.0In was increased by 34% compared to that of Sn–8Zn–3Bi. With the increase of In content, Zn-rich precipitates were smaller in size and distributed more uniformly, which might be beneficial for mechanical properties and corrosion resistance of the solders. The intermetallic compounds (IMCs) formed between Sn–8Zn–3Bi– x In solder/Cu substrate was identified as Cu–Zn with a scallop layer adjacent to the solder and a flat layer to the substrate. The addition of In slightly influenced the thickness of the IMCs. The newly developed Sn–Zn–Bi–In solder system has great potential to replace the Sn–Pb solders as low-temperature lead-free solders.