Low cycle fatigue performance of ball grid array structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints

Low cycle fatigue performance of ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu joints with different standoff heights (h, varying from 100 to 500 μm) and two pad diameters (d, d = 320 and 480 μm) under displacement-controlled cyclic loading was studied by experimental method and finite element (FE) simulation. A prediction method based on the plastic strain energy density and continuum damage mechanics (CDM) framework was proposed to evaluate the initiation and propagation of fatigue crack in solder joints. The results show that fatigue failure of solder joints is a process of damage accumulation and the plastic strain energy density performs a power function correlation with the cycle numbers of crack initiation and propagation. Crack propagation rate is affected by the stress triaxiality, which is dependent on the loading mode and increases dramatically with decreasing h under tensile loading, while the change of standoff height has very limited influence on the stress triaxiality under shear loading mode. Moreover, crack growth correlation constants identified in Cu/Sn–3.0Ag–0.5Cu/Cu joints with a specific geometry (h = 100 μm and d = 480 μm) can be well used to predict the fatigue life of BGA joints with other geometries. Furthermore, the results have also shown that the fatigue life of solder joints increases with decreasing the geometric ratio of h/d under the same nominal shear strain amplitude, while it drops with decreasing h/d under the same shear displacement amplitude in cyclic loading. When the geometric ratio (i.e., h/d ratio) is unchanged, the miniaturization of BGA joints brings about a decrease in fatigue life of the joints.     

Geometry effect on mechanical performance and fracture behavior of micro-scale ball grid array structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints

Solder joint integrity has long been recognized as a key issue affecting the reliability of integrated circuit packages. In this study, both experimental and finite element simulation methods were used to characterize the mechanical performance and fracture behavior of micro-scale ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints with different standoff heights (h, varying from 500 to 100 μm) and constant pad diameter (d, d = 480 μm) and contact angle under shear loading. With decreasing h (or the ratio of h/d), results show that the stiffness of BGA solder joints clearly increases with decreasing coefficient of stress state and torque. The stress triaxiality reflects the mechanical constraint effect on the mechanical strength of the solder joints and it is dependent on the loading mode and increases dramatically with decreasing h under tensile loading, while the change of h has very limited influence on the stress triaxiality under shear loading. Moreover, when h is decreased, the concentration of stress and plastic strain energy along the interface of solder and pad decreases, and the fracture location of BGA solder joints changes from near the interface to the middle of the solder. Both geometry and microstructure greatly affect the shear behavior of joints, the average shear strength shows a parabolic trend with decreasing standoff height. Furthermore, the brittle fracture of BGA solder joints after long-time isothermal aging was investigated. Results obtained show that, under the same shear force, the stress intensity factors, K_I and K_II, and the strain energy release rate, G_I, at the Sn–3.0Ag–0.5Cu/Cu₆Sn₅ interface and in the Cu₆Sn₅ layer obviously decrease with decreasing h, hence brittle fracture is more prone to occur in the joint with a large standoff height.     

Effect of solder resist dissolution on the joint reliability of ENIG surface and Sn–Ag–Cu solder

The electroless nickel immersion gold (ENIG) process results in surface defects, such as pinholes and black pads, which weaken the solder joint and eventually degrade the reliability of the PCB. Contamination of the plating solutions, including dissolution of the solder resist (SR), can be a cause of the pinholes and black pads. This study examined the effects of SR dissolution on the solder joint reliability and electroless Ni plating properties. Electroless Ni plating was performed by adding 1 to 10 ppm hardener (melamine) to the fresh Ni solution. Many black pads were observed in the 7 and 10 ppm hardener-added surfaces. In addition, the content of P was highest when 7 and 10 ppm hardener was added. The ball shear tests were carried out to confirm the joint reliability between the ENIG surface with hardener-added and the Sn-3.0Ag-0.5Cu solder (SAC 305). The ball shear strength decreased with increasing dissolution of the hardener. In particular, the shear strength was the lowest at 7 and 10 ppm hardener addition. In addition, the failure mode of the solder joint was changed from ductile to brittle mode with increasing hardener addition. That is, as the hardener additive increases, intermetallic compound (IMC) phases were changed from (Cu,Ni)₆Sn₅ to (Cu,Ni)₃Sn₄ and Cu₆Sn₅ (brittle structure).     

