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AGING EFFECTS IN SAC SOLDER JOINTS

Yifei Zhang, Zijie Cai, Jeffrey C. Suhling, Pradeep Lall, Michael J. Bozack Department of Mechanical Engineering, and

Center for Advanced Vehicle and Extreme Environment Electronics Auburn University Auburn, AL 36849

Phone: (334) 844-3332 Fax: (334) 844-3124

Email: [email protected] ABSTRACT 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 oC) and elevated temperature aging (125 oC) 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. In the current work, we have extended our previous studies to include a full test matrix of aging temperatures and solder alloys. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on four different SAC alloys (SAC105, SAC205, SAC305, SAC405) that were aged for various durations (0-6 months) at room temperature (25 oC), and several elevated temperatures (50, 75, 100, and 125 oC). Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. The chosen selection of SAC alloys 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). Variations of the mechanical and creep properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) have been observed and modeled as a function of aging time and aging temperature. In this paper, we report on the results of the creep experiments. INTRODUCTION Eutectic or near eutectic tin/lead (Sn/Pb) solder (melting temperature TM = 183 °C) has been the predominant choice of the electronics industry for decades due to its outstanding solderability and reliability. However, legislation that mandates the banning of lead in electronics has been actively pursued worldwide during the last 15 years due to the environmental and health concerns. Although the implementation deadlines and products covered by such legislation continue to evolve, it is clear that laws requiring conversion to lead-free electronics are becoming a reality. Other factors that are affecting the push towards the elimination of lead in electronics are the market differentiation and advantage being realized by companies producing so-called “green” products that are lead-free. A large number of research studies are currently underway in the lead-free solder area. Although no “drop in” replacement has been identified for all applications; Sn-Ag, Sn-Ag-Cu (SAC), and other alloys involving elements such as Sn, Ag, Cu, Bi, In, and Zn have been identified as potential replacements for standard 63Sn-37Pb eutectic solder. Several SAC alloys have been the proposed by various user groups and industry experts. These include 96.5Sn-3.0Ag-0.5Cu (SAC305), 95.5Sn-3.8Ag-0.7Cu (SAC387), 95.5Sn-3.9Ag-0.6Cu (SAC396) and

Proceedings of the SEM Annual ConferenceJune 1-4, 2009 Albuquerque New Mexico USA

©2009 Society for Experimental Mechanics Inc.

