Effect of isothermal storage on the microstructure and grain orientation of Cu/Cu3Sn/Cu solder joint

The paper focused on the changes in microstructure and mechanical properties of the full Cu41Sn11 solder joint (Cu/Cu41Sn11/Cu) during isothermal aging at 420 °C. It was motivated by potential applications of Cu–Sn intermetallic compounds (IMCs) solder joint in third-generation wide bandgap semiconductor devices. Experimental results revealed that the Cu41Sn11 phase was unstable under high-temperature conditions, the full Cu41Sn11 joint transformed into the full α(Cu) joint (Cu/α(Cu)/Cu) joint at 150 h during thermal aging. The formed α(Cu) phase was a Cu solid solution with inhomogeneous Sn atomic concentration, and its crystal structure and orientation were consistent with the original Cu plate. The conversion of the Cu41Sn11 to α(Cu) was accompanied by the formation of voids due to the volume shrinkage effect, predominantly near the middle of the solder joint interface. The α(Cu) solder joint presented a decrease in strength but an increase in strain rate sensitivity index compared to the Cu41Sn11 solder joint. Furthermore, the strain rate sensitivity index of α(Cu) and Cu41Sn11 is lower than that of ordinary Sn solders. After the shear test, the fractures that occurred in Cu41Sn11 grains were brittle, while the fractures in α(Cu) grains were ductile.

Compared with Sn-based solders, the In-Pb solders have better fatigue resistance. The great advantage of In-Pb solders is that gold dissolves more slowly in In-Pb solders than in Sn-based solders. To evaluate the reliability of In-Pb solders, in this paper, the microstructure and properties of In25Pb75 solder joints during aging and temperature cycling test was studied. The microstructure of the solder joints was observed by Scanning Electricity Microscope (SEM). The results show that there is a thin layers of the intermetallic compound in the middle of solder joints. The solder joints were aged at 200 ℃ for different time. After aging the shear mechanics performance of the solder joints was tested by a shear force tester. The results showed that the shear strength of the solder joints first increased and then decreased as aging time increased. The reliability of solder joints was estimated by temperature cycling test. After temperature cycling, the shear strength of the solder joints was tested by a shear force tester. The results show that the shear strength of the solder joints declines sharply after temperature cycling by a drop of 10%.KeywordsSolder jointsMicrostructureIntermetallic compoundShear strength

PurposeThe purpose of this paper is to investigate the effects of Sn-Ag-x leveling layers on the mechanical properties of SnBi solder joints. Four Sn-Ag-x (Sn-3.0Ag-0.5Cu, Sn-0.3Ag-0.7Cu, Sn-0.3Ag-0.7Cu-0.5 Bi-0.05Ni and Sn-3.0Ag-3.0 Bi-3.0In) leveling layers were coated on Cu pads to prepare SnBi/Sn-Ag-x/Cu solder joints. The microstructure, hardness, shear strength and fracture morphology of solder joints before and after aging were studied.Design/methodology/approachThe interfacial brittleness of the SnBi low-temperature solder joint is a key problem affecting its reliability. The purpose of this study is to improve the mechanical properties of the SnBi solder joint.FindingsOwing to the addition of the leveling layers, the grain size of the ß-Sn phase in the SnBi/Sn-Ag-x/Cu solder joint is significantly larger than that in the SnBi/Cu eutectic solder joint. Meanwhile, the hardness of the solder bulk in the SnBi/Cu solder joint shows a decrease trend because of the addition of the leveling layers. The SnBi/Cu solder joint shows obvious strength drop and interfacial brittle fracture after aging. Through the addition of the Sn-Ag-x layers, the brittle failure caused by aging is effectively suppressed. In addition, the Sn-Ag-x leveling layers improve the shear strength of the SnBi/Cu solder joint after aging. Among them, the SnBi/SACBN/Cu solder joint shows the highest shear strength.Originality/valueThis work suppresses the interfacial brittleness of the SnBi/Cu solder joint after isothermal aging by adding Sn-Ag-x leveling layers on the Cu pads. It provides a way to improve the mechanical performances of the SnBi solder joint.

