Effect of Interfacial Reactions on the Reliability of Lead-Free Assemblies after Board Level Drop Tests

The reliability of lead-free electronic assemblies after board level drop tests was investigated. Thin small outline package (TSOP) components with 42 FeNi alloy leads were reflow soldered on FR4 printed circuit boards (PCBs) with Sn3.0Ag0.5Cu (wt%) solder. The effects of different PCB finishes [organic solderability preservative (OSP) and electroless nickel immersion gold (ENIG)], multiple reflow (once and three times), and isothermal aging (500 h at 125°C after one time reflow) were studied. The ENIG finish showed better performance than its OSP counterparts. With the OSP finish, solder joints reflowed three times showed obvious improvement compared to those of the sample reflowed once, while aging led to apparent degradation. The results showed that intermetallic compound (IMC) types, IMC microstructure and solder microstructure compete with each other, all playing very important roles in the solder joint lifetime. The results also showed that it is important to specify adequate conditions for a given reliability assessment program, to allow meaningful comparison between results of different investigators.     

Mechanical characterization of Sn–Ag-based lead-free solders

Recently, preventing environmental pollutions, lead-free (Pb-free) solders are about to replace tin–lead (Sn–Pb) eutectic solders. However, the mechanical properties of Pb-free solders have not been clarified. Hence, the following study was conducted; first, a rate-dependent plasticity was characterized to represent the inelastic deformation behavior for Sn–Ag-based lead-free solders. The material parameters in a constitutive model were determined in a direct method combining both rate-dependent and rate-independent plastic strains. The constitutive model unifies both rate-dependent creep behavior and rate-independent plastic behavior occurring concurrently at the same time in the solders. Secondly, the strength of solders with a variety of plating materials was studied. Intermetallic compounds (IMC) between solder and electrical pads are formed during reflow process and gradually grow in service. By using the Cu-plates on which Cu or Ni or Ni/Au plating was deposited, the specimens of solder joints were fabricated with Sn–Ag-based lead-free solders. After aging the specimens in an isothermal chamber, tensile tests were performed. From scanning electron microscope (SEM) microscope observation and EDX microprobe analysis, the growth and components of the IMC layer were also examined. Based on the experimental tests, the relations between solder joint strength and the aging period were discussed. Furthermore, the validation of fracture strength of solder joints resulting from the tensile tests was verified with package-mounted board level reliability tests.     

Characteristics of Sn8Zn3Bi solder joints and crack resistance with various PCB and lead coatings

Reliability of QFP (quad flat package) solder joints after thermal shock was investigated for PCB’s and connecting leads plated with several different alloy coatings before soldering. Sn–8 wt%Zn–3 wt%Bi (hereafter, Sn–8Zn–3Bi) was selected as a solder, and FR-4 PCB’s finished with Cu/Sn, Cu/OSP and Cu/Ni/Au were used as substrates. The leads of the QFP were Cu plated with Sn–10 wt%Pb, or Sn, or Sn–3 wt%Bi. The QFP chips were mounted on the substrates using a Sn–8Zn–3Bi solder paste and reflowed in air atmosphere. The pull strength and microstructure for the soldered leads of QFP were evaluated before and after thermal shock testing. The leads plated with Sn or Sn–3Bi showed approximately 40–50% higher pull strength than the reference value of a Sn–37%Pb solder joint for all PCB-finishes. However, in the case of leads coated with Sn–10Pb, the pull strength of the leads soldered to a Sn-finished PCB was 21% lower than the reference value. In microstructure analysis of the joints with Sn–10Pb-plated leads, cracks were found along the bonded interface for Sn-finished PCB. The cracks were believed to start from the low melting temperature phase, 49.38 wt% Pb–32.58 wt%Sn–18.03 wt%Bi, found around the crack, and then propagated through Cu–Zn intermetallic compound. Meanwhile, even when using Sn–10Pb-plated leads, the PCB’s finished with Cu/Ni/Au coating had about 30% higher strength than the reference value, and cracks were hardly found on the soldered joint. Thus, even with Sn–10Pb-plated leads the Cu/Ni/Au-finished PCB’s were evaluated to be as reliable as the reference joint.     

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.     

