Microstructure and shear strength of Sn37Pb/Cu solder joints subjected to isothermal aging

The effects of isothermal aging on the microstructure and shear strength of Sn37Pb/Cu solder joints were investigated. Single-lap shear solder joints of eutectic Sn37Pb solder were aged for 1–10 days at 120 °C and 170 °C, respectively, and then loaded to failure in shear with a constant loading speed of 5 × 10⁻³ mm/s. The growth of the interfacial Cu–Sn intermetallic compounds (IMC) layer (Cu₆Sn₅ + Cu₃Sn) of Sn37Pb/Cu solder joints subjected to isothermal aging exhibited a linear function of the square root of aging time, indicating that the formation of Cu–Sn IMC was mainly controlled by the diffusion mechanism. And the diffusion coefficient (D) values of IMC layer were 1.07 × 10⁻¹⁷ and 3.72 × 10⁻¹⁷ m²/s for aged solder joints at 120 °C and 170 °C, respectively. Shear tests results revealed that as-reflowed solder joint had better shear strength than the aged solder joints and the shear strength of all aged solder joints decreased with increasing aging time. The presence of elongated dimple-like structures on the fracture surfaces of these as-reflowed or aged for short time solder joints were indicative of a ductile failure mode. As aging time further increased, the solder joints fractured in the mixed solder/IMC mode at the solder/IMC interface.     

High temperature reliability and interfacial reaction of eutectic Sn–0.7Cu/Ni solder joints during isothermal aging

The interfacial reactions and growth kinetics of intermetallic compound (IMC) layers formed between Sn–0.7Cu (wt.%) solder and Au/Ni/Cu substrate were investigated at aging temperatures of 185 and 200 °C for aging times of up to 60 days. After reflow, the IMC formed at the interface was (Cu, Ni)₆Sn₅. After aging at 185 °C for 3 days and at 200 °C for 1 day, two IMCs of (Cu, Ni)₆Sn₅ and (Ni, Cu)₃Sn₄ were observed. The growth of the (Ni, Cu)₃Sn₄ IMC consumed the (Cu, Ni)₆Sn₅ IMC at an aging temperature of 200 °C due to the restriction of supply of Cu atoms from the solder to interface. After aging at 200 °C for 60 days, the Ni layer of the substrate was completely consumed in many parts of the sample, at which point a Cu₃Sn IMC was formed. In the ball shear test, the shear strength decreased with increasing aging temperature and time. Until the aging at 185 °C for 15 days and at 200 °C for 3 days, fractures occurred in the bulk solder. After prolonged aging treatment, fractures partially occurred at the (Cu, Ni)₆Sn₅ + Au/solder interface for aging at 185 °C and at the (Ni, Cu)₃Sn₄/Ni interface for aging at 200 °C, respectively. Consequently, thick IMC layer and thermal loading history significantly affected the integrity of the Sn–0.7Cu/Ni BGA joints.     

Effect of different temperature cycling profiles on the crack initiation and propagation of Sn–3.5Ag wave soldered solder joints

Temperature cycling of a test board with different electronic components was carried out at two different temperature profiles in a single-chamber climate cabinet. The first temperature profile ranged between −55 and 100 °C and the second between 0 and 100 °C. Hole mounted components and secondary side SMD components were wave soldered with an Sn–3.5Ag alloy. Joints of both dual in line (DIL) packages and ceramic chip capacitors were investigated. Crack initiation and propagation was analysed after every 500 cycles. In total, 6500 cycles were run at both temperature profiles and the observations from each profile were compared.For both kinds of components analysed, cracks were first visible for the temperature profile ranging between −55 and 100 °C. For this temperature profile, and for DIL packages, cracks were visible already after 500 cycles, whereas for the other temperature profile, cracks initiated between 1000 and 1500 cycles. The cracks observed after 1500 cycles were visibly smaller for the temperature profile ranging between 0 and 100 °C, concluding that crack initiation and propagation was slightly slower for this temperature profile. For the chip capacitors, cracks were first visible after 2000 cycles.     

