Effect of geometry on the fracture behavior of lead-free solder joints

Copper bars were soldered along their length with a thin layer of lead-free Sn3.0Ag05.Cu alloy under standard surface mount processing conditions to prepare double cantilever beam (DCB) specimens. The geometry of the DCBs was varied by changing the thickness of the solder layer and the copper bars. These specimens were then fractured under mode-I and two mixed-mode loading conditions. The initiation strain energy release rate, G_ci, increased with the relative fraction of mode-II, but was unaffected by the changes in either the substrate stiffness or the solder layer thickness. However, the steady-state strain energy release rate, realized after several millimeters of crack growth, was found to increase with the solder layer thickness at the various mode ratios. The crack path was found to be influenced by mode ratio of loading and followed a path that maximizes the von Mises strain rather than maximum principal stress. Finally, some preliminary results indicated that the loading rate significantly affects the G_ci.     

Fracture behavior of Cu-cored solder joints

Copper-cored solder can be regarded as the next-generation solder for microelectronic semiconductors exposed to harsh operating conditions owing to its excellent sustainability under extreme thermal conditions, e.g., in microelectronic semiconductors used in transportation systems. Cu-cored solder joints with two different coating layers, Sn–3.0Ag and Sn–1.0In, were compared with the baseline Sn–3.0Ag–0.5Cu solder. The fracture strength and failure mode were examined using the high-speed ball-pull and normal-speed shear tests. The Cu-cored solder joint with the Sn–1.0In plating layer exhibited the highest ball-pull and shear strengths. In addition, it showed a much lower percentage of interface fracture between the Cu-core and plating layer than the interface fracture percentage in the Sn–3.0Ag plating layer due to the improved wettability between the Cu-core and Sn–1.0In plating layer.     

Reliability behavior of lead-free solder joints in electronic components

With more consumer products moving towards environmentally friendly packaging, making solder Pb-free has become an urgent task for electronics assemblies. Solder joints are responsible for both electrical and mechanical connections. Solder joint does not have adequate ductility to ensure the repeated relative displacements due to the mismatch between expansion coefficients of the chip carrier and the circuit board. Materials behavior of solder joints involves a creep–fatigue interaction, making it a poor material for mechanical connections. The reliability of solder joints of electronics components has been found playing a more important role in service for microelectronics components and micro-electro-mechanical systems. So many researchers in the world investigated reliability of solder joints based on finite element simulation and experiments about the electronics devices, such as CR, QFP, QFN, PLCC, BGA, CSP, FCBGA and CCGA, which were reviewed systematically and extensively. Synchronously the investigation on reliability of solder joints was improved further with the high-speed development of lead-free electronic packaging, especially the constitutive equations and the fatigue life prediction equations. In this paper, the application and research status of constitutive equations and fatigue life prediction equations were reviewed, which provide theoretic guide for the reliability of lead-free solder joints.     

The effects of suppressed beta tin nucleation on the microstructural evolution of lead-free solder joints

Most lead-free solders comprise tin (Sn) as the majority component, and nominally pure β-Sn is the majority phase in the microstructure of these solders. It is well established that nucleation of β-Sn from Sn-base liquid alloys is generally difficult. Delays in the onset of β-Sn formation have a profound effect upon the microstructural development of solidified Sn-base alloys. Utilizing stable and metastable phase diagrams, along with solidification principles, the effects of inhibited β-Sn nucleation on microstructural development are discussed, employing the widely studied Sn–Ag–Cu (SAC) alloy as a model system. This analysis shows that the main effect of suppressed β-Sn nucleation on near-eutectic SAC solders is to increase the number and/or volume fraction of primary or primary-like microconstituents, while simultaneously decreasing the volume fraction of eutectic microconstituent. General strategies are outlined for avoiding unwanted microconstituent development in these materials, including the use of metastable phase diagrams for selecting alloy compositions, employment of inoculants to promote β-Sn nucleation, and utilization of high cooling rates to limit solid phase growth. Finally, areas for future research on the development of inoculated Sn-base solder alloys are outlined.     

Role of anisotropic behaviour of Sn on thermomechanical fatigue and fracture of Sn-based solder joints under thermal excursions

Solder joints experience thermomechanical fatigue (TMF) as a consequence of thermal stresses that arise from coefficient of thermal expansion (CTE) mismatches between various entities present in the joint under thermal excursions. Sn present in solder joints made with alloys containing significant amounts of Sn, exists in a body centred tetragonal (BCT) structure, under normally realized thermal excursion regimes encountered during service. BCT Sn exhibits significant anisotropic behaviour in its physical and mechanical properties as a consequence of its highly unusual c/a ratio of about 0.5. Such severe anisotropy causes significant stresses at the Sn grain boundaries present within the solder joints during thermal excursions, resulting in damage accumulation within the solder. Stresses resulting from this anisotropy can be much larger than those that can arise from CTE mismatches between entities such as solder/substrate, solder/intermetallics etc. Damage accumulation under TMF progresses in the severely constrained region of the solder/substrate interface, and causes the initiation and propagation of the catastrophic crack. This crack propagates within the solder in a region very close to the solder/substrate interface and results in the TMF failure of the joint.     

