Comparison of Impact Toughness and Fracture Morphologies Between Pb-Containing and Pb-Free Solder Joints Subject to the Charpy Impact Test

Charpy impact tests on three kinds of as-soldered U-notch specimens were performed with reference to the American Society of Testing Materials (ASTM) standard E23-07 by using a pendulum-type impact tester at room temperature. Three kinds of solders, conventional Sn-37Pb, Sn-3.8Ag-0.7Cu, and Sn-3.8Ag-0.7Cu doped with rare-earth (RE) elements, were selected to fabricate the joint specimens for the impact test. The three joints demonstrate similar impact toughness values, with averages of 11.4 kJ/m², 10.4 kJ/m², and 11.0 kJ/m², respectively. Under observation by scanning electron microscopy (SEM), the Sn-37Pb joint exhibited mainly ductile fracture morphology. Fractographic observations of lead-free joints exhibited a mixture of ductile and brittle morphologies. The addition of RE elements resulted in an impact toughness that was slightly higher than that of the Sn-3.8Ag-0.7Cu alloy. The impact toughness and the fracture mode were notably dependent on the type of solder. Additionally, the thickness of the intermetallic compound (IMC) layer had a remarkable influence on the fracture path and impact toughness of the solder joints. An erratum to this article can be found at     

Effects of Microstructure and Loading on Fracture of Sn-3.8Ag-0.7Cu Joints on Cu Substrates with ENIG Surface Finish

When dropped, electronic packages often undergo failure by propagation of an interfacial crack in solder joints under a combination of tensile and shear loading. Hence, it is crucial to understand and predict the fracture behavior of solder joints under mixed-mode high-rate loading conditions. In this work, the effects of the loading conditions (strain rate and loading angle) and microstructure [interfacial intermetallic compound (IMC) morphology and solder yield strength] on the mixed-mode fracture toughness of Sn-3.8 wt.%Ag-0.7 wt.%Cu solder joints sandwiched between two Cu substrates with electroless nickel immersion gold (ENIG) metallization have been studied, and compared with the fracture behavior of joints attached to bare Cu. Irrespective of the surface finish, the fracture toughness of the solder joints decreased monotonically with strain rate and mode-mixity, both resulting in increased fracture proportion through the interfacial IMC layer. Furthermore, the proportion of crack propagation through the interfacial IMC layer increased with increase in the thickness and the roughness of the interfacial IMC layer and the yield strength of the solder, resulting in a decrease in the fracture toughness of the joint. However, under most conditions, solder joints with ENIG finish showed higher resistance to fracture than joints attached directly to Cu substrates without ENIG metallization. Based on the experimental observations, a fracture mechanism map is constructed correlating the yield strength of the solder, the morphology and thickness of the interfacial IMC, and the fracture mechanisms as well as the fracture toughness values for different solder joints under mode I loading.     

Improvement of High-Temperature Performance of Zn-Sn Solder Joint

Pb-based solders are used as high-temperature solders in power semiconductor devices. Although the use of Pb is globally restricted, alternative materials cannot replace the Pb-based solder. This study proposes that the Pb-based solder can be replaced by Zn-Sn alloys. Die shear tests revealed that some Zn-Sn solder joints between Cu substrates had a higher shear strength between 300 K and 543 K than those between Fe-42Ni substrates. The microstructure of the Zn-Sn solder joints between Cu substrates showed network microstructures consisting of a Zn phase and ε-CuZn₅ phase and direct connection between the network microstructures and intermetallic compound layer. These morphologies of the high melting phase should improve the shear strength even at the elevated temperature of 543 K.     