Magnetic field is believed to have a significant impact on the evolution of microstructure during solidification or service condition. Many researches apply magnetic field to obtain a material with textures or special microstructure, such as turbine blade. The magnetic field is able to influence the crystal growth process, constrain crystal arrangement direction; and effectively inhibit the conductive fluid in thermo solute convection. However, the effect of magnetic field on the reliability of solder joint or related research is rarely reported. The purpose of this research is to investigate how magnetic field impact on the microstructure and mechanical performance of Sn-based lead-free solder joint. This research contains two possible aspects. Firstly, magnetic field with different intensities was posed on Sn0.3Ag0.7Cu to analyze the solidification microstructure. The roles of the second phase particles, such as Ni, were also considered. Intensity of magnetic field ranged from 0T-7T. Secondly, the solder joint was subjected to current density with magnetic field. The coupling effect of magnetic and electric fields was studied. The result indicated that, magnetic field influenced migration rate and migration direction of atoms. Consequently, the thicknesses of the intermetallic compounds (IMCs) at two interfaces were different. The appearance of interfacial IMCs changed. The couple of magnetic and electric made the solder joint failure more quickly. This study contributes to understand the reliability of solder joint under multi-field condition.

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 (Ma, et al., ECTC 2006), we demonstrated that the observed material behavior variations of SAC405 and SAC305 lead free solders during room temperature aging (25 degC) were unexpectedly large and universally detrimental to reliability. Such effects for lead free solder materials are much more dramatic at the higher aging temperatures (e.g. 100-150 degC) typical of the harsh 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 explored the effects of elevated temperature isothermal aging on the mechanical behavior and reliability of lead free solders. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on SAC405 and SAC305 samples that were aged for various durations (0-6 months) at several elevated temperatures (80, 100, 125, and 150 degC). Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. Variations of the temperature dependent mechanical properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) were observed and modeled as a function of aging time and temperature. In this paper, we have concentrated our efforts on presenting the results for samples aged at 125 degC. In addition, the new elevated temperature aging data were correlated with our room temperature results from last year's investigation. The results obtained in this work have demonstrated the significant effects of elevated temperature exposure on solder joints. As expected, the mechanical properties evolved at a higher rate and experienced larger changes during elevated temperature aging (compared to room temperature aging). After approximately 200 hours of aging, the lead free solder joint material properties were observed to degrade at a nearly constant rate. We have developed a mathematical model to predict the variation of the properties with aging time and aging temperature. Our data for the evolution of the creep response of solders with elevated temperature aging show that the creep behavior of lead free and tin-lead solders experience a "cross-over point" where lead free solders begin to creep at higher rates than standard 63Sn-37Pb solder for the same stress level. Such an effect is not observed for solder joints aged at room temperature, where SAC alloys always creep at lower rates than Sn-Pb solder.

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 (Ma, et al., ECTC 2006), we demonstrated that the observed material behavior variations of SAC405 and SAC305 lead free solders during room temperature aging (25 °C) were unexpectedly large and universally detrimental to reliability. Such effects for lead free solder materials are much more dramatic at the higher aging temperatures (e.g. 100–150 °C) typical of the harsh 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 explored the effects of elevated temperature isothermal aging on the mechanical behavior and reliability of lead free solders. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on SAC405 and SAC305 samples that were aged for various durations (0–6 months) at several elevated temperatures (80, 100, 125, and 150 °C). Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. Variations of the temperature dependent mechanical properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) were observed and modeled as a function of aging time and temperature. In this paper, we have concentrated our efforts on presenting the results for samples aged at 125 °C. In addition, the new elevated temperature aging data were correlated with our room temperature results from last year’s investigation. The results obtained in this work have demonstrated the significant effects of elevated temperature exposure on solder joints. As expected, the mechanical properties evolved at a higher rate and experienced larger changes during elevated temperature aging (compared to room temperature aging). After approximately 200 hours of aging, the lead free solder joint material properties were observed to degrade at a nearly constant rate. We have developed a mathematical model to predict the variation of the properties with aging time and aging temperature. Our data for the evolution of the creep response of solders with elevated temperature aging show that the creep behavior of lead free and tin-lead solders experience a “crossover point” where lead free solders begin to creep at higher rates than standard 63Sn-37Pb solder for the same stress level. Such an effect is not observed for solder joints aged at room temperature, where SAC alloys always creep at lower rates than Sn-Pb solder.

Within this work, we demonstrate that an easy soldering process in combination with wet chemical coating is suitable to realize a strong and reliable solder interconnection of Al substrates, even at short soldering times <5 s in ambient air. The microstructure of solder joints on wet chemically treated aluminum foils is investigated. A single and double zincate pre-treatment are compared to activate the Al surface, followed by electroless Ni plating. The quality of the solderable Ni surface is characterized by contact angle measurements, yielding good wettability (<60°), which is also achieved after isothermally heating (250 °C) the Ni-coated Al foils for 100 min. The microstructure of the Sn62Pb36Ag2 solder joints is investigated by SEM and EDX of cross sections, directly after soldering as well as after isothermal aging at 85 °C. Under the used soldering conditions, with a soldering temperature at about 280 °C, diffusion zones <500 nm were identified. Nonetheless, high peel forces after soldering >5 N mm−1 show stable values under aging conditions of 85 °C for 1000 hours. This could be correlated to a mixed fracture pattern, promoting the high adhesion due to the absence of a dominant failure mechanism.