Morphology and Growth of Intermetallics at the Interface of Sn-based Solders and Cu with Different Surface Finishes

Several types of surface finishes have been applied on Cu substrates in an effort to facilitate bonding and improve the reliability of lead-free solder joints. In the current research, the effects of printed circuit board surface finishes on the reliability of the solder joints were investigated by examining the morphology and growth behavior of the intermetallic compounds (IMCs) between Sn-based solders and different surface finishes on Cu. Three types of Cu substrates with different surface finishes were fabricated in this study: organic solderability preservative (OSP)/Cu, Ni/Cu, and electroless nickel immersion gold (ENIG)/Cu. Sn-3.5Ag and Sn-3.0Ag-0.5Cu were used as the solders. In the experiment, the solder joint specimens were aged isothermally at 150°C for up to 1000 h. Experimental results revealed that the OSP surface finish promoted the interdiffusion between Cu and Sn during soldering. The composition and morphology of the IMC layer at the solder/Ni/Cu interface were sensitive to the Cu concentration in the solder. Meanwhile, the solder joints with different morphological features of the IMCs exhibited significant differences in shear strengths. The Au-containing ENIG surface finish affected the shear strength of the solder joint significantly at the initial stage of isothermal aging.     

Impact of Thermal Cycling on Sn-Ag-Cu Solder Joints and Board-Level Drop Reliability

The electronic packaging industry uses electroless nickel immersion gold (ENIG) or Cu-organic solderability preservative (Cu-OSP) as a bonding pad surface finish for solder joints. In portable electronic products, drop impact tests induce solder joint failures via the interfacial intermetallic, which is a serious reliability concern. The intermetallic compound (IMC) is subjected to thermal cycling, which negatively affects the drop impact reliability. In this work, the reliability of lead-free Sn-3.0Ag-0.5Cu (SAC) soldered fine-pitch ball grid array assemblies were investigated after being subjected to a combination of thermal cycling followed by board level drop tests. Drop impact tests conducted before and after thermal aging cycles (500, 1000, and 1500 thermal cycles) show a transition of failure modes and a significant reduction in drop durability for both SAC/ENIG and SAC/Cu-OSP soldered assemblies. Without thermal cycling aging, the boards with the Cu-OSP surface finish exhibit better drop impact reliability than those with ENIG. However, the reverse is true if thermal cycle (TC) aging is performed. For SAC/Cu-OSP soldered assemblies, a large number of Kirkendall voids were observed at the interface between the intermetallic and Cu pad after thermal cycling aging. The void formation resulted in weak bonding between the solder and Cu, leading to brittle interface fracture in the drop impact test, which resulted in significantly lower drop test lifetimes. For SAC/ENIG soldered assemblies, the consumption of Ni in the formation of NiCuSn intermetallics induced vertical voids in the Ni(P) layer.     

Isothermal and thermal cycling aging on IMC growth rate in lead-free and lead-based solder interface

The growth of interfacial intermetallic compounds (IMC) between Pb-free and Pb-based solders with different surface finish (Cu and Ni/Au) metallization is a major concern for long-term solder joint reliability performance in electronic assemblies. The growth rate of the IMC layer can affect the solder joint reliability. Analysis of solid-state diffusion mechanism for the growth of IMC between solder-to-substrate interface for Pb-free and Pb-based solders subject to isothermal and thermal cycling aging were conducted. Experimental study of IMC layer growth between Sn3.8Ag0.7Cu and Ni/Au surface finish by isothermal aging versus thermal cycling (TC) aging was investigated to develop a framework for correlating IMC layer growth behavior. An integrated model for IMC growth was derived to describe the Ni-Cu-Sn IMC growth behavior subject to TC aging. Comparison of modeling and test results showed that IMC layer growth rate under TC aging was accelerated. It is noted that IMC layer growth study from various references showed different experimental data and growth kinetic parameters for both liquid-state and solid-state reactions.     

