Zijie Cai

The effects of aging temperature on SAC solder joint material behavior and reliability

The effects of aging on mechanical behavior of lead free solders have been examined by performing creep tests on four different SAC alloys (SAC105, SAC205, SAC305, SAC405) that were aged for various durations (0-4 months) at room temperature (25degC), and several elevated temperatures (75, 100, and 125 degC). Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. Variations of the creep properties were observed and modeled as a function of aging time and aging temperature. In addition, the chosen selection of SAC alloys has allowed us to explore the effects of silver content on aging behavior.

The effects of SAC alloy composition on aging resistance and reliability

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 the observed material behavior variations of Sn-Ag-Cu (SAC) lead free solders during room temperature aging (25 °C) and elevated temperature aging (125 °C) were unexpectedly large and universally detrimental to reliability. Such effects for lead free solder materials are especially important for the harsh applications environments present in high performance computing and in automotive, aerospace, and defense applications. However, there has been little work in the literature, and the work that has been done has concentrated on the degradation of solder ball shear strength (e.g. Dage Shear Tester). Current finite element models for solder joint reliability during thermal cycling accelerated life testing are based on traditional solder constitutive and failure models that do not evolve with material aging. Thus, there will be significant errors in the calculations with the new lead free SAC alloys that illustrate dramatic aging phenomena.

Reduction of lead free solder aging effects using doped SAC alloys

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 have been shown to be relatively small in conventional Sn-Pb solders. Aging effects for lead free solder materials are especially important for the harsh applications environments present in high performance computing and in automotive, aerospace, and defense applications. In the current investigation, we have extended our previous studies to include a full test matrix of aging temperatures and SAC lead free solder alloys. In an attempt to reduce the aging induced degradation of the material behavior of SAC solders, we are also exploring various doped SAC-X alloys. These materials are 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. 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 room temperature (25 °C), and several elevated temperatures (50, 75, 100, and 125 °C). Four “standard” SAC alloys have been examined in this work including SAC105, SAC205, SAC305, and SAC405. This selection has allowed us to explore the effects of silver content on aging behavior (we have examined SACN05 with N= 1%, 2%, 3%, and 4% silver; with all alloys containing 0.5% copper). The doped SAC solder materials being considered in our ongoing studies include SAC0307-X, SAC105-X, and SAC305-X. In this work, we will concentrate on presenting the results for SAC0307-X (SAC-X), where X is 0.1%Bi. This alloy has been proposed as a lower cost SAC variation suitable for enhancing drop reliability. For all of the solders, 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 alloy illustrates reduced degradations with aging for all of the aging temperatures considered. The stress-strain and creep mechanical properties of SAC-X are better than those of SAC105 after short durations of aging, and approach those of SAC205 with longer aging times. After long term aging, the SAC-X alloy was found to have more stable behavior than all of the standard SACN05 alloys. Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes.

The effects of aging on the cyclic stress-strain behavior and hysteresis loop evolution of 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 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 several parameters (aging, temperature, strain/stress limits, and solder alloy composition) 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. Most tests in this investigation were performed with SAC105 solder alloy. However, the effect of solder composition was examined in a limited way by testing four SAC alloys (SAC105, SAC205, SAC305, SAC405) with varying silver content (1–4%) under strain controlled cycling. In addition, the effect of the testing temperature has also been studied by performing cyclic testing of SAC405 samples at four different temperatures (25, 50, 75, and 100 °C). 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.

Improved predictions of lead free solder joint reliability that include aging effects

It has been demonstrated that isothermal aging leads to large reductions (up to 50%) in several key material properties for lead free solders including stiffness (modulus), yield stress, ultimate strength, and strain to failure. In addition, even more dramatic evolution has been observed in the creep response of aged solders, where up to 10,000X increases have been observed in the steady state (secondary) creep strain rate (creep compliance). Such degradations in the stiffness, strength, and creep compliance of the solder material are expected to be universally detrimental to reliability of solder joints in electronic assemblies. Traditional finite element based predictions for solder joint reliability during thermal cycling accelerated life testing are based on solder constitutive equations (e.g. Anand viscoplastic model) and failure models (e.g. energy dissipation per cycle models) that do not evolve with material aging. Thus, there will be significant errors in the calculations with lead free SAC alloys that illustrate dramatic aging phenomena. In our current research, we are developing new reliability prediction procedures that utilize constitutive relations and failure criteria that incorporate aging effects, and then validating the new approaches through correlation with thermal cycling accelerated life testing experimental data. In this paper, we report on the first step of that development, namely the establishment of a revised set of Anand viscoplastic stress-strain relations for solder that include material parameters that evolve with the thermal history of the solder material. The effects of aging on the nine Anand model parameters have been examined by performing stress strain tests on SAC305 samples that were aged for various durations (0-6 months) at a temperature of 100 C. For each aging time, stress-strain data were measured at three strain rates (0.001, 0.0001, and 0.00001 1/sec) and five temperatures (25, 50, 75, 100, and 125 C). Using the measured stress-strain data, the Anand model material parameters have been determined for various aging conditions. Mathematical expressions were then developed to model the evolution of the Anand model parameter with aging time. Our results show that 2 of the 9 constants remain essentially constant during aging, while the other 6 show large changes (30-70%) with up to 6 months of aging at 100 C. Preliminary finite element simulations have also shown that the use of the modified Anand model leads to a strong dependence of the calculated plastic work dissipated per cycle on the aging conditions prior to thermal cycling.