95.5Sn-4.0Ag-0.5Cu (SAC405). For enhanced reliability during high strain rate exposures (e.g. shock and drop), several alloys with lower silver content have been recommended including 98.5Sn-1.0Ag-0.5Cu (SAC105) and 99Sn-0.3Ag-0.7Cu (SAC0307). The main benefits of the various SAC alloy systems are their relatively low melting temperatures compared with the 96.5Sn-3.5Ag binary eutectic alloy, as well as their superior mechanical and solderability properties when compared to other lead free solders. Solder joint fatigue is one of the predominant failure mechanisms in electronic assemblies exposed to thermal cycling. Reliable, consistent, and comprehensive solder constitutive equations and material properties are needed for use in mechanical design, reliability assessment, and process optimization. Mechanical characterization of solder materials has always been hampered by the difficulties in preparing test specimens that reflect the same true material making up the as actual solder joints (e.g. match the solder microstructure). Solder uniaxial samples haven been fabricated by machining of bulk solder material [1-8], or by melting of solder paste in a mold [9-18]. Use of a bulk solder bars is undesirable, because they will have significantly different microstructures than those present in the small solder joints used in microelectronics assembly. In addition, machining can develop internal/residual stresses in the specimen, and heat generated during turning operations can cause localized microstructural changes on the exterior of the specimens. Reflow of solder paste in a mold causes challenges with flux removal, minimization of voids, microstructure control, and extraction of the sample from the mold. In addition, many of the developed specimens have shapes that significantly deviate from being long slender rods. Thus, undesired non-uniaxial stress states will be produced during loading. Other investigators have attempted to extract constitutive properties of solders by direct shear or tensile loading [6, 19-28], or indenting [29-32], of actual solder joints (e.g. flip chip solder bumps or BGA solder balls). While such approaches are attractive because the true solder microstructure is involved, the unavoidable non-uniform stress and strain states in the joint make the extraction of the correct mechanical properties or stress-strain curves from the recorded load-displacement data very challenging. Also it can be difficult to separate the various contributions to the observed behavior from the solder material and other materials in the assembly (bond pads, silicon die, PCB/substrate, etc.). 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 [8, 13, 15-16, 18, 23-24, 28, 31, 33-51]. The observed material behavior variation during thermal aging/cycling is universally detrimental to reliability and includes reductions in stiffness, yield stress, ultimate strength, and strain to failure, as well as highly accelerated creep. Such aging effects are greatly exacerbated at higher temperatures typical of thermal cycling qualification tests. However, significant changes occur even with aging at room temperature [13, 15-16, 23-24, 33-41, 48]. As early as 1956, Medvedev [33] observed a 30% loss of tensile strength for bulk solder Sn/Pb solder stored for 450 days at room temperature. In addition, he reported 4-23% loss of tensile strength for solder joints subjected to room temperature storage for 280-435 days. In 1976, Lampe [34] found losses in shear strength and hardness of up to 20% in Sn-Pb and Sn-Pb-Sb solder alloys stored for 30 days at room temperature. Miyazawa and Ariga [35-36] measured significant hardness losses and microstructural coarsening for Sn-Pb, Sn-Ag, and Sn-Zn eutectic solders stored at 25 oC for 1000 hours, while Chilton and co-workers [37] observed a 10-15% decrease in fatigue life of single SMD joints after room temperature aging. Several studies [38-41] have also documented the degradation of Sn-Pb and SAC solder ball shear strength (10-35%) in area array packages subjected to room temperature aging. The effects of room temperature isothermal aging on constitutive behavior have also been investigated [13, 15-16, 48]. Chuang, et al. [13] characterized the reductions in yield stress and increases in elongations obtained in Sn-Zn eutectic solder during aging at room temperature. In addition, Xiao and Armstrong [15-16] recorded stress-strain curves for SAC 396 specimens subjected to various durations of room temperature aging, and found losses of ultimate tensile strength of up to 25%. The effects of room temperature aging on the mechanical properties and creep behavior of SAC alloys have been extensively discussed by the authors (Ma, et. al. [48]). The measured stress-strain data demonstrated large reductions in stiffness, yield stress, ultimate strength, and strain to failure (up to 40%) during the first 6 months after reflow solidification. In addition, even more dramatic evolution was observed in the creep response of aged solders, where up to 100X increases were found in the steady state (secondary) creep strain rate (creep compliance) of lead free solders that were simply allowed to sit in a room temperature environment. The SAC solder materials in room temperature aged joints were also found to enter the tertiary creep range (imminent