Solder joint reliability of indium-alloy interconnection

Recent high-density very large scale integrated (VLSI) interconnections in multichip modules require high-reliability solder interconnection to enable us to achieve small interconnect size andlarge number of input/output terminals, and to minimize soft errors in VLSIs induced by α-particle emission from solder. Lead-free solders such as indium (In)-alloy solders are a possible alternative to conventional lead-tin (Pb-Sn) solders. To realize reliable interconnections using In-alloy solders, fatigue behavior, finite element method (FEM) simulations, and dissolution and reaction between solder and metallization were studied with flip-chip interconnection models. We measured the fatigue life of solder joints and the mechanical properties of solders, and compared the results with a computer simulation based on the FEM. Indium-alloy solders have better mechanical properties for solder joints, and their flip-chip interconnection models showed a longer fatigue life than that of Pb-Sn solder in thermal shock tests between liquid nitrogen and room temperatures. The fatigue characteristics obtained by experiment agree with that given by FEM analysis. Dissolution tests show that Pt film is resistant to dissolution into In solder, indicating that Pt is an adequate barrier layer material for In solder. This test also shows that Au dissolution into the In-Sn solder raises its melting point; however, Ag addition to In-Sn solder prevents melting point rise. Experimental results show that In-alloy solders are suitable for fabricating reliable interconnections.     

Effect of displacement rate on ball shear properties for Sn–37Pb and Sn–3.5Ag BGA solder joints during isothermal aging

The interfacial reactions and ball shear properties of ball grid array (BGA) solder joints aged at 170 °C for up to 21 days were investigated with different displacement rates. Two different kinds of solders, Sn–37Pb and Sn–3.5Ag (all wt.%), and an electroplated Ni/Au BGA substrate were employed in this work. A continuous Ni₃Sn₄ intermetallic compound (IMC) layer was formed at the interfaces between both the Sn–37Pb and Sn–3.5Ag solders and the substrate during reflow. After aging, two different reaction layers, consisting of (Au_xNi_1−x)Sn₄ IMC and Pb-rich phase, were additionally observed between the Sn–37Pb solder and the Ni₃Sn₄ IMC layer. The thicknesses of these interfacial reaction layers increased with increasing aging time. After reflow, all the fractures occurred inside the bulk solder. The fracture location of the Sn–37Pb solder joints was shifted toward the solder/Ni interface with increasing aging time and displacement rate, whereas the fracture of the Sn–3.5Ag solder joints mainly occurred inside the bulk solder, irrespective of the aging time and displacement rate. Consequently, the shear properties of the Sn–37Pb solder joints significantly decreased with increasing aging time, whereas those of the Sn–3.5Ag solder joints slightly decreased. The tendency toward brittle fracture of the Sn–37Pb solder joints was intensified with increasing displacement rate. The shear properties of the ductile solder joints increased with increasing displacement rate, while the displacement until fracture, deformation energy and displacement rate sensitivity of the brittle solder joints significantly decreased with increasing displacement rate.     

Reliability of composite solder bumps produced by an in-situ process

In an attempt to develop a thermally stable solder system, an in-situ Pb-Sn solder composite reinforced with Cu₆Sn₅ dispersoids was investigated for its thermal stability. The stability was evaluated mainly by measuring the growth rate of intermetallics at in-situ composite solder/BLM interface as a function of the number of reflow soldering cycles and aging time. The rates were compared with those of the eutectic Pb-Sn and Sn-Ag solders. After the thermal treatments, the solder joints were tested for their shear strengths. The results indicated that the in-situ composite solder has a higher shear strength and better thermal stability than the eutectic Pb-Sn solder. Jointly appointed by CAAM at POSTECH     

Effect of Thermal Aging on Impact Absorbed Energies of Solder Joints Under High-Strain-Rate Conditions