Thermomigration in solder joints

In 3D IC technology, the vertical interconnection consists of through-Si-vias (TSV) and micro solder bumps. The size of the micro-bump is approaching 10 μm, which is the diameter of TSV. Since joule heating is expected to be the most serious issue in 3D IC, heat flux must be conducted away by temperature gradient. If there is a temperature difference of 1 °C across a micro-bump, the temperature gradient will be 1000 °C/cm, which can cause thermomigration at the device operation temperature around 100 °C. Thus thermomigration will become a very serious reliability problem in 3D IC technology. We review here the fundamentals of thermomigration of atoms in microbump materials; both molten state and solid state thermomigration in solder alloys will be considered. The thermomigration in Pb-containing solder joints is discussed first. The Pb atoms move to the cold end while Sn atoms move to the hot end. Then thermomigration in Pb-free SnAg solder joints is reviewed. The Sn atoms move to the hot end, but the Ag atoms migrate to the cold end. Thermomigration of other metallization elements, such as Cu, Ti and Ni is also presented in this paper. In solid state, copper atoms diffuse rapidly via interstitially to the cold end, forming voids in the hot end. In molten state, Cu thermomigration affects the formation of intermetallic compounds.     

Formation and behavior of Kirkendall voids within intermetallic layers of solder joints

The sub-micron void called “Kirkendall void” has been widely observed within intermetallic compound (IMC) layers in solder joints of semiconductor package interconnections that include both the first level interconnection for a silicon die to a substrate and the second level interconnection for the substrate to a PCB board. Based on many researches on Kirkendall void through a variety of variables, it has been demonstrated as a critical reliability risk within various binary and ternary IMC layers of solder joints in electronic packaging industry. Even, it is more crucial for fine pitch and high complexity in chip-scale electronic packaging. Hence, it is necessarily demanding to review the dependency and influence of critical variables for Kirkendall void formation and behavior in the basis of solid and solid–liquid state interdiffusion process, time and temperature-dependent kinetic process, and morphology and microstructure change of IMCs. Specifically, we reviewed the initial formation, growth and behaviors of Kirkendall void in: (1) short and long-term interfacial reaction by aging in different time and temperatures (2) multiple reflows with different peak temperature (3) annealing after reflow and (4) electromigration, within IMCs of solder joints. Probably, this study may serve as conceptually helpful references to the overall understanding of formation, growth and behavior of Kirkendall void in interfacial reaction of solder joints.     

Preparation of soft solder joints

The preparation of solder joints in electronic applications is not easy because the solder is soft and often surrounded by hard and brittle materials. Smearing, scratching, and structural changes caused by the preparation as well as destruction of the specimens during preparation due to their filigree geometries make the procedure demanding. A sequence has been developed that enables the preparation of soft solder under these difficult circumstances. The preparation and the development of the phases found in solder is explained step by step and illustrated with examples.     

Effect of water on the dielectric behavior of solder

Journal of Materials Science: Materials in Electronics - The electrical conduction behavior of solder is well-known, but the dielectric behavior has been reported only recently. Water in the...     

Microstructural effects on the behavior of Sn–Ag solder joints under repeated reverse stressing

Double shear lap joints made with eutectic Sn–Ag solder were cooled from 270 °C at different rates ranging from air-cooling to iced-brine quenching. These joints were subjected to reversed stressing with constant shear strain amplitude at room temperature. Shear strain amplitudes between 0.75 and 1 were imposed during the course of this investigation. Role of microstructural features on the surface damage accumulation and stress–strain behavior was investigated as a function of shear strain amplitude and number of cycles of repeated stressing. Shear banding was found to be along Sn dendrites in air-cooled specimens, while it cut across the small Sn grains present in the quenched specimens.     

Effect of rare earth addition on shear strength of SnAgCu lead-free solder joints

In the present study, the effect of adding trace amount of rare earth (RE) on the shear strength of Sn3.8Ag0.7Cu lead-free solder joints has been investigated. The shear strength of the solder joints as-reflowed and after aging at 150 °C for 168 and 336 h was measured at a constant loading rate of 0.3 mm/min and room temperature. The investigation indicates that the shear strength of Sn3.8Ag0.7Cu0.1RE solder joints is lower than that of Sn3.8Ag0.7Cu solder joints. The shear strength of both Sn3.8Ag0.7Cu solder joints and Sn3.8Ag0.7CuRE solder joints was reduced after aging at elevated temperature. However, the shear strength reduction rate of the Sn3.8Ag0.7Cu solder joints was much faster than that of Sn3.8Ag0.7CuRE solder joints. Moreover, the fracture surfaces were examined by scanning electron microscopy (SEM) and the thickness of intermetallic compounds layer (IML) in the solder joints that join Cu substrate was measured. The results indicated that the addition of rare earth elements suppresses the growth of the thickness of intermetallic compounds layer.     