failure) at much lower strain levels than virgin joints (non aged, immediately after reflow solidification). We also demonstrated that there are corresponding changes in the solder joint microstructure occurring during room temperature aging. The magnitudes of the material behavior evolution occurring in lead free SAC solder joints were found to be much larger (e.g. 25X) than the corresponding changes occurring in traditional Sn-Pb assemblies. The effects of aging at elevated temperature are the most widely studied due to the dramatic changes in the microstructure and mechanical properties that result. Aging effects (reduced effective stiffness and ultimate strength) have been observed for solder subjected to elevated temperature aging (e.g. 125 oC) [8, 15-16, 18, 23-24]. Pang, et al. [20] measured microstructure changes, intermetallic layer growth, and shear strength degradation in SAC single ball joints subjected to elevated temperature aging. Darveaux [21] performed an extensive experimental study on the stress-strain and creep behavior of area array solder balls subjected to shear. He found that aging for 1 day at 125 oC caused significant effects on the observed stress-strain and creep behavior. The aged specimens were also found to creep much faster than un-aged ones by a factor of up to 20 times for both SAC305 and SAC405 solder alloys. Xiao and Armstrong [13-14] measured stress-strain curves for SAC396 specimens subjected to elevated temperature aging at 180 oC. At this highly elevated temperature, they observed a quick softening of the material during the first 24 hours followed by a gradual hardening with time. Dutta, et al. [31] used impression techniques to measure the creep behavior of SAC405 solder joints and observed large increases in the secondary creep rates with aging at 180 oC. Wiese and Kolter [28] demonstrated analogous large increases in the creep rates for SAC387 joints by directly loading flip chip assemblies that were aged at 125 oC. Several studies have been performed on the degradation of BGA ball shear strength with elevated temperature aging at 125 oC or 150 oC [42-46]. All of these investigations documented both microstructure coarsening and intermetallic layer growth. In addition, Hasegawa, et al. [42] measured elastic modulus reductions with aging by testing thin solder wires, while Chiu and co-workers [46] found significant reductions in drop reliability during elevated temperature aging. Finally, Ding, et al. [47] explored the evolution of fracture behavior of SnPb tensile samples with elevated temperature aging. In our prior work on elevated temperature aging effects [49], we demonstrated that the observed material behavior variations of SAC305 and SAC405 lead free solders during isothermal aging at 125 oC were unexpectedly large and universally detrimental to reliability. The measured stress-strain data demonstrated large reductions in stiffness, yield stress, ultimate strength, and strain to failure (up to 50%) during the first 6 months after reflow solidification. After approximately 1000 hours of aging, the lead free solder joint material properties were observed to degrade at a slow but constant rate. In addition, even more dramatic evolution was observed in the creep response of aged solders, where up to 500X increases in the secondary creep rates were observed for aging up to 6 months. The solder materials in aged joints were also found to enter the tertiary creep range (imminent failure) at much lower strain levels than virgin joints (non aged, tested immediately after reflow solidification). We also correlated the changes in mechanical behavior during aging with changes that occur in the solder joint microstructure, and showed that the magnitudes of the material behavior evolution occurring in lead free SAC solder joints are much larger (e.g. 100X) than the corresponding changes occurring in traditional Sn-Pb assemblies. One of the most important observations from our prior work on creep behavior was the demonstration that a “cross-over point” occurs during the elevated temperature aging (125 oC) of lead free and tin-lead solders. This cross-over point occurred after approximately 50 hours of aging at 125 oC, and marked the point where the two lead free solders began to creep at higher rates than standard 63Sn-37Pb solder for the same stress level. Such an effect was not observed for solder joints aged at room temperature (25 oC). The presence of the cross-over point with elevated temperature aging can possibly explain existing reliability data for area array packages where lead free packaging becomes less reliable than the analogous Sn-Pb case when the upper limit of the thermal cycling test is increased. As demonstrated above, the literature has documented the dramatic changes occurring in the constitutive and failure behavior of solder materials and solder joint interfaces during isothermal aging. However, these effects have been largely ignored in most other studies involving solder material characterization or finite element predictions of solder joint reliability during thermal cycling. It is also widely acknowledged that the large discrepancies in measured solder mechanical properties from one study to another are due to differences in the microstructures of the tested samples. This problem is exacerbated by the aging issue, as it is clear that the microstructure and material behavior of the samples used in even a single investigation are moving targets that

are evolving rapidly even at room temperature. Furthermore, the effects of aging on solder behavior must be better understood so that more accurate viscoplastic constitutive equations can be developed for SnPb and SAC solders. Without such relations, it is doubtful that finite element reliability predictions can ever reach their full potential. In the current work, we have extended our previous studies [48-51] to include a full test matrix of aging temperatures and solder alloys. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on four different SAC alloys (SAC105, SAC205, SAC305, SAC405) that were aged for various durations (0-6 months) at room temperature (25 oC), and several elevated temperatures (50, 75, 100, and 125 oC). Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. The chosen selection of SAC alloys 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). Variations of the mechanical and creep properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) have been observed and modeled as a function of aging time and aging temperature. In this paper, we report on the results of the creep experiments. EXPERIMENTAL PROCEDURE Uniaxial Test Sample Preparation In the current study, mechanical measurements of aging effects and material behavior evolution of lead free solders have been performed. We have avoided the specimen preparation pitfalls present in many previous studies by a using a novel procedure where solder uniaxial test specimens are formed in high precision rectangular cross-section glass tubes using a vacuum suction process. The tubes were then cooled by water quenching and 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). The solder is first melted in a quartz crucible using a pair of circular heating elements (see Figure 1). A thermocouple attached on the crucible and a temperature control module is used to direct the melting process. One end of the glass tube is inserted into the molten solder, and suction is applied to the other end via a rubber tube connected to the house vacuum system. The suction forces are controlled through a regulator on the vacuum line so that only a desired amount of solder is drawn into the tube. The specimens are then cooled to room temperature using a user-selected cooling profile. In order to see the extreme variations possible in the mechanical behavior and microstructure, we are exploring a large spectrum of cooling rates including water quenching of the tubes (fast cooling rate), air cooling with natural and forced convection (slow cooling rates), and controlled cooling using a surface mount technology solder reflow oven. For the reflow oven controlled cooling, the tubes are first cooled by water quenching, and they are then sent through a reflow oven (9 zone Heller 1800EXL) to re-melt the solder and subject it to the desired temperature profile. Thermocouples are attached to the glass tubes and monitored continuously using a radio-frequency KIC temperature profiling system to ensure that the samples are formed using the desired temperature profile (same as actual solder joints). Figure 2 illustrates the reflow temperature profiles used in this work for SAC and SnPb solder specimens.

