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 prestrain,rate of prestrain,and temperature on the stress-relaxation behavior of eutectic Sn-3.5Ag solder joints

Stress-relaxation studies on eutectic Sn-Ag solder (Sn-3.5Ag in wt.%) joints were carried out at various temperatures after imposing different amounts and rates of simple shear strain. Stress-relaxation parameters were evaluated by subjecting geometrically realistic solder joints with a nominal joint thickness of ∼100 μm and a 1 mm × 1 mm solder-joint area. The peak shear stress during preloading and residual shear stress resulting from stress relaxation were higher at the low-temperature extremes than those at high-temperature extremes. Also, those values increased with increasing simple shear strain and the rate of simple shear strain imposed prior to the stress-relaxation events. The relaxation stress is insensitive to simple shear strain at 150°C, but at lower temperatures, a faster rate of simple shear strain causes a higher relaxed-stress value. The resulting deformation structures observed from the solder-joint side surfaces were also strongly affected by these parameters. At high temperature, grain-boundary sliding effects were commonly observed. At low temperature, intense shear bands dominated, and no grain-boundary sliding effects were observed.     

Creep deformation behavior in eutectic Sn-Ag solder joints using a novel mapping technique

Creep deformation behavior was measured for 60–100 μm thick solder joints. The solder joints investigated consisted of: (a) non-composite solder joints made with eutectic Sn-Ag solder, and (b) composite solder joints with eutectic Sn-Ag solder containing 20 vol. %, 5 μm diameter in-situ Cu₆Sn₅ intermetallic reinforcements. All creep testing in this study was carried out at room temperature. Qualitative and quantitative assessment of creep deformation was characterized on the solder joints. Creep deformation was analyzed using a novel mapping technique where a geometrical-regular line pattern was etched over the entire solder joint using excimer laser ablation. During creep, the laser-ablation (LA) pattern becomes distorted due to deformation in the solder joint. By imaging the distortion of laser-ablation patterns using the SEM, actual deformation mapping for the entire solder joint is revealed. The technique involves sequential optical/digital imaging of the deformation versus time history during creep. By tracing and recording the deformation of the LA patterns on the solder over intervals of time, local creep data are obtained in many locations in the joint. This analysis enables global and localized creep shear strains and strain rate to be determined.     

Development of nano-composite lead-free electronic solders

Inert, hybrid inorganic/organic, nano-structured chemicals can be incorporated into low melting metallic materials, such as lead-free electronic solders, to achieve desired levels of service performance. The nano-structured materials technology of polyhedral oligomeric silsesquioxanes (POSS), with appropriate organic groups, can produce suitable means to promote bonding between nano-reinforcements and the metallic matrix. The microstructures of lead-free solder reinforced with surface-active POSS tri-silanols were evaluated using scanning electron microscopy (SEM). Wettability of POSS-containing lead-free solders to copper substrate was also examined. Steady-state deformation of solder joints made of eutectic Sn-Ag solder containing varying weight fractions of POSS of different chemical moieties were evaluated at different temperatures (25°C, 100°C, and 150°C) using a rheometric solids analyzer (RSA-III). Mechanical properties such as shear stress versus simple shear-strain relationships, peak shear stress as a function of rate of simple shear strain, and testing temperature for such nano-composite solders are reported. The service reliability of joints made with these newly formulated nano-composite solders was evaluated using a realistic thermomechanical fatigue (TMF) test profile. Evolution of microstructures and residual mechanical property after different extents of TMF cycles were evaluated and compared with joints made of standard, unreinforced eutectic Sn-Ag solder.     

Residual-mechanical behavior of thermomechanically fatigued Sn-Ag based solder joints

Thermomechanical fatigue (TMF) due to the mismatch in coefficients of thermal expansion between solder and substrate gradually degrades the mechanical properties of electronic solder joints during service. This study investigated the role of TMF on the residual-mechanical behavior of solder joints made with eutectic Sn-Ag solder and Sn-Ag solder with Cu or Ni additions. The TMF tests were carried out between −15°C and +150°C with a ramp rate of 25°C/min for the heating segment and 7°C/min for the cooling segment. The hold times were 20 min at the high extreme and 300 min at the low extreme. Residual shear strength was found to drop significantly during the first 250 TMF cycles, although it did remain relatively constant between 250 and 1000 cycles. Alloying elements were found to affect the residual creep strength of solder joints after TMF.     