Determination of Anand constants for SAC solders using stress-strain or creep data

The Anand viscoplastic constitutive model is often used to represent the deformation behavior of solders in electronic assemblies. In the Anand model, plasticity and creep are unified and described by the same set of flow and evolution relations. The nine parameters of the Anand constitutive model are typically determined from uniaxial stress-strain tests at several strain rates and temperatures using a standard multistep model parameter determination procedure. Conversely, creep data are often measured for solders, but typically are not used to determine Anand model constants. In this study, the theoretical equations for the uniaxial stress-strain response (constant strain rate) and for the creep response of solder have been derived from the Anand viscoplastic model. Procedures for extracting the Anand model constants from experimental stress-strain and creep data were also established. The two developed methods were then applied to find the Anand constants for SAC305 (Sn3.0Ag-0.5Cu) lead free solder using two completely different sets of experimental test data. The first set of Anand parameters were extracted from uniaxial stress strain data measured over a wide range of strain rates (ε = 0.001, 0.0001, and 0.00001 1/sec) and temperatures (T = 25, 50, 75, 100, and 125oC). The second set of Anand parameters were calculated from creep test data measured at several stress levels ( = 10, 12 and 15 MPa and temperatures (T = 25, 50, 75, 100, and 125oC). The two sets of Anand model constants derived from the stress-strain and creep data have been compared and found to be numerically very similar in magnitude. In addition, the accuracy (goodness of fit) of the Anand model using the extracted material constants has been evaluated by comparing the responses calculated from the Anand model with the measured stress-strain and creep data. In all cases, the Anand model was shown to represent the observed data very well over a wide range of temperatures and stress/strain levels.

Evolution of the tension/compression and shear cyclic stress-strain behavior of lead-free solder subjected to isothermal aging

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, we have demonstrated that the observed material behavior variations of Sn-Ag-Cu (SAC) lead free solders during elevated temperature aging were unexpectedly large and universally detrimental to reliability. 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 recently examined in some detail, there have been no prior studies on the effects of aging on solder failure and fatigue behavior.In our current work, we have examined effects of several parameters (aging temperature and time, testing temperature, strain/stress limits, and solder alloy composition) on the cyclic stress-strain behavior of SAC lead free solders. Both uniaxial specimens subjected to cyclic tension/compression and Iosipescu lap shear samples subjected to cyclic positive/negative shear have been studied. Samples were subjected to mechanical cycling 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 energy dissipated per cycle, which is correlated to the damage accumulation in the specimen. In this paper, we report on our findings on the effects of isothermal aging on the cyclic stress-strain behavior of SAC lead free solders. Prior to cyclic testing, the specimens were aged (preconditioned) at 125oC for various durations (up to one year). 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 behavior, 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/failure behavior of the lead free solders are highly accelerated for lower silver content alloys, and in this paper we concentrate on presenting the results for SAC105 (Sn-1.0Ag-0.5Cu) lead free solder.

Mitigation of lead free solder aging effects using doped SAC-X alloys

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 SACX0307 (Sn-0.3Ag-0.7Cu-0.1Bi). This alloy is often referred to as SAC-X, where X is 0.1%Bi, and the enhancement of aging resistance for the doped lead free solder was explored. Comparisons were made to the responses of SAC105 and SAC205, which served as the low silver content reference SAC 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 room temperature (25°C), and several elevated temperatures (50, 75, 100, and 125°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.

Aging induced evolution of free solder material behavior

Solder materials demonstrate evolving microstructure and mechanical behavior that changes significantly with environmental exposures such as isothermal aging and thermal cycling. These physical aging effects are greatly exacerbated at higher temperatures typical of thermal cycling qualification tests for harsh environment electronic packaging. In the current study, mechanical measurements of thermal aging effects and material behavior evolution of lead free solders have been performed. Extreme care has been taken so that the fabricated solder uniaxial test specimens accurately reflect the solder materials present in actual lead free solder joints. A novel specimen preparation procedure has been developed where the solder uniaxial test specimens are formed in high precision rectangular cross- section glass tubes using a vacuum suction process. The tubes are then sent through a SMT reflow to re-melt the solder in the tubes and subject them to any desired temperature profile (i.e. same as actual solder joints). Using specimens fabricated with the developed procedure, isothermal aging effects and viscoplastic material behavior evolution have been characterized for Sn-Ag-Cu lead free solders, which are commonly used as the solder ball alloy in lead free BGAs and other components. Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. In our total experimental program, several different SAC alloys have been examined. Samples have been solidified with both re flowed and water quenching temperature profiles, and isothermal aging has been performed at room temperature (25degC) and elevated temperatures (100degC, 125degC and 150degC). In this paper, we have concentrated our efforts on reporting results for 95.5Sn-4.0Ag-0.5Cu (SAC405) subjected to room temperature and elevated temperature aging experiments. Variations of the temperature dependent mechanical properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) were observed and modeled as a function of room temperature aging time. Microstructural changes during room temperature aging have also been recorded for the solder alloys and correlated with the observed mechanical behavior changes.

The influence of aging on the stress-strain and creep behavior of SAC solder alloys

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 the observed material behavior variations of Sn-Ag-Cu (SAC) lead free solders during room temperature aging (25 °C) and elevated temperature aging (125 °C) were unexpectedly large and universally detrimental to reliability. Such effects for lead free solder materials are especially important for the harsh applications environments present in high performance computing and in automotive, aerospace, and defense applications. However, there has been little work in the literature, and the work that has been done has concentrated on the degradation of solder ball shear strength (e.g. Dage Shear Tester). Current finite element models for solder joint reliability during thermal cycling accelerated life testing are based on traditional solder constitutive and failure models that do not evolve with material aging. Thus, there will be significant errors in the calculations with the new lead free SAC alloys that illustrate dramatic aging phenomena.