Sn-58Bi eutectic solder is the most recommended low temperature Pb-free solder but is also limited from the interfacial embrittlement of Bi segregation. Since the quaternary Sn-38Bi-1.5Sb-0.7Ag solder provides a similar melting point as Sn-58Bi eutectic, this paper systematically investigated the properties of this solder from wettability, bulk tensile properties, interfacial microstructure in solder joints with a Cu substrate, interfacial evolution in joints during isothermal aging and the shear strength on ball solder joints with effect of aging conditions. The results were also compared with Sn-58Bi solder. The wettability of solder alloys was evaluated with wetting balance testing, and the quaternary Sn-38Bi-1.5Sb-0.7Ag solder had a better wettability than Sn-58Bi solder on the wetting time. Tensile tests on bulk solder alloys indicated that the quaternary Sn-38Bi-1.5Sb-0.7Ag solder had a higher tensile strength and similar elongation compared with Sn-58Bi solder due to the finely distributed SnSb and Ag3Sn intermetallics in the solder matrix. The tensile strength of solder decreased with a decrease in the strain rate and with an increase in temperature, while the elongation of solder was independent of the temperature and strain rate. When soldering with a Cu substrate, a thin Cu6Sn5 intermetallic compound (IMC) is produced at the interface in the solder joint. Measurement on IMC thickness showed that the quaternary Sn-38Bi-1.5Sb-0.7Ag had a lower IMC growth rate during the following isothermal aging. Ball shear test on solder joints illustrated that the quaternary Sn-38Bi-1.5Sb-0.7Ag solder joints had higher shear strength than Sn-58Bi solder joints. Compared with the serious deterioration on shear strength of Sn-58Bi joints from isothermal aging, the quaternary Sn-38Bi-1.5Sb-0.7Ag solder joints presented a superior high temperature stability. Therefore, the quaternary Sn-38Bi-1.5Sb-0.7Ag solder provides better performances and the possibility to replace Sn-58Bi solder to realize low temperature soldering.

The impact reliability of solder joints in electronic packages is critical to the lifetime of electronic products, especially those portable devices using area array packages such as ball-grid array (BGA) and chip-scale packages (CSP). Currently, SnAgCu (SAC) solders are most widely used for lead-free applications. However, BGA and CSP solder joints using SAC alloys are fragile and prone to premature interfacial failure, especially under shock loading. To further enhance impact reliability, a family of SAC alloys doped with a small amount of additives such as Mn, Ce, Ti, Bi, and Y was developed. The effects of doping elements on drop test performance, creep resistance, and microstructure of the solder joints were investigated, and the solder joints made with the modified alloys exhibited significantly higher impact reliability.

The microstructure of the flip-chip solder joints fabricated using stud bumps and Pb-free solder was characterized. The Au or Cu stud bumps formed on Al pads on Si die were aligned to corresponding metal pads in the substrate, which was printed with Sn-3.5Ag paste. Joints were fabricated by reflowing the solder paste. In the solder joints fabricated using Au stud bumps, Au-Sn intermetallics spread over the whole joints, and the solder remained randomly island-shaped. The δ-AuSn, e-AuSn2, and η-AuSn4 intermetallic compounds formed sequentially from the Au stud bump. The microstructure of the solder joints did not change significantly even after multiple reflows. The AuSn4 was the main phase after reflow because of the fast dissolution of Au. In the solder joints fabricated using Cu stud bumps, the scallop-type Cu6Sn5 intermetallic was formed only at the Cu interface, and the solder was the main phase. The difference in the microstructure of the solder joints with Au and Cu stud bumps resulted from the dissolution-rate difference of Au and Cu into the solder.