Reliability of composite solder bumps produced by an in-situ process

In an attempt to develop a thermally stable solder system, an in-situ Pb-Sn solder composite reinforced with Cu₆Sn₅ dispersoids was investigated for its thermal stability. The stability was evaluated mainly by measuring the growth rate of intermetallics at in-situ composite solder/BLM interface as a function of the number of reflow soldering cycles and aging time. The rates were compared with those of the eutectic Pb-Sn and Sn-Ag solders. After the thermal treatments, the solder joints were tested for their shear strengths. The results indicated that the in-situ composite solder has a higher shear strength and better thermal stability than the eutectic Pb-Sn solder. Jointly appointed by CAAM at POSTECH     

Partially-constrained thermomechanical fatigue of eutectic tin-bismuth/copper solder joints

Small bimetallic load-frames with reference assembly stiffness, k′, and fully-constrained shear strain, γ_fc, were used to simulate the thermo-mechanical conditions experienced by eutectic Bi-42wt.%Sn-to-Cu solder joints. Shear stress and strain were induced in the solder joint by a 45-minute, 0 to 100°C temperature cycle and were calculated from the assembly temperature, joint configuration, and measured elastic strain in the load-frame. Early in cycling, a hysteresis loop representing the maximum stress range and minimum strain range was reached. As damage accumulated in the solder, the stress range decreased and the strain range increased. The TMF life of the joints, defined by the load range drop, Φ, as a function of k′ and γ_fc, can be determined, defining an effective plastic strain range which allows data for various stiffnesses and thermal expansion mismatches to be summarized on a single Coffin-Mansion plot. The effective plastic strain range also provides an important link to conventional low cycle fatigue (LCF) data taken from an infinitely stiff load-frame.     

Scanning electron microscope in-situ investigation of fracture behavior in 96.5Sn3.5Ag lead-free solder

In-situ tensile tests of as-cast 96.5Sn3.5Ag eutectic solder were performed under the scanning electron microscope (SEM) using different strain rates at room temperature, and various crack initiation and propagation behavior was observed on the specimen surface. It was found that, due to the existence of Ag₃Sn intermetallic particles and the special microstructure of β-Sn phases in Sn3.5Ag solder, grain boundary sliding (GBS) was no longer the dominant mechanism for this Pb-free solder. In the lower strain rate regime, accompanied by partial intragranular cracks, intergranular fracture along the grain boundaries in Sn-Ag eutectic structure or along the interphase boundaries between Sn-rich dendrites and Sn-Ag eutectic phases occurred primarily for the Sn3.5Ag solder in the early tensile stage. However, significant plastic deformation was observed in large areas for the specimens tested at higher strain rates, and cracks propagated in a transgranular manner across the Sn dendrites and Sn-Ag eutectic structure.     

Mechanical properties of intermetallic compounds associated with Pb-free solder joints using nanoindentation

Mechanical properties of intermetallic compound (IMC) phases in Pb-free solder joints were obtained using nanoindentation testing (NIT). The elastic modulus and hardness were determined for IMC phases associated with insitu FeSn particle reinforced and mechanically added, Cu particle-reinforced, composite solder joints. The IMC layers that formed around Cu particle reinforcement and at the Cu substrate/solder matrix interface were probed with NIT. Moduli and hardness values obtained by NIT revealed were noticeably higher for Cu-rich Cu₃Sn than those of Cu₆Sn₅. The Ag₃Sn platelets that formed during reflow were also examined for eutectic Sn-Ag solder column joints. The indentation modulus of Ag₃Sn platelets was significantly lower than that of FeSn, SnCuNi, and CuSn IMCs. Indentation creep properties were assessed in localized microstructure regions of the as-cast, eutectic Sn-Ag solder. The stress exponent, n, associated with secondary creep differed widely depending on the microstructure feature probed by the indenter tip.     

Effect of cooling rate on microstructure and mechanical properties of eutectic Sn-Ag solder joints with and without intentionally incorporated Cu6Sn5 reinforcements

Solidification of eutectic Sn-Ag solder, with and without Cu₆Sn₅ composite reinforcements, on copper substrates, was investigated at two different cooling rates. The size, orientation, randomness, and overall morphology of the dendritic microstructure were examined as a function of cooling rate. Cu₆Sn₅ particle reinforcements were found to act as nucleation sites for dendrites, in addition to sites on the substrate/solder interface. The mechanical properties of these solders were also examined as a function of cooling rate. Solder joints with a lower load-carrying area were found to exhibit higher shear strength, but reduced ductility when compared to solder joints with more load carrying area.     

