Effect of Isothermal Aging and Electromigration on the Microstructural Evolution of Solder Interconnections During Thermomechanical Loading (original) (raw)
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IEEE 70th Electronic Components and Technology Conference (ECTC), 2020
In electronic packages, solder joints are frequently exposed to thermal cycling environment where temperature variations occur from very low to high temperature. These exposures can occur in real life applications as well as in accelerated thermal cycling tests used for the characterization of thermal-mechanical fatigue behavior. Due to temperature variations and CTE mismatches of the assembly materials, cyclic temperature leads to damage accumulation due to shear fatigue and material property evolves in the solder joints. In addition, the thermal cycling dwell periods at the high temperature extremes will cause thermal aging phenomena in the solder material. This leads to microstructural evolution and material property degradation. Further aging effects can occur during the ramp periods between the low and high temperature extremes of the cycling. While changes in solder materials during aging have been examined in detail in prior studies, there have been limited studies examining material evolution occurring during thermal cycling. In a previous study of the authors, mechanical behavior evolutions of SAC305 lead-free solder material under several different thermal cycling profiles have been reported. The results demonstrated severe degradations in the mechanical properties, especially for thermal cycles with the long ramp and dwell periods. In our other recent work, evolution of the mechanical behavior of real solder joints has been investigated. In the current investigation, these prior studies have been extended. In particular, the mechanical behavior evolutions in both bulk SAC305 solder samples and SAC305 solder joints have been investigated under the same slow thermal cycling profile, and then the results were compared. In the first part of this study, miniature bulk solder uniaxial test specimens were prepared by reflowing solder in rectangular cross-section glass tubes with a controlled temperature profile. After reflow solidification, the samples were placed into the environmental chamber and thermally cycled between-40 C to +125 o C under a stress-free condition (no load). The thermal cycle consisted of 150 minutes cycles with 45 minutes ramps and 30 minutes dwells. The test specimens were separated into groups that were subjected to various durations of cycling (e.g. 0, 10, 50, 100, 250 cycles, etc.). After the environmental exposures, stress-strain curves of the cycled uniaxial samples were recorded, and then the mechanical properties were measured including the effective elastic modulus (E), yield stress (YS), ultimate tensile strength (UTS). The evolutions of the mechanical properties were characterized as a function of number of applied thermal cycles. In the second part of this study, the evolution of the mechanical behavior in thermally cycled BGA solder joints was studied using nanoindentation. PBGA solder joint strip specimens were first prepared by cross sectioning BGA assemblies followed by surface polishing to facilitate nanoindentation testing. Single grain solder joints were tested since they had large regions of solder material with equivalent mechanical behavior, which could then be indented several times after various durations of cycling. After preparation, the solder joint strip samples were thermally cycled using the same thermal cycling profile as the bulk samples. At various points in the cycling, the package was taken out from the chamber, and nanoindentation was performed to obtain the modulus and hardness. This allowed for investigation of the evolution of the mechanical properties of the SAC305 solder joints with the duration of thermal cycling. The results for the thermally cycled bulk samples showed that the detrimental effects of aging are accelerated in a thermal cycling environment. Similar degradations were found in the BGA solder joints subjected to thermal cycling. The degradation for both bulk samples and solder joints showed exponential variation with number of cycles. However, the degradation rates were higher in the bulk solder samples relative to those in the real solder joints. For example, the effective elastic modulus and yield stress reduced by 69% and 43%, respectively, for the bulk samples; whereas for the real solder joints, these values both reduced by 26%.
THERMOMIGRATION VERSUS ELECTROMIGRATION IN MICROELECTRONICS SOLDER JOINTS
Competing mechanisms of electromigration and thermomigration in flip chip SnAgCu (SAC) solder joints was studied experimentally. A chain of solder joints were stressed at 2.0 × 10 4 Amps/cm 2 , 2.4 × 10 4 Amps/cm 2 , and 2.8 × 10 4 Amps/cm 2 current density at room temperature. In the test vehicle, some solder joints were exposed to a combination of electromigration and thermomigration, while some others were exposed to thermomigration alone. The changes in the intermetallic compound (IMC) microstructure were observed with scanning electron microscope (SEM) under thermomigration alone and when both migration processes are present. In all cases, Cu 6 Sn 5 IMC at the hot side disintegrated while at the cold side thickened. The dissolution of the IMC at the hot side and the thickening at the cold side is a result of temperature and diffusion driving force. It is shown that thermomigration driving force, when present is much larger than electromigration.
Microstructure of Solder Joints and Isothermal Aging
2014
This paper deals with some aspects of ternary solder alloy 96.5Sn3Ag0.5Cu (wt.%) – SAC305, in order to determine changes in microstructure of solder joints usually used in electronics realized by Vapour Phase Soldering as well as by accelerated isothermal aging. The samples of solder joints were prepared to evaluate changes in their microstructure due to interfacial reactions on HAL (Hot Air Levelling) and Ni/Au PCB (Printed Circuit Board) surface finishes. Emphasis was placed on studying the effect of behaviour of intermetallic compounds as well as on prediction of reliability of solder joints.