This study was concerned with the effect of thermal aging on the impact properties of solder joints. Three kinds of solders, conventional Sn-37Pb solder, Sn-3.8Ag-0.7Cu solder, and Sn-3.8Ag-0.7Cu doped with rare-earth (RE) elements, were selected to manufacture joint specimens for the Charpy impact test. U-notch specimens were adopted and isothermally aged at 150°C for 100 h and 1000 h, and then impacted by using a pendulum-type impact tester at room temperature. The Sn-37Pb solder joints exhibited higher performance in terms of impact absorbed energy in the as-soldered and 100 h thermally aged conditions. Interestingly, the Sn-3.8Ag-0.7Cu solder joints exhibited improved performance for the impact value after 1000 h of thermal aging. For the Sn-37Pb and Sn-3.8Ag-0.7Cu solder joints, the impact absorbed energies initially increased when the storage duration was limited to 100 h, and then gradually decreased with its further increase. For the Sn-3.8Ag-0.7Cu-RE specimens, impact performance decreased directly with increasing thermal aging. Furthermore, scanning electron microscopy (SEM) observation showed that the fracture paths were focused on the interface zone for the three kinds of joints in the aged conditions. For the Sn-37Pb joints, the fracture surfaces mainly presented a ductile fracture mode. For the Sn-3.8Ag-0.7Cu joints, with microstructure coarsening, crack propagation partly shifted towards the Sn/Cu₆Sn₅ interface. Compared with the 100 h aged joints, the area fraction of intergranular fracture of Sn grains on the Sn-3.8Ag-0.7Cu fracture surfaces was increased when the aging time was 1000 h. On the contrary, the fracture morphologies of Sn-3.8Ag-0.7Cu-RE were mainly brittle as thermal aging increased. Thus, an interrelationship between impact energy value and fracture morphology was observed.     

无铅焊料的蠕变行为研究进展

无铅焊料已经逐渐代替锡铅焊料广泛应用于电子产品连接技术。但其在环境中的物理和机械性能,尤其是蠕变性能却低于锡铅焊料合金,成为无铅焊料可靠性的主要问题。综述了近些年来无铅焊料蠕变性能的研究,包括蠕变机制、蠕变本构方程、焊点尺寸、无铅合金成分、金属间化合物以及微观组织结构对蠕变性能影响等主要研究热点,并对此领域的发展做出了展望。     

Brittle to Ductile Fracture Transition in Bulk Pb-Free Solders

The fracture toughness of bulk Sn, Sn-Cu, Sn-Ag, and Sn-Ag-Cu lead-free solders was measured as function of the temperature by means of a pendulum impact test (Charpy test). A ductile to brittle fracture transition was found, i.e., a sharp change in the fracture toughness. No transition was found for the eutectic Sn-Pb. The transition temperature of high purity Sn, Sn-0.5%Cu and Sn-0.5%Cu(Ni) alloys is around -125degC. The Ag-containing solders show a transition at higher temperatures: in the range of -78 to -45degC. The increase of the Ag content shifts the transition temperature towards higher values, which is related to the higher volume fraction of SnAg particles in the solder volume. At fixed volume fraction, smaller particle size shifts the transition temperature towards higher values. Therefore, a careful microstructure control is needed during the solder solidification after reflow in order to decrease the low temperature brittleness hazard.     

Sn-9Zn/Cu焊点剪切强度及断口形貌分析

研究了BGA直径分别为750μm、1 000μm、1 300μm的Sn-9Zn/Cu焊点剪切强度及其变化规律。采用SEM和EDX对剪切断口进行观察和元素成分分析。试验结果表明,随着焊球直径的增大,焊点剪切强度先减小后增大。剪切断裂位置大部分位于钎料内部,局部位于界面化合物Cu5Zn8处。在相同的剪切高度与剪切速率下,随...     