The ball grid array (BGA) component with cross-sectioned edge row was thermally shocked to investigate the activated slip systems in the Sn–3.0Ag–0.5Cu lead-free solder joint. The microstructure and crystal orientations of Sn-based solder joints in as-reflowed and thermally shocked conditions were obtained by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD), respectively. The sample was reexamined after 200, 300 and 400 thermal shock cycles without further polishing. In this paper, one single-crystal solder joint in a BGA component was selected to analyze the activated slip systems during recrystallization under the thermally shocked cycled condition. Two steps were used to determine the activated slip systems in this study. Step one, subgrain rotation angles and axes were calculated by the Euler angles obtained by EBSD before and after thermal shock. Then several slip systems were obtained according to the calculated subgrain rotation axes. Step two, in order to further determine the accurate slip system which could cause the subgrain rotation along the axes in step one, the theoretical slip traces were obtained by calculating Euler angles. Because the results contained many kinds of theoretical slip traces, so the slip bands observed by SEM were used and assisted to select the theoretical slip traces. Based on the comprehensive analysis of the two steps, slip systems that causes the serious deformation were determined. The results showed that recrystallization occurred at different areas of the solder joint and the slip bands appeared at the corners of the solder joint. The subgrain rotation behavior was variable at the recrystallization area by analyzing the subgrain orientation after 200, 300 and 400 thermal shock cycles. By compared the as-calculated slip traces with the slip bands characterized by SEM and the rotation axes calculated by the Euler angles, (0 \(\bar {1}\) 0) [0 0 1], (1 \(\bar {1}\) 0) [0 0 1] and (1 1 0) [0 0 \(\bar {1}\)] were the slip systems that caused the subgrain rotation along a certain axis in this solder joint.

Microstructure and mechanical behavior of SnBi-xAg and SnBi-xAg@P-Cu solder joints during isothermal aging

Flip-chip (FC) light-emitting diode (LED) filaments with a silver conductive layer are impeded by heat dissipation, with the flip-chip junction temperature of reaching up to 423.15 K. To overcome this problem, we simulated aging of SAC305 solder joints at a high temperature of 433.15 K. Scanning electron microscopy was performed to analyze the changes in the microstructure of the solder joints with aging time. Moreover, energy-dispersive X-ray spectroscopy was performed to analyze changes in the element content of the solder joints with aging time. The shear force of solder joints was measured using a strength tester. Results revealed that the Ag/Au content of the solder joints increased under the high-temperature conditions, resulting in the formation of cracks and holes in the solder joints. Furthermore, the reliability of the FC LED filaments was affected by the maximum shear stress that can be tolerated by the solder joints.

Since the National Electronics Manufac tu r ing Initiative recommended Sn3.9Ag-0.6Cu for refl ow soldering and Sn-3.5Ag and Sn-0.7Cu for wave soldering in 2000, great effort has been made to address the reliability issues of lead-free soldered packages. Investigation of the dependence of properties of bulk solder alloys on compositions and processes can, of course, greatly help reliability engineering of solder joints. However, because of the strong effect of the interface between solder and soldered metal on the reliability of fi ne pitch solder joints, research of bulk solders is not enough to answer all the questions. The infl uences of composition and microstructure of solder joints on the reliability of bond interface should also be studied. In this issue, fi ve papers address materials issues directly related to solder joint reliability. In the fi rst paper, I. Dutta et al. summarize their research in characterization of microstructural coarsening in Sn-Ag based solders and its effects on mechanical properties of bulk solder as well as reliability of solder joints. In the eutectic SnAgCu alloy, the volume fraction of Cu 6 Sn 5 is much smaller than that of Ag 3 Sn and Cu 6 Sn 5 coarsens more rapidly than Ag 3 Sn. Therefore, Ag 3 Sn is the major contributor to the effects of coarsening on mechanical properties of SnAgCu solders and reliability of joints. The data indicate that a lower rate of interfacial failure of solder joints can be expected if electronic devices are handled with greater caution in the fi rst several days after assembly. The second paper by F. Guo et al. is Lead-free Soldering: Materials Science and Solder Joint Reliability

The microstructure, shear behaviour and hardness of the SnBi/SACBN/Cu solder joint before and after isothermal ageing were investigated in comparison with the SnBi/Cu and SACBN/Cu solder joints. The experimental results indicated that the pre-soldered SACBN joint had a significant effect on the formation and growth of the β-Sn grains in the SnBi bulk solder. The brittleness of the SnBi/SACBN/Cu composite solder joint was also suppressed and its failure mode transformed from brittle failure to brittle-ductile failure after reflow. However, the shear strength and failure mode of the SnBi/SACBN/Cu composite solder joint became similar to those of the SnBi/Cu joints after 600 h isothermal ageing. The shear strength of the three kinds of solder joints decreased after isothermal ageing, but the SnBi/SACBN/Cu composite solder joint showed higher shear strength than SnBi/Cu did during ageing. The shear strength of the composite solder joint was 67.1MPa after ageing. Due to the diffusion of elements in the isothermal ageing process, the microstructure of the composite solder joint was significantly coarsened after ageing for 600 hours. This phenomenon further led to the decrease of the hardness and shear strength of the three kinds of solder joints.