Effect of nano-Al2O3 reinforcement on the microstructure and reliability of Sn–3.0Ag–0.5Cu solder joints

Nanoparticle reinforced lead-free solder has previously been studied by several investigators, but few studies have evaluated its reliability. In this study, resistor chip (RC) micro joints were soldered using nano-Al₂O₃ particle reinforced Sn–Ag–Cu solder paste. The microstructure and reliability of RC micro joints having different nano-Al₂O₃ contents (0, 0.25, 0.5 and 1.0 wt%) were investigated in detail. More than 40 solder joints for each condition were made and examined in order to achieve reliable data. The results indicated that nano-Al₂O₃ particles refined the β-Sn grain size and enlarged the eutectic area of the micro solder joints. Those nanoparticles also reduced the IMC thickness of the Ni-solder and Cu-solder interfaces. Those effects can be attributed to the poor-wetting behavior of nano-Al₂O₃ particles. The nano-Al₂O₃ reinforcement mainly enhanced the reliability of the micro solder joints, but did not affect the strength of as-soldered joints obviously. The improvement of reliability was proportional to the nano-Al₂O₃ content. The microstructure and fracture analysis indicated that the reinforcement and stability of Ni-IMC and Cu-IMC interfaces accounted for better reliability.     

Effect of small Sn–3.5Ag–0.5Cu additions on the structure and properties of Sn–9Zn solder in ball grid array packages

Sn–9Zn with various additions of Sn–3.5Ag–0.5Cu powder was prepared by mechanically dispersing different weight percentages (1, 3, 5 and 7) of Sn–Ag–Cu powder into Sn–9Zn solder paste. In the Sn–Zn solder, scallop-shaped AuZn₃ intermetallic compound was found at the interfaces. On the other hand, in the Sn–3.5Ag–0.5Cu content solders, an additional ε-AgZn₃ intermetallic compound layer was found to be well adhered on the top surface of the AuZn₃ layer and the ε-AgZn₃ layer thickness increased with the number of reflow cycles. In addition, fine spherical-shaped ε-AgZn₃ intermetallic compound particles as well as an acicular-shaped Zn-rich phase was clearly observed in the β-Sn matrix. On increasing the Sn–Ag–Cu content, the shear load was increased from 1.80 to 2.03 kg after one reflow cycle. In the Sn–3.5Ag–0.5Cu content solders, the fracture surfaces exhibited typical ductile behavior with very rough dimpled surfaces while the fracture surface in the Sn–Zn solder gave fractures with a brittle appearance. In the fracture surface of the Sn–3.5Ag–0.5Cu content solders, some dimples were clearly observed associated with the formation of spherical-shaped ε-AgZn₃ intermetallic compound particles.     

Impact strength of Sn–3.0Ag–0.5Cu solder bumps during isothermal aging

In order to investigate the fracture behavior of Sn–3.0Ag–0.5Cu solder bump, solder balls with the diameter of 0.76 mm were soldered on Cu pad in this study, then high speed impact test and static shear test of solder bumps were carried out to measure the joint strength of the soldering interface. The effect of isothermal aging on joint strength as well as fracture behavior of solder bumps was investigated, and the composition of the fracture surface was identified by means of EPMA. The results indicate that the fracture is inside the bulk solder in low speed shear test regardless of the aging effect, thus the maximum load reflects the solder strength rather than the interfacial strength. It is also found that under 1 m/s impact loading, the crack initiation position is changed from solder/Cu₆Sn₅ interface to Cu₃Sn/Cu interface after long time isothermal aging, and the fracture occurs inside the bulk solder accompanying with intermetallic compound in both of the as-soldered and aged joints. The thickened multiple IMC layers during isothermal aging account for the degraded impact resistance, and the change of the solder matrix is another factor for reduced impact resistance owing to Sn residue on the fracture surface.     

Experimental investigation of the failure mechanism of Cu–Sn intermetallic compounds in SAC solder joints

A detailed experimental study on the fracture mechanism of Cu–Sn intermetallic compounds (IMCs) in the Pb-free solder was presented in this paper. The growth behaviors of the Cu₆Sn₅ and Cu₃Sn IMCs were inspected and the respective evolution pattern of their microstructures was investigated. Then, a detailed fractographic analysis on brittle fractured solder joints was conducted after the high speed ball pull test. The fracture locations in the Cu–Sn IMC layers during different periods of aging process were identified. The fracture modes of Cu₆Sn₅ and Cu₃Sn were determined as well. Afterwards, the fracture energies of different Cu–Sn IMC materials were directly compared using the Charpy impact test with a specially designed specimen. It was found that the grain boundary of Cu₃Sn is the weakest link in the Cu–Sn IMC system. Finally, based on these three parts of study, a mechanism to explain the thermal degradation of Cu–Sn IMCs was proposed.     