弯曲应力状况下微型BGA封装可靠性比较

文中采用传统的表面贴装技术进行焊接,研讨μBGA的PCB装配及可靠性。弯曲循环试验（1000με～-1000με）,用不同的热因数（Qη）回流,研究μBGA、PBGA和CBGA封装的焊点疲劳失效问题。确定液相线上时间,测定温度,μBGA封装的疲劳寿命首先增大,接着随加热因数的增加而下降。当Qη接近500s·℃时,出现寿命最大值。最佳Qη范围在300s·℃～750s·℃之间,此范围如果装配是在氮气氛中回流,μBGA封装的寿命大于4500个循环。采用扫描电子显微镜（SEM）,来检查μBGA和PBGA封装在所有加热因数状况下焊点的失效。每个断裂接近并平行于PCB焊盘,在μBGA封装中裂纹总是出现在焊接点与PCB焊盘连接的尖角点,接着在Ni3Sn4金属间化合物（IMC）层和焊料之间延伸。CBGA封装可靠性试验中,失效为剥离现象,发生于陶瓷基体和金属化焊盘之间的界面处。     

Interfacial reaction and mechanical reliability of PTH solder joints with different solder/surface finish combinations

The effects of minor Ni addition (0.05 wt.%) on the microstructures and mechanical reliability of the lead-free solder joints used in the pin through hole (PTH) components were carefully investigated using a scanning electron microscope (SEM), a field-emission electron probe x-ray microanalyzer, and a pull tester. The PTH walls (i.e., Cu) of printed circuit boards (PCBs) were coated with organic solderability preservative (OSP) or electroless nickel/immersion gold (ENIG) surface finish before soldering. During soldering, the pins of the electronic components were first inserted into the PTHs deposited with OSP or ENIG, and then joined using a Sn–3Ag–0.5Cu (SAC) solder bath through a typical wave-soldering process. After wave soldering, a rework (the second wave soldering) was performed, where an SAC or Sn–0.7Cu–0.05Ni (SCN) solder bath was employed. The SCN joints were found to possess a higher tensile strength than the SAC ones in the OSP case. The sluggish growth of Cu₃Sn, along with few Kirkendall voids at the solder/Cu interface caused by minor Ni addition into the solder alloy (i.e., SCN), was believed to be the root cause responsible for the increase in the strength value. However, the mechanical strength of the PTH components was revealed to be insensitive to the solder composition in the alternative case where an ENIG was deposited over the PTH walls. The implication of this study revealed that minor addition of Ni into the solder is beneficial for the solder/Cu joints, but for the solder/Ni(P) joints.     

Interfacial Reaction between Zn-Al-Based High-Temperature Solders and Ni Substrate

The Zn-Al(-Cu) eutectic alloys (melting point 381°C) are candidates for use as Pb-free high-temperature solders as a substitute for Pb-based solders, which are suitable for severe working environments such as the engine room of hybrid vehicles equipped with an inverter system as well as a heat engine. In this study, the interfacial reaction between Zn-Al(-Cu) alloys and the Ni substrate during soldering, aging, and thermal cycling was investigated. Semiconductor chips and Ni substrates were soldered with Zn-Al(-Cu) alloys at various temperatures under a nitrogen atmosphere. The soldered assemblies were then heat-treated at 200°C and 300°C to examine the microstructural evolution at the soldered interface. The effect of severe thermal cycles between −40°C and 250°C in air on the microstructure and fracture behavior at the solder joint was investigated. Even after a 1000-cycle test, the thickness of the Al₃Ni₂ layer formed at the interface between the Zn-Al-based solder and the Ni substrate, which is responsible for the damage of the soldered assemblies, was quite small.     

弯曲应力状况下微型BGA封装可靠性比较研究

微型球栅阵列（μBGA）是芯片规模封装（CSP）的一种形式,已发展成为最先进的表面贴装器件之一。在最新的IxBGA类型中使用低共晶锡．铅焊料球,而不是电镀镍金凸点。采用传统的表面贴装技术进行焊接,研讨μBGA的PCB装配及可靠性。弯曲循环试验（1000～1000με）,用不同的热因数（Qη）回流,研究μBGA、PBGA和CBGA封装的焊点疲劳失效问题。确定液相线上时间,测定温度,μBGA封装的疲劳寿命首先增大,接着随加热因数的增加而下降。当Q。接近500S·℃时,出现寿命最大值。最佳Qη范围在300-750s·℃之间,此范围如果装配是在氮气氛中回流,μBGA封装的寿命大于4500个循环。采用扫描电子显微镜（SEM）,来检查μBGA和PBGA封装在所有加热N数状况下焊点的失效。每个断裂接近并平行于PCB焊盘,在μBGA封装中裂纹总是出现在焊接点与PCB焊盘连接的尖角点,接着在Ni3Sn4金属间化合物（IMC）层和焊料之间延伸。CBGA封装可靠性试验中,失效为剥离现象,发生于陶瓷基体和金属化焊盘之间的界面处。     