Isothermal aging effects on the microstructure and solder bump shear strength of eutectic Sn37Pb and Sn3.5Ag solders

The reliability of the eutectic Sn37Pb (63%Sn37%Pb) and Sn3.5Ag (96.5%Sn3.5%Ag) solder bumps with an under bump metallization (UBM) consisting of an electroless Ni(P) plus a thin layer of Au was evaluated following isothermal aging at 150 °C. All the solder bumps remained intact after 1500 h aging at 150 °C. Solder bump microstructure evolution and interface structure change during isothermal aging were observed and correlated with the solder bump shear strength and failure modes. Cohesive solder failure was the only failure mode for the eutectic Sn37Pb solder bump, while partial cohesive solder failure and partial Ni(P) UBM/Al metallization interfacial delamination was the main failure mode for eutectic Sn3.5Ag solder bump.     

Isothermal aging characteristics of Sn–Pb micro solder bumps

The shear strength of the under bump metallurgy (UBM) structure in both the high-melting solder bump and low-melting solder bump after aging were evaluated. Scanning electron microscopy and transmission electron microscopy were examined in the intermetallic compounds (IMCs) and bump joint profiles at the interface between solder and UBM. In 900 h aging experiments, the maximum shear strength of Sn–97wt.%Pb and Sn–37wt.%Pb decreased by 25% and 20%, respectively. The growth of Cu₆Sn₅ and Cu₃Sn was ascertained by the aging treatment. The crack path changes from the interior of a solder to the IMC interface. Compare with the Cu–Sn IMC, the amount of Ni–Sn IMC was small. The Ni layer is considered as the diffusion barrier.     

Evaluation of displacement rate effect in shear test of Sn–3Ag–0.5Cu solder bump for flip chip application

An experimental investigation was combined with a non-linear finite element analysis using an elastic–viscoplastic constitutive model to study the effect of ball shear speed on the shear forces of flip chip solder bumps. A solder composition used in this study was Sn–3mass%Ag–0.5mass%Cu. A low cost bumping process has been employed using electroless Ni and immersion Au followed by solder paste stencil printing. A thin layer of intermetallic compound, (Ni_1−xCu_x)₃Sn₄, was formed by the reaction between the solder and electroless Ni with a thickness of about 1.4 μm, while some discontinuous (Cu_1−yNi_y)₆Sn₅ particles were also formed at the interface. The compositions of the resulting compounds were identified using energy dispersive spectrometer (EDS) and electron microprobe analysis (EPMA). Shear tests were carried out over a shear speed range from 20 to 400 μm/s at a shear ram height of 20 μm. The shear force was observed to linearly increase with shear speed and reach the maximum value at the fastest shear speed in both experimental and computational results. The optimum shear speeds for the shear test of solder bumped flip chip were recommended to be not exceeding 200 μm/s. The failure mechanisms were discussed in terms of von Mises stresses and plastic strain energy density distributions.     

SnAgCu体钎料及BGA焊球焊点纳米级力学性能

为了研究低银Sn-0.3Ag-0.7Cu无铅体钎料、BGA焊料小球和BGA焊点的力学行为,基于物理反分析的方法采用纳米压痕仪对其进行实验。从压痕载荷–深度曲线提取出弹性模量、硬度和蠕变速率敏感指数。结果表明:体钎料的杨氏模量和蠕变速率敏感指数大约是BGA焊料小球和BGA焊点的2.5倍,验证了尺寸效应理论。采用纳米压痕仪测出的体钎料维氏硬度(15.101HV)小于显微硬度计的测量结果(20.660HV)。     

Phase coarsening and crack growth rate during thermo-mechanical cycling of 63Sn37Pb solder joints