Effects of isothermal aging and temperature–humidity treatment of substrate on joint reliability of Sn–3.0Ag–0.5Cu/OSP-finished Cu CSP solder joint

This study examined the effects of isothermal aging and temperature–humidity (TH) treatment of substrate on the joint reliability of a Sn–3.0Ag–0.5Cu (wt.%)/organic solderability preservative (OSP)-finished Cu solder joint. Two types of OSP-finished chip-scale-package (CSP) substrates were used, those subjected and not subjected to the TH test. This study revealed an association between the interfacial reaction behaviors, void formation and mechanical reliability of the solder joint. Many voids were formed at the interface of the OSP-finished Cu joint subjected to the TH test. These voids were caused by the oxidation of the OSP-finished Cu substrate during the TH test. In the shear tests, the shear force of the joint with the substrate not subjected to the TH test was slightly higher than that with the TH test. The mechanical reliability of the solder joint was degraded by voids at the interface.     

Microstructure of Sn–1Ag–0.5Cu solder alloy bearing Fe under salt spray test

Understanding the behavior of lead-free solder alloys within a high humidity environment is a serious topic in the deployment of products in various electronics applications. The work reported herein investigates this specific impact on Sn–1.0Ag–0.5Cu–0.5Fe solder alloy. Specimens were treated with 5% NaCl salt spray. All specimens showed strong resistance to corrosion. Microstructural deformations after the test were analyzed using Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDX). Concerns were at the localized corroded area, as this would cause significant degradation at the solder joints. The mechanisms leading to these disadvantageous results as well as the microstructural evolution and correlation with the intrinsic properties of the solder alloy are discussed.     

Evaluation of creep properties for Sn–Ag–Cu micro solder joint by multi-temperature stress relaxation test

The creep properties of a Sn–Ag–Cu micro solder joint with a solder ball with 500 μm were investigated by a multi-temperature stress relaxation test performed using a specimen at three temperatures (298, 348, and 398 K). The stress exponents in Norton’s law were 8 at 398 K, 8.8 at 348 K, and 9 at 298 K, and the activation energies were found to be 39 kJ/mol in the high-stress region and 80 kJ/mol in the low-stress region. The stress exponent in Norton’s law for a micro solder joint was lower than that for a large-scale specimen, which resulted in more coarsened intermetallics in the microstructure than in the large-scale specimen. The activation energies for the micro solder joint were almost equal to those for the large-scale specimen in the high- and low-stress regions. These results reflect the microstructure of the micro solder joint, and the creep constitutive equation for the Sn–Ag–Cu joint could be derived by the multi-temperature stress relaxation test proposed in this study.     

Microstructure and mechanical properties of Sn–Zn–Bi–Cr lead-free solder

In this paper, the effect of trace addition of Cr on the mechanical properties and reliability on Sn–8Zn–3Bi solder alloys was investigated. It has been demonstrated that the microstructure of solder alloys was refined after doping traces of Cr. The elongation reaches up to 40.63% after doping 0.1% Cr; and the fracture mechanism converts from quasi-cleavage fracture into ductile fracture. With aging time of 0, 4, 9 and 16 days, mechanical property of Sn–8Zn–3Bi–0.3Cr alloy was improved slightly. It was found that the Sn–Zn–Cr phase was increased and Zn in alloy was consumed after aging, so that the amount of primary Zn phase was reduced and microstructure was improved.     

Impact of Ni concentration on the intermetallic compound formation and brittle fracture strength of Sn–Cu–Ni (SCN) lead-free solder joints

Cu₆Sn₅ and Cu₃Sn are common intermetallic compounds (IMCs) found in Sn–Ag–Cu (SAC) lead-free solder joints with OSP pad finish. People typically attributed the brittle failure to excessive growth of IMCs at the interface between the solder joint and the copper pad. However, the respective role of Cu₆Sn₅ and Cu₃Sn played in the interfacial fracture still remains unclear. In the present study, various amounts of Ni were doped in the Sn–Cu based solder. The different effects of Ni concentration on the growth rate of (Cu, Ni)₆Sn₅/Cu₆Sn₅ and Cu₃Sn were characterized and compared. The results of characterization were used to evaluate different growth rates of (Cu, Ni)₆Sn₅ and Cu₃Sn under thermal aging. The thicknesses of (Cu, Ni)₆Sn₅/Cu₆Sn₅ and Cu₃Sn after different thermal aging periods were measured. High speed ball pull/shear tests were also performed. The correlation between interfacial fracture strength and IMC layer thicknesses was established.     