The characteristics of vibration fracture of Pb-Sn and lead-free Sn-Zn eutectic solders

This work investigated the fatigue fracture characteristics of Sn-7∼11wt.%Zn and Sn-30∼50wt.%PPb solder alloys under resonant vibration. For the alloys containing proeutectic Sn-rich grains, the results show stratum appearance on the deformation of coarse proeutectic grains. This stratum-type deformation will reduce the vibration resistance of Pb-Sn solder by inducing cracks. For the Sn-Zn system, fine eutectic structure will cause cracks to form and coalesce easily in the regions concentrated with small fibrous deformation grains, whereas hypereutectic structure shows coarse proeutectic Zn-rich particles as the crack initiation sites. Sn-t-Zn solders have better damping capacity than Pb-Sn solders. With hypoeutectic composition to induce stratum-type deformation and under lower vibration strain, these lead-free solders tend to exhibit superior crack propagation resistance, and, in that respect, are possible to replace Pb-Sn solders.     

Diffusion and phase transformations during interfacial reaction between lead-tin solders and palladium

The interfacial reaction between Pb-Sn solders and bulk Pd substrate is studied both in the liquid- and solid-state of the solder. The interfacial microstructures are characterized by imaging and energy dispersive x-ray analysis in scanning electron microscope. The correlation between the diffusion path and interfacial microstructure in 62Sn38Pb/Pd, 95Pb5Sn/Pd, and Pb/Pd diffusion couples is demonstrated by means of calculated isothermal sections of the Pb-Sn-Pd system and the metastable phase diagram of the Pb-Pd system.     

Interfacial fracture toughness of Pb-free solders

Increasing environmental concerns and pending government regulations have pressured microelectronic manufacturers to find suitable alternatives to Pb-bearing solders traditionally used in electronics packaging. Over recent years, Sn-rich solders have received significant attention as suitable replacements for Pb-bearing solders. Understanding the behavior of intermetallics in Sn-rich solders is of particular concern as the microelectronics industry progresses towards Pb-free packaging. The formation of intermetallic compounds results from the reaction of the solder with the metallization on the substrate in the electronic package. While the presence of the intermetallic is an indication of good wetting, excessive growth of the intermetallic can have a dramatically adverse effect on the toughness and reliability of the solder joint. Understanding their fracture behavior will lend insight to their reliability under mechanical and thermomechanical strains.We investigated the intermetallic compound growth associated with Sn-0.7Cu and Sn-4.0Ag-0.5Cu solders on Ni-Au, Ni-Pd, and Cu substrates. (Ni,Cu)₃Sn₄ was present at the Ni interface for both solders but was coarser in the case of Ni-Pd. Cu₆Sn₅ and Cu₃Sn were observed for both solder types. The Cu₃Sn layer was similar in thickness and appearance for both solders, but the Cu₆Sn₅ was smoother and rounder in the case of Sn-0.7Cu. Additional time above liquidus resulted in growth of the Cu₆Sn₅ layer and eventual spalling of the IMC grains. The effect of the intermetallic on the toughness (K_Q) of the solder joint was investigated using a modified compact tension specimen. Typical failure modes included bulk solder failure, intergranular separation, and intermetallic fracture, or cleavage. In some cases, additional time above solder liquidus was used to shift the dominant failure mode from that dominated by the bulk solder to interfacial delamination through the intermetallics. Solder joint fracture toughness was different between Ni-Sn and Cu-Sn interfacial intermetallics and was also affected by the relative intermetallic thickness. The relationship between solder and intermetallic microstructure and mechanical properties is discussed.     

Effect of reaction time on mechanical strength of the interface formed between the Sn-Zn(-Bi) solder and the Au/Ni/Cu bond pad

In this work, the shear strengths and the interfacial reactions of Sn-9Zn, Sn-8Zn-1Bi, and Sn-8Zn-3Bi (wt.%) solders with Au/Ni/Cu ball grid array (BGA) pad metallization were systematically investigated after extended reflows. Zn-containing Pb-free solder alloys were kept in molten condition (240°C) on the Au/electrolytic Ni/Cu bond pads for different time periods ranging from 1 min. to 60 min. to render the ultimate interfacial reaction and to observe the consecutive shear strength. After the shear test, fracture surfaces were investigated by scanning electron microscopy equipped with an energy dispersive x-ray spectrometer. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces. The solder ball shear load for all the solders during extended reflow increased with the increase of reflow time up to a certain stage and then decreased. It was found that the formation of thick Ni-Zn intermetallic compound (IMC) layers at the solder interface of the Au/electrolytic Ni/Cu bond pad with Sn-Zn(-Bi) alloys deteriorated the mechanical strength of the joints. It was also noticed that the Ni-Zn IMC layer was larger in the Sn-Zn solder system than that in the other two Bi-containing solder systems.     