The effects of phase coarsening in 63Sn37Pb solder joints produced by isothermal annealing and during thermo-mechanical cycling (TMC) on the fatigue cracked area growth rate dA_c/dN were investigated. The phase coarsening by isothermal annealing was in accord with the mechanism of interphase boundary diffusion; that during TMC was significantly greater than by isothermal annealing. The phase coarsening in the range considered (D=1–5 μm, N<150 cycles) had only little, if any, influence on dA_c/dN, which occurred mainly in Stage I of the fatigue crack growth rate regime. Results obtained previously, however, indicate that for N>150 cycles dA_c/dN is significantly decreased by a reduction in the as-reflowed phase size. Reasonable agreement occurred between the calculated fatigue lifetime vs initial as-reflowed phase size and that measured.     

Effects of TMF heating rates on damage accumulation and resultant mechanical behavior of Sn–Ag based solder joints

Solder joint reliability depends on several service parameters such as temperature extremes encountered, dwell times at these temperatures, and the ramp-rates representing the rate at which the temperature changes are imposed. TMF of Sn–Ag based solder alloy joints of realistic dimensions were carried out with dwell of 115 min and 20 min at 150 °C and −15 °C, respectively. Different heating rates were obtained by controlling the power input during heating part of TMF cycles. Surface damage and residual mechanical strength of these solder joints were characterized after 0, 250, 500, and 1000 TMF cycles to evaluate the role of TMF heating rate on the solder joint integrity.     

Influence of current density on mechanical reliability of Sn-3.5Ag BGA solder joint

In this study, we investigated the effect of the current density on the interfacial reaction and mechanical reliability of an electroless Ni/immersion Au (ENIG) substrate with Sn-3.5Ag solder. We first evaluated the interfacial reactions of the solder joint under aging for up to 800 h and current stressing with current densities of 3 × 10² A/cm² and 5 × 10³ A/cm². Also, we successfully revealed the correlation between the interfacial reaction behavior and mechanical reliability under current stressing. With increasing aging time, the thickness of the Ni₃Sn₄ layer increased. At both low and high current densities, the thickness of the Ni₃Sn₄ layer increased up to 400 h and decreased thereafter at the cathode, while that of the IMC increased up to 800 h at the anode. After the die shear test, the ductile fracture was observed in the as-reflowed joint without current stressing. The fracture mode changed from ductile fracture to brittle fracture when thermal aging and current flow were simultaneously applied. The combination of the current stressing and isothermal aging at high temperature significantly deteriorated the mechanical reliability of the solder joint.     

冷却介质对BGA焊球质量的影响

采用均匀液滴喷射法制备BGA焊球,以硅油和花生油作为冷却介质,研究了冷却介质在150,200和250℃时对Sn-3.0Ag-0.5Cu焊球球形度和表面质量的影响,对焊球的显微组织进行了分析.结果表明,硅油和花生油作为冷却介质,温度为200℃时得到的焊球球形度最好,球形度分别为92.2%和98.2%.另外,提高冷却介质温...     

Electromigration of Sn–37Pb and Sn–3Ag–1.5Cu/Sn–3Ag–0.5Cu composite flip–chip solder bumps with Ti/Ni(V)/Cu under bump metallurgy

We examine electromigration fatigue reliability and morphological patterns of Sn–37Pb and Sn–3Ag–1.5Cu/Sn–3Ag–0.5Cu composite solder bumps in a flip–chip package assembly with Ti/Ni(V)/Cu UBM. The flip–chip test vehicle was subjected to test conditions of five combinations of applied electric currents and ambient temperatures, namely, 0.4 A/150 °C, 0.5 A/150 °C, 0.6 A/125 °C, 0.6 A/135 °C, and 0.6 A/150 °C. The electrothermal coupling analysis was employed to investigate the current crowding effect and maximum temperature in the solder bump in order to correlate with the experimental electromigration reliability using the Black’s equation as a reliability model. From available electromigration reliability models, we also present a comparison between fatigue lives of Sn–37Pb solder bumps with Ti/Ni(V)/Cu and those with Al/Ni(V)/Cu UBM under different current stressing conditions.     