(a) SAC (105/205/305/405)

(b) Sn-Pb Figure 1 - Specimen Preparation Hardware Figure 2 - Solder Reflow Temperature Profiles

Typical glass tube assemblies filled with solder and a final extracted specimen are shown in Figure 3. For some cooling rates and solder alloys, the final solidified solder samples can be easily pulled from the tubes due to the differential expansions that occur when cooling the low CTE glass tube and higher CTE solder alloy. Other options for more destructive sample removal involve breaking the glass or chemical etching of the glass. The final test specimen dimensions are governed by the useable length of the tube that can be filled with solder, and the cross-sectional dimensions of the hole running the length of the tube. In the current work, we formed uniaxial samples with nominal dimensions of 80 x 3 x 0.5 mm. A thickness of 0.5 mm was chosen because it matches the height of typical BGA solder balls. The specimens were stored in the aging oven immediately after the reflow process to eliminate possible room temperature aging effects. The described sample preparation procedure yielded repeatable samples with controlled cooling profile (i.e. microstructure), oxide free surface, and uniform dimensions. By extensively cross-sectioning several specimens, we have verified that the microstructure of any given sample is consistent throughout the volume of the sample. In addition, we have established that our method of specimen preparation yields repeatable sample microstructures for a given solidification temperature profile. Samples were inspected using a micro-focus x-ray system to detect flaws (e.g. notches and external indentations) and/or internal voids (non-visible). With proper experimental techniques, samples with no flaws and voids were generated. Mechanical Testing System and Experimental Test Matrix A MT-200 tension/torsion thermo-mechanical test system from Wisdom Technology, Inc., as shown in Figure 4, has been used to test the samples in this study. The system provides an axial displacement resolution of 0.1 micron and a rotation resolution of 0.001°. Testing can be performed in tension, shear, torsion, bending, and in combinations of these loadings, on small specimens such as thin films, solder joints, gold wire, fibers, etc. Cyclic (fatigue) testing can also be performed at frequencies up to 5 Hz. In addition, a universal 6-axis load cell was utilized to simultaneously monitor three forces and three moments/torques during sample mounting and testing. Environmental chambers added to the system allow samples to be tested over a temperature range of -185 to +300 °C.

(a) Within Glass Tubes

(b) After Extraction Figure 3 - Solder Uniaxial Test Specimens Figure 4 - MT-200 Testing System with Solder Sample Figure 5 illustrates a typical solder creep curve (strain vs. time response for a constant applied stress). The response begins with a quick transition to the initial “elastic” strain level, followed by regions of primary, secondary, and tertiary creep. Depending on the applied stress level, the primary creep region can be more extensive for the SAC alloys relative to Sn-Pb solders. The secondary creep region is typically characterized by a very long duration of nearly constant slope. This slope is referred to as the “steady state” secondary creep rate or creep compliance, and it is often used by practicing engineers as one of the key material parameters for solder in finite element simulations used to predict solder joint reliability. In this work, the measured creep rates were taken to be the minimum slope values in the secondary creep regions of the observed ε versus t responses. The tertiary creep region occurs when rupture is imminent, and typically features an abrupt change to a nearly constant but significantly increased creep rate.

&

Using specimens fabricated with the casting procedure described above, thermal aging effects and viscoplastic material behavior evolution have been characterized for SAC105, SAC205, SAC305, and SAC405 lead free solders. Such alloys are commonly used for solder balls in lead free BGAs, CSPs, and flip chip die, as well as for solder pastes used in SMT processes with other components. The lead free solder test results presented in this paper are all for samples solidified with the reflowed cooling profile shown in Figure 2a (mimics that seen by actual solder joints during PCB assembly). Analogous experiments were also performed with reflowed 63Sn-37Pb eutectic solder samples for comparison purposes. Creep curves have been characterized for five different aging temperatures including T = 25 (RT), 50, 75, 100, and 125 °C, with aging durations up to 6 months. We have also tested SAC405 with aging at T = 150 oC [51]. The test matrix of aging temperatures and aging times for the completed creep experiments is shown in Figure 6.