Experimental studies of board-level reliability of chip-scale packages subjected to JEDEC drop test condition

We investigate in this paper board-level drop reliability of chip-scale packages subjected to JEDEC drop test condition B, which features an impact pulse profile with a peak acceleration of 1500G and a pulse duration of 0.5 ms. Effects of Sn–Ag–Cu or Sn–Pb solder joint compositions, fluxes, and substrate pads with Ni/Au surface finish or OSP coating on the drop reliability of the board-level test vehicle are compared. Locations and modes of the failed solder joints are examined using the dye stain test. The results indicate that solder joints with a low Ag weight content and substrate pads with OSP coating both enhance the drop resistance of the board-level test vehicle.     

Effects of process conditions on reliability, microstructure evolution and failure modes of SnAgCu solder joints

In this study, microstructure evolution at intermetallic interfaces in SnAgCu solder joints of an area array component was investigated at various stages of a thermal cycling test. Failure modes of solder joints were analyzed to determine the effects of process conditions on crack propagation. Lead-free printed-circuit-board (PCB) assemblies were carried out using different foot print designs on PCBs, solder paste deposition volume and reflow profiles. Lead-free SnAgCu plastic-ball-grid-array (PBGA) components were assembled onto PCBs using SnAgCu solder paste. The assembled boards were subjected to the thermal cycling test (−40 °C/+125 °C), and crack initiation and crack propagation during the test were studied. Microstructure analysis and measurements of interface intermetallic growth were conducted using samples after 0, 1000, 2000 and 3000 thermal cycles. Failures were not found before 5700 thermal cycles and the characteristic lives of all solder joints produced using different process and design parameters were more than 7200 thermal cycles, indicating robust solder joints produced with a wide process window. In addition, the intermetallic interfaces were found to have Sn–Ni–Cu. The solder joints consisted of two Ag–Sn compounds exhibiting unique structures of Sn-rich and Ag-rich compounds. A crystalline star-shaped structure of Sn–Ni–Cu–P was also observed in a solder joint. The intermetallic thicknesses were less than 3 μm. The intermetallics growth was about 10% after 3000 thermal cycles. However, these compounds did not affect the reliability of the solder joints. Furthermore, findings in this study were compared with those in previous studies, and the comparison proved the validity of this study.     

Joint reliability of various Pb-free solders under harsh vibration conditions for automotive electronics

These days, realization of technology for automotive electronics is important for convenience and safety in automobile industries. Although technology development is continuously progressing, various problems associated with the reliability of automotive electronics have arisen. In this study, combined vibration tests were performed to determine the reliability of various solders under harsh environments: Sn-3.0Ag-0.5Cu (SAC305), Sn0.7Cu and Sn-0.5Cu-0.01Al-0.005Si-0.008Ge (SnCuAl(Si)) solder (in wt%). The Pb-free solder balls were used on electroplating nickel finished Cu pads of a fine ball grid array (FBGA) package. The BGA packages mounted with solder balls were set up on electroless nickel-immersion gold (ENIG) finished Cu pads of a daisy-chained printed circuit board (PCB). The combined random vibration test was performed under 2.5 G_rms in the range of 400 to 2000 Hz in the temperature range of −45 to 125 °C and was continued until 500 cycles. The reliability of the solder joint was determined by measuring the electrical resistance using a multi-meter. The resistance gradually increased and finally approached infinity. In addition, the IMC thicknesses increased during the combined random vibration test, which affected the fracture behavior. To determine the reliability of the solders, the number of failures of solders and the crack morphology and propagation in each solder were evaluated. Among the three solders, the SnCuAl(Si) solder demonstrated the best reliability.     

Reliability study of Sn–Ag–Cu–Ce soldered joints in quad flat packages

The reliability of Sn–Ag–Cu–Ce lead-free soldered joints in quad flat packages under thermal cycling was investigated based on finite element simulation and experiments. The stress and strain response of fine pitch QFP device lead-free soldered joints were analyzed using finite element method based on Garofalo–Arrhenius model. The simulated results indicate that the creep distribution is not uniform, the heel of joints is the maximum creep strain concentrated sites. And comparisons were then made with experimental results of the cracks observed in the Sn–Ag–Cu–Ce soldered joints subjected to the temperature cycled experiment. In addition, the relative mechanical and metallurgical factors, which dominate the failure of soldered joints, were utilized to analyze the phenomena. The fracture surfaces indicate that crack initiate and propagate along the interface among bulk Cu₆Sn₅ phases in Sn–Ag–Cu–Ce soldered joints.     

