Impact Testing of Sn-3.0Ag-0.5Cu Solder with Ti/Ni(V)/Cu Under Bump Metallization After Aging at 150°C

Nonmagnetic Ni(V) metal and low consumption rate with solders are the advantages of sputtered Ti/Ni(V)/Cu under bump metallization (UBM). However, a Sn-rich phase (“Sn-patch” herein) can form in the Ni(V) layer after reflow and aging. In lead-free solder, Sn-patches form and grow more quickly than in Sn-Pb solder. Thus, the effect of Sn-patches on solder joint reliability becomes critical. In this study, Sn-3.0Ag-0.5Cu solder was reflowed with Ti/Ni(V)/Cu UBM at 250°C for 60 s, and then aged at 150°C for various durations. A high-speed impact test was introduced to evaluate solder joint reliability. After impact testing, it was found that, the larger the Sn-patch, the greater the propensity of the solder joint to suffer brittle fracture. The correlation between Sn-patch and solder joint reliability is discussed.     

Board-level reliability of Pb-free solder joints of TSOP and various CSPs

To evaluate various Pb-free solder systems for leaded package, thin small outline packages (TSOPs) and chip scale packages (CSPs) including leadframe CSP (LFCSP), fine pitch BGA (FBGA), and wafer level CSP (WLCSP) were characterized in terms of board level and mechanical solder joint reliability. For board level solder joint reliability test of TSOPs, daisy chain samples having pure-Sn were prepared and placed on daisy chain printed circuit board (PCB) with Pb-free solder pastes. For CSPs, the same composition of Pb-free solder balls and solder pastes were used for assembly of daisy chain PCB. The samples were subjected to temperature cycle (T/C) tests (-65/spl deg/C/spl sim/150/spl deg/C, -55/spl deg/C/spl sim/125/spl deg/C, 2 cycles/h). Solder joint lifetime was electrically monitored by resistance measurement and the metallurgical characteristics of solder joint were analyzed by microstructural observation on a cross-section sample. In addition, mechanical tests including shock test, variable frequency vibration test, and four point twisting test were carried out with daisy chain packages too. In order to compare the effect of Pb-free solders with those of Sn-Pb solder, Sn-Pb solder balls and solder paste were included. According to this paper, most Pb-free solder systems were compatible with the conventional Sn-Pb solder with respect to board level and mechanical solder joint reliability. For application of Pb-free solder to WLCSP, Cu diffusion barrier layer is required to block the excessive Cu diffusion, which induced Cu trace failure.     

The effect of intermetallic compound morphology on Cu diffusion in Sn-Ag and Sn-Pb solder bump on the Ni/Cu Under-bump metallization

The eutectic Sn-Ag solder alloy is one of the candidates for the Pb-free solder, and Sn-Pb solder alloys are still widely used in today’s electronic packages. In this tudy, the interfacial reaction in the eutectic Sn-Ag and Sn-Pb solder joints was investigated with an assembly of a solder/Ni/Cu/Ti/Si₃N₄/Si multilayer structures. In the Sn-3.5Ag solder joints reflowed at 260°C, only the (Ni_1−x,Cu_x)₃Sn₄ intermetallic compound (IMC) formed at the solder/Ni interface. For the Sn-37Pb solder reflowed at 225°C for one to ten cycles, only the (Ni_1−x,Cu_x)₃Sn₄ IMC formed between the solder and the Ni/Cu under-bump metallization (UBM). Nevertheless, the (Cu_1−y,Ni_y)₆Sn₅ IMC was observed in joints reflowed at 245°C after five cycles and at 265°C after three cycles. With the aid of microstructure evolution, quantitative analysis, and elemental distribution between the solder and Ni/Cu UBM, it was revealed that Cu content in the solder near the solder/IMC interface played an important role in the formation of the (Cu_1−y,Ni_y)₆Sn₅ IMC. In addition, the diffusion behavior of Cu in eutectic Sn-Ag and Sn-Pb solders with the Ni/Cu UBM were probed and discussed. The atomic flux of Cu diffused through Ni was evaluated by detailed quantitative analysis in an electron probe microanalyzer (EPMA). During reflow, the atomic flux of Cu was on the order of 10¹⁶−10¹⁷ atoms/cm²sec in both the eutectic Sn-Ag and Sn-Pb systems.     