Secondary Creep(Steady-State)

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25 C (RT) 50 C 75 C 100 C

Figure 5 - Typical Solder Creep Curve Figure 6 - Aging Test Matrix for the Creep Tests For the creep data presented in this paper, the applied stress was σ = 15 MPa, which is approximately 25%-33% of the non-aged UTS values for the various alloys tested. The gage length of the specimens in this study was 60 mm, so that the specimen length to width aspect ratio was 20 to 1 (insuring true uniaxial stress states). All creep tests in this paper were conducted at room temperature (25 °C). Due to the long test times involved, only 5 specimens were tested for each alloy for any given set of aging conditions. The curves for each set of testing conditions were fit with an empirical strain-time model to generate an “average” representation of the creep response for those conditions. For the range of test conditions considered in this work, the raw strain versus time data in the primary and secondary creep regions were found to be well fit by creep response of the four parameter Burger’s (spring-dashpot) model:

( )tk210

3e1ktkk)t( −−++=ε=ε (1) From the recorded strain vs time curves under constant stress, the “steady state” creep strain rates (k1) have been extracted. In practice, the measured creep rate for each curve was evaluated numerically by calculating the minimum slope value in the secondary creep region of the observed ε& versus t response. Variations of the average creep rates with aging were determined and then modeled as a function of aging time. CREEP TESTING RESULTS Effects of Aging on Solder Creep Response Uniaxial specimens were formed for the various SAC solder alloys using the methods described in previous sections, and then aged at T = 25, 50, 75, 100, and 125 oC for up to six months. As mentioned above, specimens for each alloy were prepared in sets of five, which were then subjected to a specific set of aging conditions (aging temperature and aging time). Figure 7 illustrates the typical recorded creep curves for the SAC205 solder samples. The five graphs are for the five different aging temperatures (T = 25, 50, 75, 100, and 125 oC). In each graph, the various creep curves are for different aging times, illustrating the evolution of the creep response with

duration of aging. For brevity and clarity of the presentation, only one of the five available creep curves is shown in each plot for each set of aging conditions. Analogous results have also been recorded for the SAC105, SAC305, SAC405, and Sn-Pb solder samples.

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(c) Aging at T = 75 oC (d) Aging at T = 100 oC

Figure 7 - Creep Curves for SAC205 Aging for 0-6 Months

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(e) Aging at T = 125 oC

The plots in Figure 7 clearly indicate a dramatic evolution of the creep response of the SAC205 alloy when it is exposed to aging at elevated temperatures of T = 50, 75, 100, and 125 oC. The evolution of the creep response at room temperature is not as immediately obvious (first graph in Figure 7) due to the fact that common scales were chosen for the vertical (strain) axes in all of the graphs. However, the evolution was indeed present although it was relatively small compared to the observed variations with elevated temperature aging. Evolution of Creep Rate with Aging Exposure The effects of aging on the creep rate can be better seen by plotting the extracted secondary creep rates versus the aging time for each alloy. Such graphs are presented for SAC105, SAC205, SAC305, and SAC405 in Figures 8-11, respectively. In each plot, the creep rate evolution is indicated for each of the five aging temperatures. Each data point represents the average creep rate measured for the 5 samples tested at a given set of aging conditions. For each alloy and aging temperature, the data points were fit well with an exponential relationship: (2) )e1(CtCC t3C

210e−−++=ε&

(3) )e1(CtCClog tC

2103−−++=ε&

If the strain rate versus aging time data are plotted with a log scale on the vertical axis (as in Figures 8-11), constant C0 is the intercept and constant C1 is the slope of the linear part of the curve for large aging times. Constants C2 and C3 are associated with the nonlinear transition region in the first 30-50 days of aging.

