The Effects of Aging of the Cyclic Stress-Strain and Fatigue Behaviors of Lead Free Solders

2013 ◽  
Author(s):  
Muhannad Mustafa ◽  
Jordan C. Roberts ◽  
Jeffrey C. Suhling ◽  
Pradeep Lall
Keyword(s):  
Cyclic Loading ◽  
Damage Accumulation ◽  
Solder Joints ◽  
Peak Load ◽  
Cyclic Stress ◽  
Lead Free ◽  
Failure Behavior ◽  
Stress Strain ◽  
Effects Of Aging ◽  
Lead Free Solders

Solder joints in electronic assemblies are typically subjected to thermal cycling, either in actual application or in accelerated life testing used for qualification. Mismatches in the thermal expansion coefficients of the assembly materials cause the solder joints to be subjected to cyclic (positive and negative) mechanical strains and stresses. This cyclic loading leads to thermomechanical fatigue damage that involves damage accumulation, crack initiation, crack propagation, and failure. In addition, the microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. While the effects of aging on solder constitutive behavior (stress-strain and creep) have been examined in some detail, there have been no prior studies on the effects of aging on solder failure and fatigue behavior. Aging leads to both grain and phase coarsening, and can cause recrystallization at Sn grain boundaries. Such changes are closely tied to the damage that occurs during cyclic mechanical loading. In this investigation, we have examined the effects of aging on the cyclic stress-strain behavior and fatigue life of lead free solders. Uniaxial solder test specimens (SAC105 and SAC305) have been prepared and subjected to cyclic stress/strain loading at different aging conditions. A four-parameter hyperbolic tangent empirical model has been used to fit the entire cyclic stress-strain curve and the hysteresis loop size (area) was calculated using definite integration for a given strain limit. This area represents the energy dissipated per cycle, which is correlated to the damage accumulation in the joint. Using the recorded cyclic stress-strain curves, the evolution of the solder hysteresis loops with aging have been characterized and empirically modeled. Similar to solder stress-strain and creep behavior, there is a strong effect of aging on the hysteresis loop size (and thus the rate of damage accumulation) in the solder specimens. Fatigue experiments were also performed, where the uniaxial specimens were subjected to cyclic loading over a particular strain range until failure. Fatigue failure in the experiments was defined to occur when there was a 50% peak load drop during mechanical cycling. Prior to testing, the specimens were aged (preconditioned) at 125 °C for various aging times, and then the samples were subjected to cyclic loading at room temperature (25 °C). It was found that aging decreased the mechanical fatigue life, and the effects of aging on the peak load drop have been studied. It has also been observed that degradations in the fatigue/failure behavior of the lead free solders with aging are highly accelerated for lower silver content alloys (e.g., SAC105). Various empirical failure criteria such as the Coffin-Manson model and the Morrow model have been used to fit the measured data, and the parameters in the models have been determined as a function of the aging conditions.

Characterization of Hysteresis Loop Evolution in Aged Lead Free Solders

2011 ◽  
Author(s):  
Muhannad Mustafa ◽  
Zijie Cai ◽  
Jeffrey C. Suhling ◽  
Pradeep Lall
Keyword(s):  
Cyclic Loading ◽  
Hysteresis Loop ◽  
Solder Alloy ◽  
Damage Accumulation ◽  
Cyclic Stress ◽  
Lead Free ◽  
Stress Strain ◽  
Effects Of Aging ◽  
Lead Free Solders ◽  
Control Constant