Effect of current stressing on the reliability of 63Sn37Pb solder joints

The effect of current stressing on the reliability of 63Sn37Pb solder joints with Cu pads was investigated at temperatures of −5 °C and 125 °C up to 600 h. The samples were stressed with 3 A current (6.0 × 10² A/cm² in the solder joint with diameter of 800 μm and 1.7 × 10⁴ A/cm² in the Cu trace with cross section area of 35 × 500 μm). The temperatures of the samples and interfacial reaction within the solder joints were examined. The microstructural change of the solder joints aged at 125 °C without current flow was also evaluated for comparison. It was confirmed that the current flow could cause the temperature of solder joints to rise rapidly and remarkably due to accumulation of massive Joule heat generated by the Cu trace. The solder joints stressed at 125 °C with 3 A current had an extensive growth of Cu₆Sn₅ and Cu₃Sn intermetallic compounds (IMC) at both top and bottom solder-to-pad interfaces. It was a direct result of accelerated aging rather than an electromigration or thermomigration effect in this experiment. The kinetic is believed to be bulk diffusion controlled solid-state reaction, irrespective of the electron flow direction. When stressed at −5 °C with 3 A current, no significant change in microstructure and composition of the solder joints had occurred due to a very low diffusivity of the atoms as most Joule heat was eliminated at low temperature. The IMC evolution of the solder joints aged at 125 °C exhibited a subparabolic growth behavior, which is presumed to be a combined mechanism of grain boundary diffusion and bulk diffusion. This is mainly ascribed to the retardant effect against the diffusion course by the sufficiently thick IMC layer that was initially formed during the reflow soldering.     

Electromigration issues in lead-free solder joints

As the microelectronic industry advances to Pb-free solders due to environmental concerns, electromigration (EM) has become a critical issue for fine-pitch packaging as the diameter of the solder bump continues decreasing and the current that each bump carries keeps rising owing to higher performance requirement of electronic devices. As stated in 2003 International Technology Roadmap for Semiconductors (ITRS), the EM is expected to be the limiting factor for high-density packages. This paper reviews general background of EM, current understanding of EM in solder joints, and technical hurdles to be addressed as well as possible solutions. It is found that the EM lifetimes of Pb-free solder bumps are between the high-Pb and the eutectic composition under the same testing condition. However, our simulation results show that the electrical and thermal characteristics remain essentially almost the same during accelerated EM tests when the Pb-containing solders are replaced by Pb-free solders, suggesting that the melting points of the solders are likely the dominant factor in determining EM lifetimes. The EM behavior in Pb-free solder is a complicated phenomenon as multiple driving forces coexist in the joints and each joint contains more than four elements with distinct susceptibility to each driving force. Therefore, atomic transport due to electrical and thermal driving forces during EM is also investigated. In addition, several approaches are presented to reduce undesirable current crowding and Joule heating effects to improve EM resistance.     

Role of imposed cyclic straining on the stress relaxation behavior of eutectic Sn-3.5Ag solder joints

Solder joints experience repeated reverse straining during thermal excursions encountered in service, as a consequence of stresses that arise due to coefficient of thermal expansion (CTE) mismatches between entities present in the joint. They also undergo stress relaxation under fixed strain during dwell times at temperature extremes encountered during service. In order to understand the fundamental processes involved under such conditions, cyclic shear straining with associated stress relaxation at the shear strain extremes were imposed during stress relaxation of pre-strained solder joints at various temperatures. Results of such studies were compared with previously reported findings from monotonic shear stressing and stress relaxation tests. Residual stress during stress relaxation under repeated reverse straining exhibited significant decrease for specimens deformed to a higher pre-strain at a higher pre-strain rate, at lower temperature. Stress relaxation during subsequent cycles of straining was found to be strongly dependant on the test temperature and the imposed pre-strain amplitude and pre-strain rate.     

Solderability of SnBi-nano Cu solder pastes and microstructure of the solder joints

The solderability of the Sn58Bi (SnBi)-nano Cu solder pastes and the microstructure of the solder joints were investigated. Experimental results indicated that the addition of the nano Cu particles in the SnBi solder paste shows limited effect on the solidus. The liquidus of the SnBi-3nano Cu solder paste was 1 °C higher than the SnBi solder paste. Solid Cu₆Sn₅ intermetallic particles formed in the SnBi-3nano Cu solder paste during the heating process. The Cu₆Sn₅ intermetallic particles decreased the mobility and wettability of the molten solder. Meanwhile, the Cu₆Sn₅ nano particles worked as nucleation sites for the formation of Bi grains and Sn–Bi eutectic phase during the cooling process and led to the grain refinement of the solder bulk. The SnBi-1nano Cu solder paste showed the smallest grain size in this research. Additionally, the SnBi-3nano Cu/Cu solder joint showed a eutectic microstructure of Sn–Bi system at the center of the solder bulk but a hypereutectic microstructure with polygon Bi grains near the margin in the solder bulk.     