Sn addition on the tensile properties of high temperature Zn–4Al–3Mg solder alloys

The Zn–4Al–3Mg based solder alloy is a promising candidate to replace the conventional Pb–5Sn alloy in high-temperature electronic packaging. In this study, the tensile properties of Zn–4Al–3Mg–xSn alloys (x = 0, 6.8 and 13.2 wt.%) at high temperatures (e.g., 100 °C, and 200 °C) were investigated. It was found that the uniaxial tensile strength (UTS) of Zn–4Al–3Mg–xSn solder alloys all decrease monotonously with the increment of temperature. The elongation ratio at 100 °C is superior to that at room temperature whereas follows a significant drop at 200 °C. The microstructure observations show that a typical brittle fracture of Zn–4Al–3Mg alloy occurs at room temperature and 200 °C under normal tension, whereas a ductile fracture is found at 100 °C. The 6.8 wt.% Sn addition in Zn–4Al–3Mg alloy causes a dramatic decrease of yield strength, and a slight deterioration of the ductility.     

Microstructure and morphology of interfacial intermetallic compound CoSn3 in Sn–Pb/Co–P solder joints

A systematical microscopic analysis on structure, morphology, and growth of CoSn₃ intermetallic compound (IMC) that formed at the interface between Sn–Pb alloy and Co–P films was carried out using scanning electron microscopy with back-scattered electron imaging and high-resolution transmission electron microscopy with energy dispersive spectrometry. CoSn₃ IMC with two kinds of morphology was found out after Sn–Pb alloy reacted with Co–P films with different microstructures. One kind of CoSn₃ with stacking fault was distributed densely on nanocrystalline and amorphous Co–P films, and the other kind of CoSn₃ without stacking fault existed sparsely on the Co–P film with nanocrystalline/amorphous mixed structure. The stacking fault was caused by the fast growth of CoSn₃ for the cases of Co-7 at.% P and Co-23 at.% P. Co-12 at.% P film with a nanocrystalline/amorphous mixed microstructure had the best diffusion-barrier property among Co–P films with different compositions, because the diffusion of Sn into Co–P was the least. Our study shed light on diffusion-barrier performance of Co-based metallization.     

Effects of Cu on the interfacial reactions between Sn–8Zn–3Bi–xCu solders and Cu substrate

The microstructure, thermal property, and interfacial reaction with Cu substrate of Sn–8Zn–3Bi–xCu (x = 0, 0.5, 1) lead-free solders were investigated in this work. Cu–Zn intermetallics formed in the solder matrix and the melting temperature increases slightly with Cu addition. After soldering at 250 °C for 90 s, a flat Cu₅Zn₈ layer and a scallop CuZn₅ layer formed at the interfaces of all samples. The CuZn₅ intermetallic compound (IMC) transformed to Cu₅Zn₈ IMC with longer reaction time due to the diffusion of Cu atoms from Cu substrate. The interfacial IMC layer grew thicker with the reaction time following a parabolic law which suggested the interfacial reactions were diffusion controlled. The calculation results show that the activation energy of IMC growth for Cu-containing solders is larger than that of Sn–8Zn–3Bi solder, which demonstrated that a small amount of Cu addition to the solder can effectively suppressed the growth of the interfacial IMC.     

Mechanical reliability of Sn-rich Au–Sn/Ni flip chip solder joints fabricated by sequential electroplating method

In this study, we evaluated the mechanical reliability of Sn-rich, Au–Sn/Ni flip chip solder bumps by using a sequential electroplating method with Sn and Au. After reflowing, the average diameter of the solder bump was approximately 80 μm and only a (Ni,Au)₃Sn₄ intermetallic compound (IMC) layer was formed at the interface. Due to the preferential consumption of Sn atoms within the solder matrix during aging, the solder matrix was transformed sequentially in the following order: β-Sn and η-phase, η-phase, and η-phase and ε-phase. In the bump shear test, the shear force was not significantly changed despite aging at 150 °C for 1000 h and most of the fractures occurred at the interfaces. The interfacial fracture was significantly related to the formation of brittle IMCs at the interface. The Sn-rich, Au–Sn/Ni flip chip joint was mechanically much weaker than the Au-rich, Au–Sn/Ni flip chip joint. The study results demonstrated that the combination of Sn-rich, Au–Sn solder and Ni under bump metallization (UBM) is not a viable option for the replacement of the conventional, Au-rich, Au–20Sn solder.     