Superplastic creep of eutectic tinlead solder joints

This paper presents experimental evidence that as-solidified eutectic Pb-Sn solder joints can exhibit superplastic behavior in shear creep loading. Stepped load creep tests of as-solidified joints show a change in the stress exponent from a high value typical of con-ventional creep at high stress and strain rate to a superplastic value near 2 at lower stress and strain rates. In addition, the change in stress exponent is accompanied by a change in the activation energy for creep from a value near that for bulk self-diffusion (20 kcal/mol) to a value near that for grain boundary diffusion (12 kcal/mol). The total shear deformation of joints in stress-rupture tests performed at 65° C are found to ex-ceed 150%. The concomitant observation that quenched solder joints creep faster than air-cooled ones is attributed to a grain, or phase, size dependence of the strain rate. The source of superplastic behavior is a fine, equiaxed microstructure. It is not yet clear whether the superplastic microstructure is present in the as-solidified joint, or develops during the early stages of plastic deformation.     

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.     

Stress relaxation behavior of eutectic tin-lead solder

The creep behavior of eutectic tin-lead solder was investigated using stress relaxation techniques. Stress relaxation experiments were performed on cast tensile specimens of commercial eutectic tin-lead solder, SN63. The sample casting conditions were controlled to produce microstructures similar to those found in typical solder joints on electronic assemblies. The stress relaxation data was analyzed to extract constitutive relations for creep. The strain rate during relaxation was found to follow two power law expressions, one with n = 3.2 at low stress levels and the other with n = 6.2 at higher stress levels. The apparent activation energy for creep and the power law exponent are discussed with relation to the published data for this alloy.     

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.     

Modeling thermomechanical fatigue behavior of Sn-Ag solder joints

Stresses that develop because of the coefficient of thermal expansion (CTE) mismatch between solder and substrate/components contribute to thermomechanical fatigue (TMF) of the solder joints. However, the relative importance of several processes that contribute to damage accumulation and its role in affecting the reliability of the solder joints are far from being understood. Aging, creep/stress relaxation, and stress/strain reversals are some of the important processes. These processes are affected by service conditions, such as the temperature extremes experienced, rates of heating and cooling, dwell times at the extreme temperatures, and so on. In addition, the elastic and plastic anisotropy of tin could also contribute to the damage accumulation during TMF of Sn-based solders. This preliminary effort to model TMF in Sn-Ag solder joints will consider the role of each of these parameters, with significant emphasis on the anisotropic-elastic behavior of Sn grains.     

Effect of dwell times on thermomechanical fatigue behavior of Sn-Ag-based solder joints

Thermomechanical fatigue (TMF) caused by the mismatch in the coefficient of thermal expansion (CTE) between solder and substrate gradually degrades the mechanical properties of solder joints during service. Solder joints fabricated with eutectic Sn-Ag and Sn-Ag solder with Cu or Ni were subjected to TMF between −15°C and +150°C with dwell times of 115 min at high-temperature extreme and 20 min at low-temperature extreme. Characterization of surface damage and residual-mechanical strength of these solder joints were carried out after 0, 250, 500, and 1,000 TMF cycles. Results obtained from this study were compared with those obtained with longer dwell time at lower temperature extreme. The solder joints that experienced longer dwell times at high-temperature extreme exhibited less surface-damage accumulation and less decrease in simple-shear strength as compared to those that experienced longer dwell times at low-temperature extreme. Quaternary alloys containing small amounts of Cu and Ni exhibit better TMF performance than binary and ternary alloys under TMF cycling with longer dwell times at high-temperature extreme.     

Eutectic Sn-Ag solder bump process for ULSI flip chip technology

A novel eutectic Pb-free solder bump process, which provides several advantages over conventional solder bump process schemes, has been developed. A thick plating mask can be fabricated for steep wall bumps using a nega-type resist with a thickness of more than 50 μm by single-step spin coating. This improves productivity for mass production. The two-step electroplating is performed using two separate plating reactors for Ag and Sn. The Sn layer is electroplated on the Ag layer. Eutectic Sn-Ag alloy bumps can be easily obtained by annealing the Ag/Sn metal stack. This electroplating process does not need strict control of the Ag to Sn content ratio in alloy plating solutions. The uniformity of the reflowed bump height within a 6-in wafer was less than 10%. The Ag composition range within a 6-in wafer was less than ±0.3 wt.% Ag at the eutectic Sn-Ag alloy, analyzed by ICP spectrometry. SEM observations of the Cu/barrier layer/Sn-Ag solder interface and shear strength measurements of the solder bumps were performed after 5 times reflow at 260°C in N₂ ambient. For the Ti(100 nm)/Ni(300 nm)/Pd(50 nm) barrier layer, the shear strength decreased to 70% due to the formation of Sn-Cu intermetallic compounds. Thicker Ti in the barrier metal stack improved the shear strength. The thermal stability of the Cu/barrier layer/Sn-Ag solder metal stack was examined using Auger electron spectrometry analysis. After annealing at 150°C for 1000 h in N₂ ambient, Sn did not diffuse into the Cu layer for Ti(500 nm)/Ni(300 nm)/Pd(50 nm) and Nb(360 nm)/Ti(100 nm)/Ni(300 nm)/Pd(50 nm) barrier metal stacks. These results suggest that the Ti/Ni/Pd barrier metal stack available to Sn-Pb solder bumps and Au bumps on Al pads is viable for Sn-Ag solder bumps on Cu pads in upcoming ULSIs     