The effect of soldering process variables on the microstructure and mechanical properties of eutectic Sn-Ag/Cu solder joints

Fundamental understanding of the relationship among process, microstructure, and mechanical properties is essential to solder alloy design, soldering process development, and joint reliability prediction and optimization. This research focused on the process-structure-property relationship in eutectic Sn-Ag/Cu solder joints. As a Pb-free alternative, eutectic Sn-Ag solder offers enhanced mechanical properties, good wettability on Cu and Cu alloys, and the potential for a broader range of application compared to eutectic Sn-Pb solder. The relationship between soldering process parameters (soldering temperature, reflow time, and cooling rate) and joint microstructure was studied systemati-cally. Microhardness, tensile shear strength, and shear creep strength were measured and the relationship between the joint microstructures and mechani-cal properties was determined. Based on these results, low soldering tempera-tures, fast cooling rates, and short reflow times are suggested for producing joints with the best shear strength, ductility, and creep resistance.     

Failure Morphology After Drop Impact Test of Ball Grid Array (BGA) Package With Lead-Free Sn–3.8Ag–0.7Cu and Eutectic SnPb Solders

High strain-rate drop impact tests were performed on ball grid array (BGA) packages with solder compositions of (in wt%) Sn-3.8Ag-0.7Cu (SnAgCu) and eutectic Sn-37Pb (SnPb). Solder balls were joined to the metallizations of plated Ni on the device side and plated Cu on the board side. The BGA packages were tested at 1500 g within 0.5 ms, resulting in an imposed bending strain of 0.2-0.3%. Both SnAgCu and SnPb joints failed at the interface at the device side but the detailed failure morphology differed significantly. The crack location for the eutectic SnPb was primarily through the solder and seldom extended through an entire bump. The SnPb joints also exhibited bulk solder deformation. The SnAgCu joints showed extremely brittle behavior with an interfacial failure at the (Ni,Cu)₃Sn₄ intermetallics/Ni under bump metallization (UBM) interface. The strain rate sensitivity of bulk solder defines the drop test performance and the eutectic SnPb solder showed better drop impact performance due to a less strain rate sensitivity     

Metallurgical reaction of the Sn-3.5Ag solder and Sn-37Pb solder with Ni/Cu under-bump metallization in a flip-chip package

Several international legislations recently banned the use of Pb because of environmental concerns. The eutectic Sn-Ag solder is one of the promising candidates to replace the conventional Sn-Pb solder primarily because of its excellent mechanical properties. In this study, interfacial reaction of the eutectic Sn-Ag and Sn-Pb solders with Ni/Cu under-bump metallization (UBM) was investigated with a joint assembly of solder/Ni/Cu/Ti/Si₃N₄/Si multilayer structures. After reflows, only one (Ni,Cu)₃Sn₄ intermetallic compound (IMC) with faceted and particlelike grain feature was found between the solder and Ni. The thickness and grain size of the IMC increased with reflow times. Another (Cu,Ni)₆Sn₅ IMC with a rod-type grain formed on (Ni,Cu)₃Sn₄ in the interface between the Sn-Pb solder and the Ni/Cu UBM after more than three reflow times. The thickness of the (Ni,Cu)₃Sn₄ layer formed in the Sn-Pb system remained almost identical despite the numbers of reflow; however, the amounts of (Cu,Ni)₆Sn₅ IMC increased with reflow times. Correlations between the IMC morphologies, Cu diffusion behavior, and IMC transformation in these two solder systems will be investigated with respect to the microstructural evolution between the solders and the Ni/Cu UBM. The morphologies and grain-size distributions of the (Ni,Cu)₃Sn₄ IMC formed in the initial stage of reflow are crucial for the subsequent phase transformation of the other IMC.     