Effect of cooling rate on microstructure and shear strength of pure Sn,Sn-0.7Cu,Sn-3.5Ag,and Sn-37Pb solders

The microstructure and shear strength characteristics of pure Sn and the eutectic compositions of Sn-37Pb, Sn-0.7Cu, and Sn-3.5Ag prepared under identical reflow conditions but subjected to two different cooling conditions were evaluated at room temperature. For the four solders, the ultimate shear strength increased with increasing strain rate from 10⁻⁵ s⁻¹ to 10⁻¹ s⁻¹. Decreasing the cooling rate tended to decrease the ultimate shear strength for both the Sn-0.7Cu and Sn-3.5Ag solders. The effects of work hardening resulting from increased strain rate were more prevalent in quench-cooled (QC) samples.     

Effect of bismuth on the isothermal fatigue properties of Sn-3.5mass%Ag solder alloy

Sn-3.5mass%Ag eutectic solder is selected as a candidate base alloy for replacing the eutectic Sn-Pb, and the effect of bismuth (2, 5, 10mass%) on the fatigue life of bulk Sn-3.5mass%Ag eutectic at room temperature has been studied over the total strain range from 0.3 to 3 percent in tension-tension mode. Fatigue life is defined as the number of cycles at which the load decreases to a half of the initial value. The fatigue life dramatically decreases with increasing contents of bismuth and adding this element over 2% makes fatigue life shorter than that of tin-lead eutectic alloy. Tensile strength of the alloy significantly increases with an increase in bismuth contents due to solid solution hardening (<5%Bi) or dispersion strengthning of fine bismuth particles, while ductility of this system dramatically decreases with increasing bismuth contents. Fatigue life of these alloys depends on ductility obtained by tensile test. The fatigue life of Bi containing Sn-3.5%Ag alloys can be described by, (Δε_p/2D)·N _f ^0.59 =0.66 where N_f is fatigue life defined by number of cycles to one-half load reduction, Δε_p is the plastic strain range for initial cycles, D is the ductility as measured by reduction in area.     

Creep properties of eutectic Sn-3.5Ag solder joints reinforced with mechanically incorporated Ni particles

The creep deformation behavior of eutectic Sn-3.5Ag based Ni particle rein forced composite solder joints was investigated. The Ni particle reinforced composite solder was prepared by mechanically dispersing 15 vol.% of Ni particles into eutectic Sn-3.5Ag solder paste. Static-loading creep tests were carried out on solder joint specimens at 25 C, 65 C, and 105 C, representing homologous temperatures ranging from 0.6 to 0.78. A novel-design, miniature creep-testing frame was utilized in this study. Various creep parameters such as the global and localized creep strain, steady-state creep rate, onset of tertiary creep and the activation energy for creep were quantified by mapping the distorted laser ablation pattern imprinted on the solder joint prior to testing. The Ni-reinforced composite solder joint showed improved creep resistance compared to the results previously reported for eutectic Sn-3.5Ag solder, Sn-4.0Ag-0.5Cu solder alloys, and for eutectic Sn-3.5Ag solder reinforced with Cu or Ag particle reinforcements. The activation energy for creep was ∼0.52 eV for Sn-3.5Ag and Sn-4Ag-0.5Cu solder alloys. The activation energies ranged from 0.55–0.64 eV for Cu, Ag, and Ni reinforced composite solder joints, respectively. Most often, creep fracture occurred closer to one side of the solder joint within the solder matrix.     