Residual shear strength of Sn-Ag and Sn-Bi lead-free SMT joints after thermal shock

In this study, two lead-free solder alloys, namely 50 tin-50 bismuth (Sn-Bi) and 96.5 tin-3.5 silver (Sn-Ag), were studied for their use in surface mount solder joints. They have been considered as potential replacements for 63 tin-37 lead (Sn-Pb) solder. All joints were subjected to various cycles of thermal shock with temperature ranging from -25 to 125/spl deg/C. Shear tests were conducted on joints with and without thermal shock treatment. Another thermal shock cycle (-25 to 85/spl deg/C) was carried out on Sn-Bi solder joints for comparison. Their performance against thermal shock was compared with eutectic Sn-Pb solder by evaluating their residual shear strength and studying their microstructural change. For the Sn-Ag solder, a fine rod-like Ag/sub 3/Sn intermetallic was formed in the solder matrix after the thermal shock. On the other hand, Bi-rich and Sn-rich phases appeared in the Sn-Bi solder after the -25 to 125/spl deg/C thermal shock. Moreover, fine cracks were observed along the Bi-rich grain-like phase boundary. These were not observed in the Sn-Bi solder with the -25 to 85/spl deg/C thermal shock treatment. Voids and cracks were also observed in the joint of Sn-Bi solder alloy after 1000 thermal shock cycles. In addition, the thickness of intermetallic compound (IMC) of three solder alloys gradually grew with the number of thermal shock cycles. These defects reduced the strength of solder joint and led to thermal fatigue failure. In general, the shear strength is found to decrease with increasing number of thermal shock cycles. The Sn-Ag solder was better than the Sn-Bi solder in terms of residual thermal shock shear strength. Sn-Bi solder showed good properties when it was treated with the -25 to 85/spl deg/C thermal shock. It has a strong potential to replace Sn-Pb solder in low temperature applications such as consumer electronics. The Sn-Ag solder is suitable for high temperature applications.     

Analysis of Slip Behavior in a Single Shear Lap Lead-Free Solder Joint During Simple Shear at 25°C and 0.1/s

The relative activity of the potential slip systems in Sn is examined by comparing an experiment of a single shear lap deformation with simulations using the viscoplastic self-consistent crystal plasticity model developed by Lebensohn and Tomé. In a single shear lap specimen made using eutectic Sn-Ag solder on copper pull tabs, the initially polished side was characterized using orientation mapping before and after 0.8 shear deformation at 25°C at a shear strain rate of 0.1/s. The critical resolved shear stress of potential slip systems and the rate sensitivity was altered by trial and error until good agreement between experimentally observed and computed texture was obtained. This result indicated that slip on {101) and {211) planes is much more difficult to activate than on other slip systems for the grain orientations present in this sample. This particular sample showed much activity on the {010)〈101] slip system, but the activation of this slip system may be related to the initial dominant orientation in the specimen. This result is compared with literature and related experiments on ball grid arrays in a companion paper in this volume that show similar trends. As lead-free solder joints are commonly single crystals or multicrystals, this particular result is not indicative of lead-free solder joints as a whole, but must be interpreted in the context of a larger data set.     

Microstructure evolution of eutectic Sn-Ag solder joints

Laser and infrared reflow soldering methods were used to make Sn-Ag eutectic solder joints for surface-mount components on printed wiring boards. The microstructures of the joints were evaluated and related to process parameters. Aging tests were conducted on these joints for times up to 300 days and at temperature up to 190°C. The evolution of microstructure during aging was examined. The results showed that Sn-Ag solder microstructure is unstable at high temperature, and microstructural evolution can cause solder joint failure. Cu-Sn intermetallics in the solder and at copper-solder interfaces played an important role in both the microstructure evolution and failure of solder joints. Void and crack formation in the aged joints was also observed.