Interfacial reaction and joint reliability of fine-pitch flip-chip solder bump using stencil printing method

This study was focused on the formation and reliability evaluation of solder joints with different diameters and pitches for flip chip applications. We investigated the interfacial reaction and shear strength between two different solders (Sn-37Pb and Sn-3.0Ag-0.5Cu, in wt.%) and ENIG (Electroless Nickel Immersion Gold) UBM (Under Bump Metallurgy) during multiple reflow. Firstly, we formed the flip chip solder bumps on the Ti/Cu/ENIG metallized Si wafer using a stencil printing method. After reflow, the average solder bump diameters were about 130, 160 and 190 μm, respectively. After multiple reflows, Ni₃Sn₄ intermetallic compound (IMC) layer formed at the Sn-37Pb solder/ENIG UBM interface. On the other hand, in the case of Sn-3.0Ag-0.5Cu solder, (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ IMCs were formed at the interface. The shear force of the Pb-free Sn-3.0Ag-0.5Cu flip chip solder bump was higher than that of the conventional Sn-37Pb flip chip solder bump.     

Effect of Ag micro-particles content on the mechanical strength of the interface formed between Sn–Zn binary solder and Au/Ni/Cu bond pads

In this study, addition of Ag micro-particles with a content in the range between 0 and 4 wt.% to a Sn–Zn eutectic solder, were examined in order to understand the effect of Ag additions on the microstructural and mechanical properties as well as the thermal behavior of the composite solder formed. The shear strengths and the interfacial reactions of Sn–Zn micro-composite eutectic solders with Au/Ni/Cu ball grid array (BGA) pad metallizations were systematically investigated. Three distinct intermetallic compound (IMC) layers were formed at the solder interface of the Au/electrolytic Ni/Cu bond pads with the Sn–Zn composite alloys. The more Ag particles that were added to the Sn–Zn solder, the more Ag–Zn compound formed to thicken the uppermost IMC layer. The dissolved Ag–Zn IMCs formed in the bulk solder redeposited over the initially formed interfacial Au–Zn IMC layer, which prevented the whole IMC layer lifting-off from the pad surface. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces.     

The reliability study of selected Sn–Zn based lead-free solders on Au/Ni–P/Cu substrate

Since both Ag and In are important melting point depressants in Sn–Zn based solders, a series Sn–Zn based solders with various amounts of Ag and In additions was studied in the experiment. The melting behavior of solder alloys, wetting characteristics, coefficients of thermal expansion, microstructural evolution and long-term reliability of the selected Sn–Zn based solder on Au/Ni–P metallized copper substrate were examined. Based on the experimental result, there is little change in the melting range of Sn–Zn based solder alloys by minor addition of Ag. On the contrary, the melting point of Sn–Zn based alloys can be effectively decreased by In additions. However, the difference between solidus and liquidus temperature is broadened as the increment of In into Sn–Zn based solders. 76Sn–9Zn–15In has the lowest liquidus temperature among all alloys, and it can effectively bond the Au/Ni–P metallized copper substrate. The microstructure of 76Sn–9Zn–15In alloy soldered at 200 °C for 20 min is primarily comprised of Sn–In γ phase and needle-like ZnO₂. Since there is no flux usage during soldering, zinc oxide cannot be avoided even the process performed under 2×10⁻² mbar vacuum environment. It is also noted that there is no interfacial reaction layer between 76Sn–9Zn–15In and Au/Ni–P metallized copper substrate after soldering. However, there is a reaction layer between 76Sn–9Zn–15In and substrate as the soldered specimen aged at 90 °C for 168 h. Its chemical composition is close to Zn-rich γ phase (NiZn₃) alloyed with minor Sn, In, Cu and P. For the specimen further aged at 90 °C for 336 h, there are cracks along the interface between solder alloy and electroless Ni–P layer. The oxidation of the interfacial Zn-rich γ phase plays an important role in deterioration of the bonding between 76Sn–9Zn–15In and Au/Ni–P metallized copper substrate.     