Characterization of Stress–Strain Response of Lead-Free Solder Joints Using a Digital Image Correlation Technique and Finite-Element Modeling

The stress–strain response of miniaturized Sn-Ag-Cu (SAC) lead-free solder joints in the thickness range of 80 μm to about 1.1 mm was studied. A high-resolution three-dimensional (3D) digital image correlation system was used for in situ measurement of displacement and strain fields in the solder joints during tensile testing. These measurements showed that the localization of plastic strain and stress buildup occurs mainly at the interface of the solder. With increasing solder gap thickness the size of the plastically deformed zone in the solder increases, resulting in transformation of a brittle interfacial fracture to a ductile fracture within the bulk of the solder. The experimental deformation plots of solder joints and strain-rate-dependent tensile tests on bulk solder material were used to establish a new constitutive material model for the solder. This strain-rate- and pressure-dependent material model was implemented in ABAQUS through the user subroutine CREEP. In agreement with the experiments, the finite-element method simulation revealed a pronounced thickness effect leading to higher tensile strength of thinner solder joints.     

Effect of additions of ZrO2 nano-particles on the microstructure and shear strength of Sn-Ag-Cu solder on Au/Ni metallized Cu pads

Nano-sized, nonreacting, noncoarsening ZrO₂ particles reinforced Sn-3.0 wt%Ag-0.5 wt%Cu composite solders were prepared by mechanically dispersing ZrO₂ nano-particles into Sn-Ag-Cu solder. The interfacial morphology of unreinforced Sn-Ag-Cu solder and solder joints containing ZrO₂ nano-particles with Au/Ni metallized Cu pads on ball grid array (BGA) substrates and the distribution of reinforcing particles were characterized metallographically. At their interfaces, a Sn-Ni-Cu intermetallic compound (IMC) layer was found in both unreinforced Sn-Ag-Cu and Sn-Ag-Cu solder joints containing ZrO₂ nano-particles and the IMC layer thickness increased with the number of reflow cycles. In the solder ball region, AuSn₄, Ag₃Sn, Cu₆Sn₅ IMC particles and ZrO₂ nano-particles were found to be uniformly distributed in the β-Sn matrix of Sn-Ag-Cu solder joints containing ZrO₂ nano-particles, which resulted in an increase in the shear strength, due to a second phase dispersion strengthening mechanism. The fracture surface of unreinforced Sn-Ag-Cu solder joints exhibited a brittle fracture mode with a smooth surface while Sn-Ag-Cu solder joints containing ZrO₂ nano-particles ductile failure characteristics with rough dimpled surfaces.     

Fracture of Sn-Ag-Cu Solder Joints on Cu Substrates: I. Effects of Loading and Processing Conditions

During service, microcracks form inside solder joints, making microelectronic packages highly prone to failure on dropping. Hence, the fracture behavior of solder joints under drop conditions at high strain rates and under mixed-mode conditions is a critically important design consideration for robust joints. This study reports on the effects of joint processing and loading conditions on the microstructure and fracture response of Sn-3.8%Ag-0.7%Cu (SAC387) solder joints attached to Cu substrates. The impact of parameters which control the microstructure (reflow condition, aging) as well as loading conditions (strain rate and loading angle) are explicitly studied. A methodology based on the calculation of the critical energy release rate, G _C, using compact mixed-mode (CMM) samples was developed to quantify the fracture toughness of the joints under conditions of adhesive (i.e., interface-related) fracture. In general, higher strain rate and increased mode-mixity resulted in decreased G _C. G _C also decreased with increasing dwell time at reflow temperature, which produced a thicker intermetallic layer at the solder–substrate interface. Softer solders, produced by slower cooling following reflow, or post-reflow aging, showed enhanced G _C. The sensitivity of the fracture toughness to all of the aforementioned parameters reduced with an increase in the mode-mixity. Fracture mechanisms, elucidating the effects of the loading conditions and process parameters, are briefly highlighted.     