Effect of burn-in on shear strength of 63Sn-37Pb solder joints on an Au/Ni/Cu substrate

A study was performed to examine the effect of burn-in and Au-plating thickness on the shear strength of 63Sn-37Pb solder joints in ball grid array (BGA) packages. The Au-plating thicknesses of 0.3 μm, 0.6 μm, 0.9 μm, and 1.4 μm were evaluated. An isothermal aging temperature of 150°C was employed to simulate burn-in conditions. The evolution of the bulk solder microstructure and intermetallic compounds at the solder joint interface were characterized and correlated to the measured shear strength. The strength of the solder joints with 0.3-μm Au plating was approximately three times higher than the thicker platings after aging. Solder joints with 0.3-μm Au plating failed within the solder matrix, and their strength was dependent upon the bulk solder microstructure and composition. The weakness of the solder joints with thicker Au platings was attributed to the formation of a brittle AuSn₄/Ni₃Sn₄ interface and a ductile Pb-rich layer at the interface.     

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.     

The influence of solder volume and pad area on Sn–3.8Ag–0.7Cu and Ni UBM reaction in reflow soldering and isothermal aging

This paper examines various aspects of SAC (Sn–3.8Ag–0.7Cu wt.%) solder and UBM interactions which may impact interconnection reliability as it scales down. With different solder joint sizes, the dissolution rate of UBM and IMC growth kinetics will be different. Solder bumps on 250, 80 and 40 μm diameter UBM pads were investigated. The effect of solder volume/pad metallization area (V/A) ratio on IMC growth and Ni dissolution was investigated during reflow soldering and solid state isothermal aging. Higher V/A ratio produced thinner and more fragmented IMC morphology in SAC solder/Ni UBM reflow soldering interfacial reaction. Lower V/A ratio produced better defined IMC layer at the Ni UBM interface. When the ratio of V/A is constant, the IMC morphology and growth trend was found to be similar. After 250 h of isothermal aging, the IMC growth rate of the different bump sizes leveled off. No degradation in shear strength was observed in these solder bump after 500 h of isothermal aging.     

Effect of aging on mechanical properties of high temperature Pb-rich solder joints

The effect of aging on the evolution of interfacial microstructure and mechanical properties of Pb-rich PbSnAg solder joints with different Sn content was investigated. Tensile samples were prepared by soldering two pieces of Ni or Cu strips resulting in joints with gap sizes of about 250 μm. Multi-layered structures composed of Ni coated Si-chips soldered onto Ni/Cu metallized ceramic substrates were used for fatigue testing. All samples were subjected to thermal aging at 250 °C up to 500 h. Microstructural investigations revealed that independent from the substrate material, increased Sn content of the solder alloy results in improvement of the tensile and fatigue properties of the joints and a higher growth rate of the interfacial intermetallic compound (IMC) layers. It was found that the thickness of the wettable substrate especially in the case of low-Sn alloys, also affects the interfacial properties of high temperature PbSnAg solder joints. In this case, a reduced Sn content in solder joints results in weakening of the interface during the reflow process and subsequent thermal aging. The dominant failure mode of the solder joints subjected to cyclic loading was delamination of the interfacial IMC layer from the substrate. Finite element simulations were conducted by using a strain rate and pressure dependent material model for PbSnAg solder in order to analyse the states of stress and strain during static and cyclic loading.     

Effect of voids on the reliability of BGA/CSP solder joints

Voids in solder joints have been considered as a defect in electronics assembly. The factors that affect void formation are complex and involve the interaction of many factors. There are no established standards for void size and void area in a solder joint for it to be deemed defective. Inspection criteria have been very subjective. The effect of voids on the reliability of solder joint may depend not only on the size, but also on frequency and location. This study is focussed on investigating the effect of voids on the reliability of solder joints. The size, location and frequency effects on the reliability were studied. Testing was done by mechanical deflection testing (torsion) system and air to air thermal cycling (−40 °C/125 °C). Failures were analyzed for the failure modes by cross sectional analysis. The results indicate that voids reduce the life of the solder joint. Voids which are greater than 50% of the solder joint area, decrease the mechanical robustness of the solder joints. Small voids also have an effect on the reliability, but it is dependent on the void frequency and location.