Solder joints in electronic assemblies are typically subjected to thermal cycling, either in actual application or in accelerated life testing used for qualification. Mismatches in the thermal expansion coefficients of the assembly materials leads to the solder joints being subjected to cyclic (positive/negative) mechanical strains and stresses. This cyclic loading leads to thermomechanical fatigue damage that involves damage accumulation, crack initiation, crack propagation, and failure. While the effects of aging on solder constitutive behavior (stress-strain and creep) have been examined in some detail, there have been no prior studies on the effects of aging on solder failure and fatigue behavior. In this investigation, we have examined the effects of aging on the cyclic stress-strain behavior of lead free solders. Uniaxial SAC lead free solder specimens were subjected to cyclic (tension/compression) mechanical loading. Samples were cyclically loaded under both strain control (constant positive and negative strain limits) and stress control (constant positive and negative stress limits). The hysteresis loop size (area) was calculated from the measured cyclic stress-strain curves for a given solder alloy and temperature. This area represents the strain energy density dissipated per cycle, which can be typically correlated to the damage accumulation in the joint. The tests in this investigation were performed with SAC105 solder alloy. Prior to cyclic loading, the specimens in this study were aged (preconditioned) at 125 °C for various aging times (0–6 months). From the recorded cyclic stress-strain curves, we have been able to characterize and empirically model the evolution of the solder hysteresis loops with aging. Similar to solder stress-strain and creep behaviors, there is a strong effect of aging on the hysteresis loop size (and thus the rate of damage accumulation) in the solder specimens. The observed degradations in the fatigue/cyclic behavior of the lead free solders are highly accelerated for lower silver content alloys (e.g., SAC105), and for aging and testing at higher temperatures. In our current work, we are also subjecting aged solder samples to cyclic loading until failure occurs. Our ultimate goal is to understand the effects of aging on the thermomechanical fatigue life.

Effects of Test Temperature and Prior Aging on the Cyclic Stress-Strain Behavior of Lead Free Solders

2019 ◽  
Author(s):  
Mohammad Ashraful Haq ◽  
Mohd Aminul Hoque ◽  
Jeffrey C. Suhling ◽  
Pradeep Lall
Keyword(s):  
Strain Range ◽  
Peak Stress ◽  
Cyclic Stress ◽  
Lead Free ◽  
Stress Strain ◽  
Plastic Strain Range ◽  
Strain Behavior ◽  
Loop Area ◽  
Prior Aging ◽  
Lead Free Solders

Abstract In temperature changing environments, solder joints often experience fatigue failure due to cyclic mechanical stresses and strains induced by mismatches in the coefficients of thermal expansion. These stresses and strains lead to damage accumulation and contribute to the crack initiation, crack propagation, and eventually to failure. In this study, we have investigated the cyclic stress-strain behavior of SAC305 and SAC_Q reflowed lead free solders that occur at various testing temperatures and with various prior aging conditions. Lead free solder uniaxial test specimens with circular cross-section have been prepared using vacuum suction method and then were aged for 0 to 20 days at 125 °C. The samples were then subjected to cyclic stress-strain loading using a Micro-mechanical tester at different testing temperatures from T = 25 C to T = 100 C. The evolution of hysteresis loops with duration of prior aging was characterized by measuring the strain energy density dissipated per cycle (loop area), peak stress, and plastic strain range. It was observed that aging degrades the mechanical fatigue properties due to microstructural coarsening. At elevated temperatures, a drop in the loop area and peak stress and an increase in the plastic strain range for both lead free reflowed solder materials were obtained. In addition, SAC_Q samples had a higher loop area and peak stress compared to SAC305.

Aging Induced Mechanical Property Degradation and Microstructure Evolution of SAC+X Lead Free Solder Alloys

2013 ◽  
Author(s):  
Zijie Cai ◽  
Jeffrey C. Suhling ◽  
Pradeep Lall ◽  
Michael J. Bozack
Keyword(s):  
Thermal Cycling ◽  
Aging Time ◽  
Aging Temperature ◽  
Lead Free ◽  
Lead Free Solder ◽  
Solder Alloys ◽  
Stress Strain ◽  
Free Solder ◽  
Effects Of Aging ◽  
Lead Free Solders