Effect of UBM Thickness on the Mean Time to Failure of Flip-Chip Solder Joints under Electromigration

Flip-chip solder joints with Cu/Ni/Al underbump metallurgy (UBM) on the chip and an Au/Ni surface finish on the substrate were studied under current stressing at an ambient temperature of 150°C. Three different Ni thicknesses in the Cu/Ni/Al UBM (0.3, 0.5, and 0.8 μm) were used in order to investigate the effect of the Ni thickness on reliability. The solder used was eutectic Pb-Sn, and the applied current density was 5 × 10³ A/cm². The results show that the combined effect of current crowding and the local Joule heating near the entry points of electrons into the joints induced asymmetric Ni UBM consumption. Once the Ni was exhausted in a certain region, this region became nonconductive and the flow of electrons was diverted to the neighboring region. This neighboring region then became the place where electrons entered the joint, and the Ni UBM there was consumed at an accelerated rate. This process repeated itself, and the Ni-depleted region continued to extend, creating an ever larger nonconductive region. The solder joints eventually failed when the nonconductive region extended across the entire contact window of the joints. This failure model supports the observation that joints with a thicker Ni tend to have a longer average lifetime.     

Reliability of Au-Ge and Au-Si Eutectic Solder Alloys for High-Temperature Electronics

High-temperature electronics will facilitate deeper drilling, accessing harder-to-reach fossil fuels in oil and gas industry. A key requirement is reliability under harsh conditions for a minimum continuous operating time of 500?h at 300°C. Eutectic solder alloys are generally favored due to their excellent fatigue resistance. Performance of Au-Ge and Au-Si eutectic solder alloys at 300°C up to 500?h has been evaluated. Nanoindentation results confirm the loss of strength of Au-Ge and Au-Si eutectic solder alloys during thermal aging at 300°C, as a result of grain coarsening. However, the pace at which the Au-Ge eutectic alloy loses its strength is much slower when compared with Au-Si eutectic alloy. The interfacial reactions between these eutectic solder alloys and the underbump metallization (UBM), i.e., electroless nickel immersion gold (ENIG) UBM and Cu/Au UBM, have been extensively studied. Spalling of Au₃Cu intermetallic compound is observed at the interface between Au-Ge eutectic solder and the Cu/Au UBM, when aged at 300°C for 500?h, while the consumption of ENIG UBM is nominal. Unlike the Au-Si solder joint, hot ball shear testing at high temperature confirmed that the Au-Ge joint on ENIG UBM, when aged at 300°C for 500?h, could still comply with the minimum qualifying bump shear strength based on the UBM dimension used in this work. Thus, it has been determined that, among these two binary eutectic alloys, Au-Ge eutectic alloy could fulfill the minimum requirement specified by the oil and gas exploration industry.     

Effects of load and thermal conditions on Pb-free solder joint reliability

Reliability of lead-free solder joints has been a hot topic widely debated in the electronic industry. Most published data indicate that a change to lead-free soldering has the potential benefit of more reliable solder joints than the current Sn-Pb eutectic solder joints. However, in reality many mechanical, metallurgical, thermal, and environmental factors affect the service reliability of solder joints. This paper tries to shed some light on the effects of mechanical loading and thermal conditions on solder joint reliability. These conditions are determined not only by external environments but also by the solder alloy itself and the joint geometry. Analyses with first principles are carried out on solder joints of both areal array and peripheral packages. Effects on fatigue life of solder joint geometry, thermal and mechanical characteristics of components and substrate materials, and application conditions are discussed. The analysis helps explain why lead-free solder joints may not be more reliable in certain application conditions than the current Sn-Pb eutectic solder joints.     