Lifetime of excess electrons in Cu–Zn–Sn–Se powders

The method of time-resolved microwave photoconductivity at a frequency of 36 GHz in the range of temperatures of 200–300 K is used to study the kinetics of the annihilation of charge carriers in Cu–Zn–Sn–Se powders obtained by the solid-phase method of synthesis in cells. The lifetime of excess electrons at room temperature is found to be shorter than 5 ns. The activation energy for the process of recombination amounted to E _a ~ 0.054 eV.     

Effect of Isothermal Aging on the Long-Term Reliability of Fine-Pitch Sn–Ag–Cu and Sn–Ag Solder Interconnects With and Without Board-Side Ni Surface Finish

The combined effects on long-term reliability of isothermal aging and chemically balanced or unbalanced surface finish have been investigated for fine-pitch ball grid array packages with Sn–3.0Ag–0.5Cu (SAC305) (wt.%) and Sn–3.5Ag (SnAg) (wt.%) solder ball interconnects. Two different printed circuit board surface finishes were selected to compare the effects of chemically balanced and unbalanced structure interconnects with and without board-side Ni surface finish. NiAu/solder/Cu and NiAu/solder/NiAu interconnects were isothermally aged and thermally cycled to evaluate long-term thermal fatigue reliability. Weibull plots of the combined effects of each aging condition and each surface finish revealed lifetime for NiAu/SAC305/Cu was reduced by approximately 40% by aging at 150°C; less degradation was observed for NiAu/SAC305/NiAu. Further reduction of characteristic life-cycle number was observed for NiAu/SnAg/NiAu joints. Microstructure was studied, focusing on its evolution near the board and package-side interfaces. Different mechanisms of aging were apparent under the different joint configurations. Their effects on the fatigue life of solder joints are discussed.     

Effects of Current Stressing on Formation and Evolution of Kirkendall Voids at Sn–3.5Ag/Cu Interface

Kirkendall voids (KVs) are known to be formed at the Cu/Cu₃Sn interface, which can remarkably weaken solder joints. In this paper, the formation and evolution processes of KVs at Sn–3.5Ag/Cu joints were systematically investigated under isothermal aging and current stressing. It was found that the processes develop faster when joints are subjected to current stressing as opposed to thermal aging. This can be illuminated by the high KV densities caused by current stressing at both cathode and anode Cu/Cu₃Sn interfaces. Moreover, KVs formed under current stressing showed some polarity characteristics, namely that higher KV density was observed on the anode side compared with the cathode side. The interfacial reaction generated at the Cu₃Sn/Cu₆Sn₅ interface, which was partly affected by current stressing, contributed to this polarity effect. As the holding time was prolonged, microvoids coalesced into larger porosities and microcracks. These defects will greatly threaten the reliability of the interface.     

Effect of nano Al2O3 particles doping on electromigration and mechanical properties of Sn–58Bi solder joints

In the present study, the effect of Al₂O₃ nanoparticles on performances of Sn–58Bi solder were investigated in aspects of electro-migratio, shear strength and microhardness. The experimental results show that the Al₂O₃ nanoparticles significantly improved microstructure and mechanical performances of solder joints. With the addition of 0.5 wt% Al₂O₃, the intermetallic compounds (IMC) reduced from 2.5 μm to 1.27 μm after 288 aging hours at 85 °C. Furthermore, after electromigration test under a current density of 5 × 10³ A/cm² at 85 °C, Bi-rich layers formed at the anode side of both Al₂O₃ doped and plain solder. Moreover, the addition of Al₂O₃ nanoparticles reduced the mean thickness of Bi-rich layer. In addition, the growth rate of the IMC layer of Al₂O₃ doped solder decreased by 8% compared with the plain solder. Besides, the Al₂O₃ doped solder exhibited better performance than plain solder in microhardness after different aging times. While, the addition of Al₂O₃ significantly impeded the degradation of the shear strength of solder joint after aging for 48 and 288 h. Furthermore, it was worth noting that the fracture surface of doped solder showed a typical rough and ductile structure. However, plain solder exhibited a relatively smooth and fragile surface.