Thermomechanical fatigue behavior of Sn-Ag solder joints

Microstructural studies of thermomechanically fatigued actual electronic components consisting of metallized alumina substrate and tinned copper lead, soldered with Sn-Ag or 95.5Ag/4Ag/0.5Cu solder were carried out with an optical microscope and environmental scanning electron microscope (ESEM). Damage characterization was made on samples that underwent 250 and 1000 thermal shock cycles between −40°C and 125°C, with a 20 min hold time at each extreme. Surface roughening and grain boundary cracking were evident even in samples thermally cycled for 250 times. The cracks were found to originate on the free surface of the solder joint. With increased thermal cycles these cracks grew by grain boundary decohesion. The crack that will affect the integrity of the solder joint was found to originate from the free surface of the solder very near the alumina substrate and progress towards and continue along the solder region adjacent to the Ag₃Sn intermetallic layer formed with the metallized alumina substrate. Re-examination of these thermally fatigued samples that were stored at room temperature after ten months revealed the effects of significant residual stress due to such thermal cycles. Such observations include enhanced surface relief effects delineating the grain boundaries and crack growth in regions inside the joint.     

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.     

Microstructural characterization of damage in thermomechanically fatigued Sn-Ag based solder joints

Sn-Ag based solder joints of 100-μm thickness were thermomechanically fatigued 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 high temperature extreme and 300 min at the low temperature extreme. Surface damage accumulation predominantly consisted of shear banding, surface relief due to Sn-grain extrusion, grain boundary sliding, and grain decohesion usually near the solder/substrate interface. Small alloy additions were found to affect the extent of this surface damage accumulation.     

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.     

Damage accumulation under repeated reverse stressing of Sn-Ag solder joints

To better understand the effect of repeated reverse stress in solder joints, a new testing method was developed. Tin-silver solder joints were fabricated, constrained between Cu blocks, and then subjected to repeated shear loading in a tensile tester. Constant strain amplitudes were applied to simulate service conditions. However, large loads were used to accelerate the damage accumulation. Microstructural features of the damage were very similar to those found with studies on thermomechanical fatigue (TMF) of small, single shear lap samples. Concentrated-shear banding or striations were observed to form along Sn dendrites. The load behavior of the solder with each cycle and during hold times at the extreme strain amplitude was consistent with damage accumulating with each successive cycle. Effects of strain amplitude, hold times at the stress extremes, number of cycles, and solder-joint thickness were found to play significant roles on the stress-strain behavior and surface damage.     

Influence of microstructure on fatigue crack growth behavior of Sn-Ag solder interfaces

The relationship between microstructure and fatigue crack growth behavior was examined at Sn-Ag solder interfaces on copper and electroless-nickel metallizations. On copper metallization, the solder interface was lined with a coarse Ag₃Sn intermetallic phase in addition to the Cu₆Sn₅ intermetallic and the adjacent solder alloy contained nodular Ag₃Sn phase. This interfacial microstructure was shown to result in inferior fatigue resistance, with the fatigue crack path following the interfacial Ag₃Sn intermetallic phase. In contrast, the solder interface on the electroless-nickel metallization was covered with a thin layer of Ni₃Sn₄ intermetallic phase, and the solder microstructure was composed of fine needles of Ag₃Sn phase dispersed in the Sn-rich matrix. This solder interface was found to be significantly more resistant to fatigue than the copper/Sn-Ag solder interface.     

Stress relaxation behavior of composite and eutectic Sn-Ag solder joints

Stress relaxation experiments were carried out at 25 C and 150 C on 96.5Sn-3.5Ag eutectic solder and Sn-Ag composite solder joints (Sn-Ag eutectic solder with 20 vol.% Cu₆Sn₅ reinforcements incorporated by in-situ methods). The magnitude of the stress drop during relaxation depends primarily upon the plastic shear strain imposed prior to the stress relaxation process. For sequential stress relaxation experiments that include unloading, the stress drop is nearly independent of the accumulated plastic shear strain. However, for sequential stress relaxation that does not include unloading, the stress relaxation is more dependent upon the cumulative plastic shear strain history. The stress in single shear lap joints does not relax to zero stress, as is observed in stress relaxation of bulk tension specimens, even at 150 C. Creep strain rates extracted from the relaxation data were much lower with smaller pre-strains in both eutectic Sn-Ag and composite solder joints. The stress exponent values (n) calculated from the stress relaxation test data ranged from 7 to 15 for both eutectic and composite solder joints, which were consistent with conventional creep data. These stress-relaxation behaviors can be explained on the basis of dislocation recovery processes that occur during relaxation and when specimens are unloaded.     

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.     

低银无铅微互连焊点的振动疲劳行为研究

通过采用一系列与集成电路BGA(球栅阵列)、Flip Chip(倒装焊芯片)真实焊点体积接近的不同尺寸的典型“三明治”结构Sn0.3Ag0.7Cu低银无铅微互连焊点,基于动态力学分析的精密振动疲劳试验与微焊点疲劳断口形貌观察相结合的方法,研究了微焊点振动疲劳变形曲线的形成机制、裂纹萌生扩展与断裂机理、温度对振动疲劳行为的影响及微焊点振动疲劳行为的尺寸效应问题。结果表明,保持焊点直径恒定,随着焊点高度的减小,焊点的疲劳寿命增加,而疲劳断裂应变降低,同时焊点的疲劳断裂模式由韧性断裂转变为脆性断裂。     

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.     

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.     

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.     