The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In our prior work on aging effects, we have demonstrated that large degradations occur in the material properties (stiffness and strength) and creep behavior of Sn-Ag-Cu (SAC) lead free solders during aging. These effects are universally detrimental to reliability and are exacerbated as the aging temperature and aging time increases. Conversely, changes due to aging are relatively small in conventional Sn-Pb solders. In our current work, we are exploring several doped SAC+X alloys in an attempt to reduce the aging induced degradation of the material behavior of SAC solders. The doped materials are lead free SAC solders that have been modified by the addition of small percentages of one or more additional elements (X). Using dopants (e.g. Bi, In, Ni, La, Mg, Mn, Ce, Co, Ti, Zn, etc.) has become widespread to enhance shock/drop reliability, wetting, and other properties; and we have extended this approach to examine the ability of dopants to reduce the effects of aging and extend thermal cycling reliability. In this paper, we concentrate on presenting the results for SAC+X (X = Zn, Co, Ni). The enhancement of aging resistance for the doped lead free solders was explored. Comparisons were made to the responses of non-doped SAC lead free solder alloys. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on solder samples that were aged for various durations (0–6 months) at elevated temperature (100 °C). Variations of the mechanical and creep properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) were observed and modeled as a function of aging time and aging temperature. Our findings show that the doped SAC+X alloys illustrate reduced degradations with aging for all of the aging temperatures considered. Also, the stress-strain and creep mechanical properties of doped solders are better than those of reference solders after short durations of aging. After long term aging, doped solder alloys were found to have more stable behaviors than those of the standard SAC alloys. A parallel microstructure study has shown that less degradation and coarsening of the phases occurs in doped solder materials relative to non-doped solders after severe aging.

Evolution of the Microstructure of Lead Free Solders Subjected to Both Aging and Cyclic Loading

2019 ◽  
Author(s):  
Md Mahmudur R. Chowdhury ◽  
Mohd Aminul Hoque ◽  
Jeffrey C. Suhling ◽  
Sa’d Hamasha ◽  
Pradeep Lall
Keyword(s):  
Cyclic Loading ◽  
Electronic Packaging ◽  
Solder Joints ◽  
Regions Of Interest ◽  
Lead Free ◽  
Microstructural Changes ◽  
Mechanical Fatigue ◽  
Mechanical Cycling ◽  
Lead Free Solders ◽  
Electronic Assemblies

Abstract Currently, lead-free solders are being widely used as an alternative to traditional Sn-Pb solders in micro-electronic packaging industry due to the environmental concern of lead. Fatigue failure of solder joints is one of the common failure modes in electronic packaging which might be attributed to the experiences of thermo-mechanical fatigue (e.g. Power switching) or mechanical fatigue (e.g. vibration) loading. To design these lead-free solders more strategically for specific applications, it is important to understand the failure mechanism of lead-free solders under fatigue loading. Moreover, the microstructure and constitutive properties of conventional lead free solder joints in electronic assemblies such as SAC305 changes when exposed to isothermal aging. These changes consequently reduce the reliability of lead free electronic assemblies significantly due to aging. In this study, we have examined the effects of prior aging on damage accumulation occurring in SAC305 and SAC_Q (SAC+Bi) solder materials subjected to mechanical cycling (fatigue testing). Uniaxial samples have been prepared and polished so that the microstructural changes could be tracked after the initial aging, and then subsequently with mechanical cycling. In particular, we have examined the microstructural changes that occurred in small fixed regions in the solder samples, rather than using several different regions. Regions of interest near the center of the sample were marked using small indents formed with a nanoindentation system. Samples were then subjected to aging at 125 °C for various durations to produce several different initial microstructures. Scanning electron microscopy (SEM) were used to investigate the aging induced microstructural changes in the regions of interest in the solder sample. After aging, the samples were then subjected to mechanical cycling. After various durations of cycling (e.g. 0, 10, 25, 50, 75, 100, 200, 300 cycles) that were below the fatigue life of the materials, the regions of interest were again examined using SEM. Using the recorded images, the microstructural evolutions in the fixed regions were observed, and the effects of the initial aging on the results were determined. In case of SAC305, It was found that the number of IMC particles decreased while the average diameter of the particles increases significantly due to the initial aging. The distribution and size of the intermetallic particles in the inter-dendritic regions were observed to remain essentially unchanged with the application of the mechanical cyclic load. Relative to the non-aged samples, there were significant differences observed in the rate and intensity of the micro crack growth occurring in the heavily aged samples that began with much coarser microstructures. Later, the cycling induced microstructure evolutions observed in the SAC_Q lead free alloy has been compared with the observed changes in the microstructure of SAC305 that occurred during the cyclic loading. Due to the presence of bismuth, significant difference in the microstructural evolution of the SAC_Q alloy during cycling were observed. Thus, the doped alloys have shown a high potential for use in thermal cycling conditions because of their improved resistance to aging-induced evolution.

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