Drop impact reliability testing for lead-free and lead-based soldered IC packages

Board-level drop impact testing is a useful way to characterize the drop durability of the different soldered assemblies onto the printed circuit board (PCB). The characterization process is critical to the lead-free (Pb-free) solders that are replacing lead-based (Pb-based) solders. In this study, drop impact solder joint reliability for plastic ball grid array (PBGA), very-thin quad flat no-lead (VQFN) and plastic quad flat pack (PQFP) packages was investigated for Pb-based (62Sn–36Pb–2Ag) and Pb-free (Sn–4Ag–0.5Cu) soldered assemblies onto different PCB surface finishes of OSP (organic solderability preservative) and ENIG (electroless nickel immersion gold). The Pb-free solder joints on ENIG finish revealed weaker drop reliability performance than the OSP finish. The formation of the brittle intermetallic compound (IMC) Cu–Ni–Sn has led to detrimental interfacial fracture of the PBGA solder joints. For both Pb-based and Pb-free solders onto OSP coated copper pad, the formation of Cu₆Sn₅ IMC resulted in different failure sites and modes. The failures migrated to the PCB copper traces and resin layers instead. The VQFN package is the most resistant to drop impact failures due to its small size and weight. The compliant leads of the PQFP are more resistant to drop failures compared to the PBGA solder joints.     

Interfacial reaction studies on lead (Pb)-free solder alloys

Recently, the research and development activities for replacing Pb-containing solders with Pb-free solders have been intensified due to both competitive market pressures and environmental issues. As a result of these activities, a few promising candidate solder alloys have been identified, mainly, Sn-based alloys. A key issue affecting the integrity and reliability of solder joints is the interfacial reactions between a molten solder and surface finishes in the solder joint structures. In this paper, a fundamental study of the interfacial reactions between several Pb-free candidate solders and surface finishes commonly used in printed-circuit cards is reported. The Pb-free solders investigated include Sn-3.5 Ag, Sn-3.8 Ag-0.7 Cu, and Sn-3.5 Ag-3.0 Bi. The surface finishes investigated include Cu, Au/Ni(P), Au/Pd/Ni(P), and Au/Ni (electroplated). The reaction kinetics of the dissolution of surface finishes and intermetallic compound growth have been measured as a function of reflow temperature and time. The intermetallic compounds formed during reflow reactions have been identified by SEM with energy dispersive x-ray spectroscopy.     

Reliability issues in Pb-free solder joint miniaturization

As solder joints become increasingly miniaturized to meet the severe demands of future electronic packaging, it is vitally important to consider whether the solder joint size and geometry could become reliability issues and thereby affect implementation of the Pb-free solders. In this study, three bumping techniques, i.e., solder dipping, stencil printing followed by solder reflow, and electroplating of solders with subsequent reflow, were used to investigate the interfacial interactions of molten Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and pure Sn solders on a copper pad at 240°C. The resultant interfacial microstructures, coming from a variety of Cu pads, with sizes ranging from 1 mm to 25 μm, and representing different solder bump geometries, have been investigated. In addition, a two-dimensional thermodynamic/kinetic model has been developed to assist the understanding of the kinetics of interdiffusion and the formation of interfacial intermetallic compounds. Experimental results and theoretical predictions both suggest that the solder bump size and geometry can influence the as-soldered microstructure; therefore, this factor should be taken into consideration for the design of future reliable ultrafine Pb-free solder joints.