Interfacial Reaction Between 95Pb-5Sn Solder Bump and 37Pb-63Sn Presolder in Flip-Chip Solder Joints

Interdiffusion and interfacial reaction of 95Pb-5Sn solder bumps and 37Pb-63Sn presolder in flip-chip solder joints during high-temperature storage were studied. Reaction temperatures included 100°C, 130°C, 150°C, and 175°C. It was found that Cu₆Sn₅ and Cu₃Sn formed on the board side and (Ni,Cu)₃Sn₄ formed on the chip side after 100 h of aging. After 2000 h of aging at 175°C, the Ni under-bump metallization (UBM) was exhausted. This caused the (Ni,Cu)₃Sn₄ layer at the chip-side interface to be gradually converted into (Cu_0.6Ni_0.4)₆Sn₅. It was also found that the consumption of the Ni UBM was faster than the case where eutectic Sn-Pb solder was used for the entire joint. Nevertheless, the consumption of the Cu on the substrate side was slower than the case where pure eutectic Sn-Pb solder was used for the entire joint.     

Interfacial reaction and solder joint reliability of Pb-free solders in lead frame chip scale packages (LF-CSP)

Chip scale packages (CSP) have essential solder joint quality problems, and a board level reliability is a key issue in design and development of the CSP type packages. There has been an effort to eliminate Pb from solder due to its toxicology. To evaluate the various solder balls in CSP package applications, Pb-free Sn-Ag-X (X=In, Cu, Bi) and Sn-9Zn-1Bi-5In solder balls were characterized by melting behavior, phases, interfacial reaction, and solder joint reliability. For studying joint strength between solders and under bump metallurgy (UBM) systems, various UBMs were prepared by electroplating and electroless plating. After T/C (temperature cycle) test, Sn3.5Ag8.5In solder was partially corroded and its shape was distorted. This phenomenon was observed in a Sn3Ag10In 1Cu solder system, too. Their fractured surface, microstructure of solder joint interface, and of bulk solder ball were examined and analyzed by optical microscopy, SEM and EDX. To simulate the real surface mounting condition and evaluate the solder joint reliability on board level, Daisy chain test samples using LF-CSP packages were prepared with various Pb-free solders, then a temperature cycle test (−65∼ 150°C) was performed. All tested Pb-free solders showed better board level solder joint reliability than Sn-36Pb-2Ag. Sn-3.5Ag-0.7Cu and Sn-9Zn-1Bi-5In solders showed 35%, 100% superior solder joint reliability than Sn-36Pb-2Ag solder ball, respectively.     

Effect of Gold Content on the Microstructural Evolution of SAC305 Solder Joints Under Isothermal Aging

Au over Ni on Cu is a widely used printed circuit board (PCB) surface finish, under bump metallization (UBM), and component lead metallization. It is generally accepted that less than 3 wt.% Au in Sn-Pb solder joints inhibits formation of detrimental intermetallic compounds (IMC). However, the critical limit for Au content in Pb-free solder joints is not well established. Three surface-mount package platforms, one with a matte Sn surface finish and the others with Ni/Au finish, were soldered to Ni/Au-finished PCB using Sn-3.0Ag-0.5Cu (SAC305) solder, in a realistic manufacturing setting. The assembled boards were divided into three groups: one without any thermal treatment, one subjected to isothermal aging at 125°C for 30 days, and the third group aged at 125°C for 56 days. Representative solder joints were cross-sectioned and analyzed using scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDX) to investigate the evolution of the solder joint morphology as a function of Au content and isothermal aging. It was found that, if Cu is available to dissolve in the solder joint, the migration of AuSn₄ from the bulk to the interface as a result of thermal aging is mitigated.     

Corrosion behavior of Sn-3.0Ag-0.5Cu lead-free solder joints

The aim of this study is to evaluate the corrosion behavior of Sn-3.0Ag-0.5Cu (SAC305) lead-free solder joint using salt spray test. The presence of Cu pad accelerates the dissolution of Sn from solder joints into corrosive medium because of galvanic corrosion mechanism. So, the solder joint was easily corroded in corrosive environment than SAC305 solder bar. During salt spray test, pitting corrosion begin from the solidification cracks in the solder joints, which will lead to a decrease of the reliability of solder joints and shorten the life of electronic devices.     