elemental redistribution and interfacial reaction mechanism for the flip chip Sn-3.0Ag-(0.5 or 1.5)Cu solder bump with Al/Ni(V)/Cu under-Bump metallization during aging

In flip chip technology, Al/Ni(V)/Cu under-bump metallization (UBM) is currently applicable for Pb-free solder, and Sn−Ag−Cu solder is a promising candidate to replace the conventional Sn−Pb solder. In this study, Sn-3.0Ag-(0.5 or 1.5)Cu solder bumps with Al/Ni(V)/Cu UBM after assembly and aging at 150°C were employed to investigate the elemental redistribution, and reaction mechanism between solders and UBMs. During assembly, the Cu layer in the Sn-3.0Ag-0.5Cu joint was completely dissolved into solders, while Ni(V) layer was dissolved and reacted with solders to form (Cu_1−y,Ni_y)₆Sn₅ intermetallic compound (IMC). The (Cu_1−y,Ni_y)₆Sn₅ IMC gradually grew with the rate constant of 4.63 × 10⁻⁸ cm/sec^0.5 before 500 h aging had passed. After 500 h aging, the (Cu_1−y,Ni_y)₆Sn₅ IMC dissolved with aging time. In contrast, for the Sn-3.0Ag-1.5Cu joint, only fractions of Cu layer were dissolved during assembly, and the remaining Cu layer reacted with solders to form Cu₆Sn₅ IMC. It was revealed that Ni in the Ni(V) layer was incorporated into the Cu₆Sn₅ IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. During the period of 2,000 h aging, the growth rate constant of (Cu_1−y,Ni_y)₆Sn₅ IMC was down to 1.74 × 10⁻⁸ cm/sec^0.5 in, the Sn-3.0Ag-1.5Cu joints. On the basis of metallurgical interaction, IMC morphology evolution, growth behavior of IMC, and Sn−Ag−Cu ternary isotherm, the interfacial reaction mechanism between Sn-3.0Ag-(0.5 or 1.5)Cu solder bump and Al/Ni(V)/Cu UBM was discussed and proposed.     

Experimental characterization and mechanical behavior analysis of intermetallic compounds of Sn–3.5Ag lead-free solder bump with Ti/Cu/Ni UBM on copper chip

Due to today’s trend towards ‘green’ products, the environmentally conscious manufacturers are moving toward lead-free schemes for electronic devices and components. Nowadays the bumping process has become a branch of the infrastructure of flip chip bonding technology. However, the formation of excessively brittle intermetallic compound (IMC) between under bump metallurgy (UBM)/solder bump interface influences the strength of solder bumps within flip chips, and may create a package reliability issue. Based on the above reason, this study investigated the mechanical behavior of lead-free solder bumps affected by the solder/UBM IMC formation in the duration of isothermal aging. To attain the objective, the test vehicles of Sn–Ag (lead-free) and Sn–Pb solder bump systems designed in different solder volumes as well as UBM diameters were used to experimentally characterize their mechanical behavior. It is worth to mention that, to study the IMC growth mechanism and the mechanical behavior of a electroplated solder bump on a Ti/Cu/Ni UBM layer fabricated on a copper chip, the test vehicles are composed of, from bottom to top, a copper metal pad on silicon substrate, a Ti/Cu/Ni UBM layer and electroplated solder bumps. By way of metallurgical microscope and scanning-electron-microscope (SEM) observation, the interfacial microstructure of test vehicles was measured and analyzed. In addition, a bump shear test was utilized to determine the strength of solder bumps. Different shear displacement rates were selected to study the time-dependent failure mechanism of the solder bumps. The results indicated that after isothermal aging treatment at 150 °C for over 1000 h, the Sn–Ag solder revealed a better maintenance of bump strength than that of the Sn–Pb solder, and the Sn–Pb solder showed a higher IMC growth rate than that of Sn–Ag solder. In addition, it was concluded that the test vehicles of copper chip with the selected Ti/Cu/Ni UBMs showed good bump strength in both the Sn–Ag and Sn–Pb systems as the IMC grows. Furthermore, the study of shear displacement rate effect on the solder bump strength indicates that the analysis of bump strength versus thermal aging time should be identified as a qualitative analysis for solder bump strength determination rather than a quantitative one. In terms of the solder bump volume and the UBM size effects, neither the Sn–Ag nor the Sn–Pb solders showed any significant effect on the IMC growth rate.