Fracture load prediction of lead-free solder joints

Continuous and discrete SAC305 solder joints of different lengths were made between copper bars under standard surface mount (SMT) processing conditions, and then fractured under mode-I loading. The load-displacement behavior corresponding to crack initiation and the subsequent toughening before ultimate failure were recorded and used to calculate the critical strain energy release rates. The fracture of the discrete solder joints was then simulated using finite elements with two different failure criteria: one in terms of the critical strain energy release rate at initiation, G_ci, and another based on a cohesive zone model at the crack tip (CZM). Both criteria predicted the fracture loads reasonably well. In addition, the CZM was able to predict accurately the overall load-displacement behavior of the discrete joint specimen. It could also predict the load sharing that occurred between neighboring solder joints as a function of joint pitch and adherend stiffness. This has application in the modeling of the strength of solder joint arrays such as those found in ball grid array packages.     

Electrochemical migration of lead free solder joints

Electrochemical migration (ECM) tests on lead bearing and lead free solder joints on Cu lamination on FR-4 board were conducted by applying constant voltage. This paper first studied the ECM of the soldered joints under distilled water after removing the fluxes. In addition, conditions under high temperature and high humidity were also set up to investigate the changes of the initial surface insulation resistance (SIR) with the residues of no clean fluxes. It is found under distilled water that dendrites of the solder joints take on different morphologies with the different migration elements. For the joints of Sn-37Pb and Sn-36Pb-2Ag solders, the main migration element is Pb. While that of Sn-3.5Ag and Sn-4Ag-0.5Cu solders, it is Cu that usually migrates and forms dendrites due to the poor wettability of the solder paste leads to part exposure of Cu substrate at the wetting brim. For Sn-3Ag-0.5Cu solder joints, Sn leads the migration. While for Sn-Zn-Bi solder joints, it is always Zn to migrate which means Zn is more mobile than Cu. The investigation on SIR shows the fluxes have great effect on the migration behavior. The failure time of the joints with the same solder alloy compositions have different failure time due to the different fluxes. The effect of the wettability and the role of Cu substrate on the ECM behavior of the solder joints are discussed in detail.     

Effect of tin on the corrosion behavior of sea-water corrosion-resisting steel

This paper investigated the effect of tin on the corrosion resistance of tin-containing steel and tin-free steel using electrochemical measurements in seawater. Results showed that tin-containing steel had lower corrosion current and higher impedance than tin-free steel. Surface analyses of X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) indicated that tin could form SnO₂ and SnO in rust layer, and both of them could improve impedance and corrosion resistance of rust layer. Besides, the coprecipitation process of tin oxides with iron oxides could make the rust layer more uniform and compact, which could make the tin-containing steel have better corrosion resistance than tin-free steel. Secondary ion mass spectrometer (SIMS) showed that there was no obvious segregation of tin on substrate steel when tin addition was 0.038 wt.%, and tin could improve the oxidation resistance of substrate steel evenly by lowering the steel's Fermi energy from − 9.276 eV to − 14.445 eV.     

Effect of strain rate on interfacial fracture behaviors of Sn-58Bi/Cu solder joints

The interfacial reactions and mechanical properties of Sn-58Bi/Cu solder joints reflowed at different temperatures ranging from 180 to 220 °C for constant time of 10 min were investigated with various strain rates. Only a continuous Cu₆Sn₅ intermetallic compound (IMC) layer was formed at the interface between the Sn-58Bi solder and the Cu substrate during reflow. The equivalent thickness of the Cu₆Sn₅ layer increased with increasing reflow temperature, and the relationship between Cu₆Sn₅ layer equivalent thickness (X) and reflow temperature (T) is obtained by using method of linear regression and presented as $ X = 0.01 \times T + 0.187 $ . For the tensile property, the tensile strength of solder joint gradually decreased as the reflow temperature it increased, whereas it increased with increasing strain rate. Moreover, the fracture behavior of Sn-58Bi/Cu solder joint indicated the ductile fracture with low strain rate (5 × 10^?4 and 1 × 10^?3 s^?1), while toward brittle fracture with high strain rate (2 × 10^?3 and 1 × 10^?2 s^?1). The strain rate sensitivities of the solder joints fractured with various modes were also investigated, and it is found that the tensile strength of the solder is more sensitive to the strain rate than that of the IMC layer.