The influence of Sn–Cu–Ni(Au) and Sn–Au intermetallic compounds on the solder joint reliability of flip chips on low temperature co-fired ceramic substrates

The formation of intermetallic compounds in the solder joint of a flip chip or chip scale package depends on the under bump metallurgy (UBM), the substrate top surface metallisation, the solder alloy and the application conditions. To evaluate the influence of intermetallic compounds on the solder joint reliability, a detailed study on the influence of the UBM, the gold finish thickness of the substrate top surface metallisation, the solder alloy and the aging conditions has been conducted. Flip chips bumped with different solder alloys were reflow-mounted on low temperature co-fired ceramic substrates. The flip chip package was then aged at high temperature and a bump shear test followed to examine the shear strength of the solder joint at certain aging intervals. It was found that the type of UBM has a great impact on the solder joint reliability. With Ni(P)/Au as the UBM, well-documented gold embrittlement was observed when the gold concentration in the eutectic SnPb solder was about 3 wt%. When Al/Ni(V)/Cu was used as the UBM, the solder joint reliability was substantially improved. Copper dissolution from the UBM into the solder gives different intermetallic formations compared to Ni(P)/Au as UBM. The addition of a small amount of copper in the solder alloy changed the mechanical property of the intermetallic compound, which is attributed to the formation of Sn–Cu–Ni(Au) intermetallic compounds. This could be used in solving the problem of the AuSn₄ embrittlement. The formation and the influence of this Sn–Cu–Ni(Au) intermetallic phase are discussed. The gold concentration in the solder joint plays a role in the formation of intermetallic compounds and consequently the solder joint reliability, especially for the Sn–Ag–Cu soldered flip chip package.     

Failure mechanism of lead-free solder joints in flip chip packages

The failure mechanisms of SnAgCu solder on Al/Ni(V)/Cu thin-film, underbump metallurgy (UBM) were investigated after multiple reflows and high-temperature storage using a ball shear test, fracture-surface analysis, and cross-sectional microstructure examination. The results were also compared with those of eutectic SnPb solder. The Al/Ni (V)/Cu thin-film UBM was found to be robust enough to resist multiple reflows and thermal aging at conditions used for normal production purposes in both SnAgCu and eutectic SnPb systems. It was found that, in the SnAgCu system, the failure mode changed with the number of reflows, relating to the consumption of the thin-film UBM because of the severe interfacial reaction between the solder and the UBM layer. After high-temperature storage, the solder joints failed inside the solder ball in a ductile manner in both SnAgCu and SnPb systems. Very fine Ag₃Sn particles were formed during multiple reflows in the SnAgCu system. They were found to be able to strengthen the bulk solder. The dispersion-strengthening effect of Ag₃Sn was lost after a short period of thermal aging, caused by the rapid coarsening of these fine particles.     

Characterization of the growth of intermetallic interfacial layers of Sn-Ag and Sn-Pb eutectic solders and their composite solders on Cu substrate during isothermal long-term aging

Single shear lap joints were made with four different solders, Sn-Pb and Sn-Ag eutectic solders, and their composites containing about 20 vol.% in-situ Cu6Sn5 intermetallic phases about 3–8 micrometers in diameter. Two sets of experiments were performed: In the first set, all of the above four solder joints were aged at 150°C for periods ranging to 5000 h and the intermetallic growth was monitored periodically. In the second set, each of the above four solder joints was aged at five different temperatures for 4000 h. The interfacial layers between solders and the Cu substrate were examined using optical and scanning electron microscopy. The growth kinetics of intermetallic interfacial layers formed between solder and Cu substrate was characterized. The effect of in-situ Cu₆Sn₅ intermetallic phases on the growth rate is discussed. The growth rate of the intermetallic layers in the eutectic Sn-Pb composite was slower for the first 150 h as compared to the eutectic Sn-Pb non-composite. The growth rate of the intermetallic layers were similar for both the eutectic Sn-Ag and eutectic Sn-Ag composite throughout the aging duration. The activation energies for Cu₆Sn₅ layer growth for the eutectic Sn-Pb and Sn-Ag solder joints are evaluated to be 111 kJ/mol and 116 kJ/mol, respectively. The eutectic Sn-Pb and Sn-Ag composite solder joints exhibit higher activation energies of 161 kJ/mol and 203 kJ/mol.     