Copper substrate dissolution in eutectic Sn-Ag solder and its effect on microstructure

The dissolution of Cu into molten Sn-3.8at.%Ag (Sn-3.5wt.%Ag) solder and its effect on microstructure were studied by light microscopy, scanning microscopy, and x-ray microanalysis. X-ray microanalysis of the average Cu content of samples soldered under various conditions showed that the amount of Cu dissolved during soldering increased with increasing soldering temperature and time and that the rate of dissolution could be described by a Nernst-Brunner equation. Microstructurally it was found that the volume fractions of primary β(Sn) dendrites and η-phase dendrites increase with increasing soldering temperature and time. The microstructural changes can be explained using Sn-Ag-Cu phase equilibrium data. A numerical method was developed for calculating the amount of Cu dissolved under non-isothermal conditions, which describes dissolution reasonably well.     

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.     

电迁移作用下的微焊点振动疲劳行为研究

研究了不同电迁移时间(0～96 h)和电流密度(0～1.52×104A/cm2)对Sn-3.0Ag-0.5Cu微焊点振动疲劳行为的影响.结果显示,在电迁移时间和电流密度均为0时,微焊点的振动疲劳循环次数大于1 170次,疲劳寿命大干234 min;而在125℃服役温度下,当振动频率为0.8 Hz,交变应力为0～20 M...     

Analysis of thermomechanical interactions in a miniature solder system under cyclic fatigue loading

This paper discusses the possible thermomechanical interaction (coupling) phenomena of a miniature solder system in electronic packaging application similar to those which have been identified for some metallic material systems in aerospace and nuclear structures under cyclic fatigue loads at different frequencies. The main objective is to investigate the heat generated by the viscoplastic deformations, and vice versa, especially on the thermal transient and the gradient induced viscoplastic ratchetting response of cyclic creep. A literature review was conducted to focus on the temperature-dependent, strain rate-sensitive stress-strain response of the eutectic or near-eutectic lead-tin (Pb37-Sn63 or Pb40-Sn60) solder alloys. The result was used to develop and apply a simple overstress constitutive theory for modeling the coupled, isotropic thermoviscoplasticity of the eutectic lead-tin solder alloy. A fully coupled heat transfer and mechanical finite element model is used to simulate possible thermal-mechanical interactions of temperature rise and viscoplastic ratchetting of the miniature solder systems in a C4/BGA chip scale package (CSP) under cyclic fatigue loads at different frequencies. The results of analysis are discussed to compare between a coupled thermomechanical model and that of a pure mechanical model.     

Observations of microstructural coarsening in micro flip-chip solder joints

Coarsening of solder microstructures dramatically affects fatigue lifetimes. This paper presents a study of microstructural evolution due to thermal cycling and aging of small solder joints. The lead-tin solder joints in this study have a height of 55 5 m and a tin content of 65–70 wt.%, with a degenerate eutectic microstructure. The joint microstructure coarsens more rapidly during aging at 160°C than cycling from 0–160°C. No coarsened bands are observed. The cycling data scales with standard coarsening equations, while the aging data fits to an enhanced trend. The joints experiencing 2.8% strain during cycling fail by 1000 cycles.     

Low-cycle fatigue characteristics of Sn-based solder joints

Low-cycle, lap-shear fatigue behavior of Sn-based, Pb-free solder alloys, Sn-3.5Ag, Sn-3.5Ag-Cu, Sn-3.5Ag-Bi, and Sn-0.7Cu, were studied at room temperature using specimens with printed circuit board (PCB)/solder/PCB structure under total displacement of ±10 μm, 12 μm, 15 μm, and 20 μm. The fatigue lives of various solder joint materials, defined as 50% load drop, were correlated with the fracture paths and analyzed using the Coffin-Manson relation, Morrow’s plastic-energy dissipation model, and Solomon’s load-drop parameter. The Sn-3.5Ag, Sn-0.7Cu eutectics, and Sn-3.5Ag-Cu ternary alloys showed the same level of fatigue resistance, while Bi-containing alloys showed substantially worse fatigue properties. Cross-sectional fractography revealed cracks initiated at the solder wedge near the solder mask and subsequently propagated into the solder matrix in the former group of alloys, in contrast with the crack propagation along the solder/under bump metallurgy (UBM) interfaces in the Sn-3.5Ag-Bi alloys. Inferior fatigue resistance of Bi-containing alloys was ascribed to high matrix hardness, high stiffness, possible Bi segregation to the interface, and high residual stress in the interfacial area.     

Parameters affecting thermal fatigue behavior of 60Sn-40Pb solder joints

Solder joints in electronic packages experience cyclical thermally induced strain when temperature fluctuations are encountered in service. This study investigates three parameters that affect the microstructure and therefore the thermal fatigue behavior of 60Sn-40Pb solder joints. These parameters are: 1) the effect of a tensile component in thermal fatigue, 2) solder joint thickness variations, and 3) hold time variations at the elevated temperature portion of the thermal cycle. Solder joints were thermally fatigued in a tension/compression deformation mode. Cracks developed both in the interfacial intermetallic layer (early in thermal fatigue) and in the coarsened regions of the microstructure of the solder joint (after many more cycles). The effect of joint thickness on solder joints thermally fatigued in shear was also explored. Solder joint thickness was found not to significantly affect fatigue lifetimes. The effect of an increase in the hold time at the elevated temperature portion of the thermal fatigue cycle was also investigated. It was found that time spent at the high temperature end of the fatigue cycle does not determine solder joint lifetime, rather it is the combination of the amount of deformation induced during thermal fatigue in concert with the elevated temperature.     

Forming solder joints by sintering eutectic tin-lead solder paste

It is possible to form solder joints with mechanical integrity, but not mechanical strength comparable to that achieved by melting the solder, by sintering eutectic tin-lead solder paste where small amounts of eutectic Sn-Bi powder are added to the paste. This increases the rate of sintering through liquid-phase sintering.