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Figure 8 - Evolution of Creep Strain Rate (SAC105) Figure 9 - Evolution of Creep Strain Rate (SAC205)

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Figure 10 - Evolution of Creep Strain Rate (SAC305) Figure 11 - Evolution of Creep Strain Rate (SAC405)

From the results in Figures 8-11, it is apparent that all four of the SAC alloys experience dramatic changes in their creep rates for elevated temperature aging (200-10,000X). It is observed that the functional variations with aging at 50, 75, 100, and 125 oC become approximately “in parallel” with long term aging and are closely spaced. They are also significantly separated from the variation occurring with room temperature aging. After the large changes that occur during the first month of aging, the variation of the creep rate (log scale) for all four SAC alloys becomes nearly linear with longer aging times. For the data we have currently (up to 6 months of aging), there is no indication that an aging saturation point will be reached where the creep rate stabilizes. This suggests that there will potentially be reliability problems for SAC solder joints subjected to long term exposures at temperatures T > 50 oC. We have additional samples currently aging for 12 months to further explore this phenomenon. A final observation from the creep rate data in Figures 8-11 is that the aging effects on the creep responses are much stronger for the lower silver content alloys (SAC105 and SAC205). Considering all of the tests performed so far, the maximum recorded creep rate for each alloy was obtained with 6 months aging at 125 oC (most severe aging conditions considered). Figure 12 contains numerical values of the creep rates for non-aged samples and also the values of the maximum creep rates found for the samples aged for six months at 125 oC. We have also calculated the increases that have occurred by taking the ratios of the maximum creep rates to the corresponding creep rates for the non-aged (as reflowed) materials. These ratios represent the worst cases of the observed creep rate increases with aging. The changes for SAC105 and SAC205 were very large at 9700X and 2895X, respectively. The changes for SAC305 and SAC405 were significantly lower at 361X and 211X, respectively. Although a 200-time increase (SAC405) is still a very large change, the aging effect on the creep response is significantly smaller when the SAC alloy contains more silver. As in our prior work [49], the SAC305 and SAC405 alloys have similar creep-aging behavior.

Alloy

Strain Rate (Initial)

Strain Rate (After 6 Months Aging at 125 C)

Increase (Ratio)

SAC105 10.0 x 10-8 9.7 x 10-4 9700X SAC205 3.8 x 10-8 1.1 x 10-4 2895X SAC305 3.6 x 10-8 0.130 x 10-4 361X SAC405 3.5 x 10-8 0.074 x 10-4 211X

Figure 12 - Worst Case Increases in Creep Strain Rate with Aging [6 Months at 125 oC]

Comparison of SAC Aging and Sn-Pb Aging As observed in our previous studies [48-51], the aging induced changes in the creep strain rates of the SAC alloys are much larger than the analogous changes observed for conventional eutectic 63Sn-37Pb solder. Figure 13 contains the measured creep rate evolution curves for the Sn-Pb solder samples at the various aging temperatures. It is observed that the creep rates for Sn-Pb are restricted to a very narrow range of values between ε = 2 x 10-6 and = 1.5 x 10-5. & ε& For the same stress level, Figures 8-11 and Figure 13 illustrate that the SAC alloys begin with creep rates lower than Sn-Pb solder immediately after reflow (zero aging). However, the changes in creep rate are much larger for the SAC alloys with elevated temperature aging relative to those experienced by Sn-Pb under the same conditions. Thus, “cross-over points” will occur where the SAC alloys begin to creep faster that Sn-Pb after certain durations of aging. These cross over points are illustrated in Figures 14-17. In these plots, the shaded regions represent the total extent of the creep rate variations for the Sn-Pb and SAC alloys. The crossovers occur most quickly for SAC105 and SAC205, but eventually the creep rates of the SAC305 and SAC405 also begin to exceed those of Sn-Pb after longer durations of aging. For the stress level under consideration, crossovers did not occur for room temperature aging, which is similar to our previous results [49]. In general, the amount of aging required to obtain the creep rate cross-over depends in a complicated manner on the SAC alloy under consideration, the reflow profiles utilized, the applied stress level, and the aging temperature.

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Figure 13 - Evolution of Creep Strain Rate with Aging (Sn-Pb)

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Figure 14 - Creep Strain Rate Comparison (SAC105) Figure 15 - Creep Strain Rate Comparison (SAC205)

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Figure 16 - Creep Strain Rate Comparison (SAC305) Figure 17 - Creep Strain Rate Comparison (SAC405) Comparisons of the Alloys at Each Aging Temperature In prior investigations, it has been observed that lowering of the silver content of a SAC alloy leads to increases in the creep rate [27, 31]. However, these prior studies did not examine the effects of silver content on aging. The data presented in this paper also suggest that lowering of the silver content of a SAC alloy leads to increases in the creep rate for all aging conditions. That is, the creep rates for a given applied stress, aging temperature, and aging time are ordered by: 405SAC305SAC205SAC105SAC ε>ε>ε>ε &&&& (4)