Effects of strain rates and biaxial stress conditions on plastic yielding and flow stress of solder alloys

The rate-dependent mechanical properties of Sn3.8Ag0.7Cu (SAC387) Pb-free alloy and Sn-Pb eutectic alloy were investigated in this study under pure shearing and biaxial stress conditions with thin-walled specimens using a servo-controlled tension-torsion material testing system. The pure shearing tests were conducted at strain rates between 6.7 × 10⁻⁷ and 1.3 × 10⁻¹/sec. In addition, axial tensile stresses were superimposed onto the shearing samples to examine the effects of biaxial stress conditions on the yielding and on post-yielding plastic flow of the solder alloys. Strain hardening is observed for the Pb-free alloy at all the tested strain rates, while strain softening happens with the Sn-Pb eutectic solder at low strain rates. Special tests were also conducted for sudden strain-rates changes and stress relaxation for the purpose to develop a viscoplastic model to simulate time-dependent multiaxial deformation and to assess damage and fatigue life of general solder interconnections.     

Electromigration Reliability and Morphologies of Cu Pillar Flip-Chip Solder Joints with Cu Substrate Pad Metallization

The Cu pillar is a thick underbump metallurgy (UBM) structure developed to alleviate current crowding in a flip-chip solder joint under operating conditions. We present in this work an examination of the electromigration reliability and morphologies of Cu pillar flip-chip solder joints formed by joining Ti/Cu/Ni UBM with largely elongated ∼62 μm Cu onto Cu substrate pad metallization using the Sn-3Ag-0.5Cu solder alloy. Three test conditions that controlled average current densities in solder joints and ambient temperatures were considered: 10 kA/cm² at 150°C, 10 kA/cm² at 160°C, and 15 kA/cm² at 125°C. Electromigration reliability of this particular solder joint turns out to be greatly enhanced compared to a conventional solder joint with a thin-film-stack UBM. Cross-sectional examinations of solder joints upon failure indicate that cracks formed in (Cu,Ni)₆Sn₅ or Cu₆Sn₅ intermetallic compounds (IMCs) near the cathode side of the solder joint. Moreover, the ~52-μm-thick Sn-Ag-Cu solder after long-term current stressing has turned into a combination of ~80% Cu-Ni-Sn IMC and ~20% Sn-rich phases, which appeared in the form of large aggregates that in general were distributed on the cathode side of the solder joint.     

Characterization of the Effect of Shape Memory Alloy on Solder Joint Strength Through Modeling and Testing

Currently some of the most common problems that surface mount technology encounters are warpage, delamination, and inelastic strain concentration accumulated in the solder joint during thermal cycling because of mismatch of thermal expansion coefficient between the package and chip side. Material as well as package structure are the critical issues with respect to these problems. The objective of this research is to investigate how shape memory alloy (SMA) applied in the under bump metallization (UBM) can affect solder joint reliability under thermal mechanical stress. Joint strength tests revealed the better strength of solder joints with SMA UBM after accelerated thermal cycling test. Finite element modeling as well as multilayer stress calculations revealed less strain accumulated in the solder and more stress concentrated in Si in the solder joint with SMA UBM. A mechanism by which the SMA accommodates most of the stress and strain caused by the mismatch of the thermal expansion coefficients was proposed to explain the reinforcement of the solder joint by the SMA UBM.     

Kinetics of Au-containing ternary intermetallic redeposition at solder/UBM interface

In this study, the effects of the under bump metallurgy (UBM) structure and Cu content in solders on the redeposition rate of Au-containing ternary intermetallics at the solder/UBM interface were investigated. A UBM structure with a Ni diffusion barrier, Au/Ni/Cu, appeared to promote the redeposition of ternary Au-containing intermetallics at the solder/UBM interface of the ternary during the solid-state aging treatment and the Au-embrittlement of the solder interconnections. Copper added to the eutectic Sn-Pb and Sn-Ag solders was observed to be very effective in retarding the redeposition by forming the ternary intermetallics in solder matrices and preventing the Au-embrittlement. These phenomena were discussed with the microstructures observed. Jointly appointed by CAAM at POSTECH