This expression is illustrated graphically in Figures 18-22, where the data from Figures 8-11 has been re-organized so that each plot is for a fixed aging temperature, and includes the results for all four of the SAC alloys. As mentioned above, it is not possible to insert the creep rate of Sn-Pb into the expression in eq. (4), since crossover points exist. With no aging, Sn-Pb has the highest creep rate and is at the left of the expression in eq. (4). However, after significant aging durations at elevated temperatures, Sn-Pb has the lowest creep rate and is at the right of the expression in eq. (4). While the position of Sn-Pb changes, the relative positions of the various SAC alloys remain fixed in eq. (4).

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Figure 18 - Creep Rate Comparisons (Aging at 25 oC) Figure 19 - Creep Rate Comparisons (Aging at 50 oC)

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Figure 20 - Creep Rate Comparisons (Aging at 75 oC) Figure 21 - Creep Rate Comparisons (Aging at 100 oC)

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Figure 22 - Creep Rate Comparisons (Aging at 125 oC)

Effects of Aging on Microstructure Microstructure evolution during aging is the underlying reason for the changes in the stress-strain and creep properties of the solder alloys. The typical microstructure of SAC alloys consists of a Sn matrix (dendrites), and Ag3Sn and Cu6Sn5 second phases (intermetallics). During aging, the dendrites grow larger and merge, while at the same time the second phases develop into much larger particles (often needle-like in shape). This coarsening of the second phase particles is caused by diffusion, and is exacerbated at higher aging temperatures and longer aging durations. The diffusion rate of Ag and Cu at elevated temperatures will be much higher than at room temperature. Coarsened second phases will not be able to as effectively block dislocation movements, and thus there is a resulting loss of strength. At the same time the large secondary phases themselves become weak points in the materials. The creep deformation mechanisms for solders are mainly dislocation creep and grain sliding. The coarsened particles will also lose the ability to block grain boundary sliding. Thus the coarsening of the secondary phase particles leads to a dramatic loss of creep resistance in SAC alloys during aging. We have several parallel studies underway examining microstructural evolution during thermal aging of both unloaded solder specimens and of solder joints in various area array components (which are also subjected to stress during aging). SUMMARY AND CONCLUSIONS 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 (0-6 months) at room temperature (25 oC), and several elevated temperatures (50, 75, 100, and 125 oC). 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 (we have examined SACN05 with N = 1%, 2%, 3%, and 4% silver; with all alloys containing 0.5% copper). The results obtained in this work have demonstrated the significant effects of elevated temperature exposure on the creep behavior of solder joints. As expected, the creep rates evolved (degraded) more dramatically when the aging temperature was increased. In addition, the effects of aging were shown to be significant even for aging temperature slightly above room temperature (e.g. T = 50 oC). The creep rate evolves (increases) in an exponential manner, and the behaviors of lead free and tin-lead solders experience a “cross-over point” where the lead free solders begin to creep at higher rates than standard 63Sn-37Pb solder for the same stress level. In addition, the creep behaviors of the lower silver content alloys (e.g. SAC105) were observed to be more much more sensitive to aging (have greater changes in the creep rate for a given aging time) than the higher silver content alloys (e.g. SAC405). The times required before the cross-over occurred were reduced when considering higher aging temperatures or SAC alloys with lower silver content. It was also observed that lowering of the silver content of a SAC alloy leads to increases in the creep rates for all aging conditions. The degradation of the creep properties of lead free SAC solders during aging are caused by microstructural evolution. In particular, there is dramatic coarsening of the secondary intermetallic particles. When the particles are small and fine precipitations, they can effectively block the movement of dislocations and reduce grain boundaries sliding, thus strengthening the materials and enhancing creep resistance. When the second phase particles grow larger, their ability to block the dislocation movements and grain boundary sliding are significantly reduced leading to reduced strength and to degraded resistance to creep deformations. ACKNOWLEDGMENTS This work was supported by Basic Research Grant # IIP-0434909-015 from the National Science Foundation, as well as the NSF Center for Advanced Vehicle and Extreme Environment Electronics. REFERENCES 1. McCormack, M., Kammlott, Chen, H. S., Jin, S., “New Lead-Free Sn-Ag-Zn-Cu Solder Alloys with Improved

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