Thermomigration in lead-free solder joints
In the next generation nanoelectronics and SiC based electronic packaging, current density and temperature gradient will be larger in orders of magnitude. Electromigration and thermomigration are considered to be major road blocks leading to realisation of nanoelectronics and SiC based high temperature power electronics. In this paper, damage mechanics of 95.5Sn4Ag0.5Cu (SAC405) lead-free solder joints under high temperature gradients have been studied. This paper presents observations on samples which were subjected to 1000°C/cm thermal gradient for two hours, 286 hours, 712 hours and 1156 hours. It was observed that samples subjected to thermal gradient did not develop a Cu 3 Sn intermetallic compound (IMC) layer at the hot side due to Cu migration to the cold side thus causing insufficient Cu mass concentration to form Cu 3 Sn. On the other hand, in samples subjected to isothermal annealing exhibited IMC growth. In samples subjected to thermomigration, near the cold side the Cu concentration is significantly higher, compared to hot side. Extensive surface hardness testing showed increase in hardness from the hot to cold sides, which indicates vacancy migration and Sn grain coarsening are in the opposing direction.. He specialises in experimental and computational damage mechanics of nano and power electronics packaging. He has authored more than 140 publications in the field of high sensitivity moiré interferometry inspection and
2011
Unlike SnPb solders, the thermal fatigue reliability of the Sn-Ag-Cu (SAC) solders is believed to be influenced significantly by both the initial and evolving microstructures. This paper presents a phenomenological study of the relationship between the initial SAC solder joint microstructure, the evolving microstructure, and the thermal fatigue performance measured by accelerated temperature cycling (ATC). To reflect the board assemblies that are in field use, commercial surface mount components with multiple geometries and materials and from different package assemblers were joined to the board with different lead free SAC alloys. The initial microstructures of the board level solder joints were altered in a variety of ways including: 1) varying the solder joint cooling rate; 2) varying the number of solder reflow exposures; and 3) exposure to different isothermal temperature exposures. In all cases the solder joint microstructure was exposed to one or more of these treatments prior to exposure to temperature cycling. In addition, some of the test boards were exposed to different cycling dwell times to determine if the microstructural evolution that occurred during ATC testing effected the respective characteristic lifetimes of the joints. The microstructural evolution was tracked and characterized with optical metallography and scanning electron microscopy. These results could have practical implications in terms of limiting the ability to develop acceleration factors and effective strain-based models for predicting Pb-free solder joint life.
Thermo-mechanical analysis of various solder materials via finite element method
The thermal environment may cause the unintended thermal stress to the solder joint which could lead to the reliability issue. This study aims to study the temperature cycling analysis of various solder materials (i.e., Sn-58Bi, Sn-3.5Ag and Sn-9Zn) by using the finite element method. A leadless solder joint of the surface mount component was considered and the three-dimensional model was created in the simulation software. The thermo-mechanical aspects (i.e., maximum displacement, stress and strain) of lead-free solder materials were investigated. The simulation results revealed the solder material significantly affects the thermo-mechanical aspects during the temperature cycling test. The maximum stress of the leadless solder joint was concentrated around the interface region of the solder and pad. The lowest stress (195MPa) was noticed on the solder joint when the Sn-58Bi material was applied. This study is expected to provide the understanding of the thermo-mechanical aspects of various solder materials during the temperature cycling test.
Thermomigration induced degradation in solder alloys
Journal of Applied Physics, 2008
Miniaturization of electronics to the nanoscale brings new challenges. Because of their small size and immense information and power processing capacity, large temperature gradients exist across nanoelectronics and power electronics solder joints. In this paper, a fully coupled thermomechanical-diffusion model is introduced to study the thermomigration induced strength degradation. A nonlinear viscoplastic material model with kinematic and isotropic hardening features is utilized. The model takes into account microstructural evolution of the material. A grain coarsening capability is built into the model to study its influence on thermomigration in solder alloys. The model is validated by comparing the simulation results with experimental data.
Localized recrystallization and cracking of lead-free solder interconnections under thermal cycling
Journal of Materials Research, 2011
The failure mechanism of lead-free solder interconnections of chip scale package-sized Ball Grid Array (BGA) component boards under thermal cycling was studied by employing cross-polarized light microscopy, scanning electronic microscopy, electron backscatter diffraction, and nanoindentation. It was determined that the critical solder interconnections were located underneath the chip corners, instead of the corner most interconnections of the package, and the highest strains and stresses were concentrated at the outer neck regions on the component side of the interconnections. Observations of the failure modes were in good agreement with the finite element results. The failure of the interconnections was associated with changes of microstructures by recrystallization in the strain concentration regions of the solder interconnections. Coarsening of intermetallic particles and the disappearance of the boundaries between the primary Sn cells were observed in both cases. The nanoindentation results showed lower hardness of the recrystallized grains compared with the nonrecrystallized regions of the same interconnection. The results show that failure modes are dependent on the localized microstructural changes in the strain concentration regions of the interconnections and the crack paths follow the networks of grain boundaries produced by recrystallization.
Influence of Thermomigration on Lead-Free Solder Joint Mechanical Properties
Journal of Electronic Packaging, 2009
Thermomigration experiments were conducted to study the change in mechanical properties of 95.5Sn–4Ag–0.5Cu (SAC405) lead-free solder joint under high temperature gradients. This paper presents some observations on samples that were subjected to 1000°C/cm thermal gradient (TG) for 286 h, 712 h, and 1156 h. It was observed that samples subjected to thermal gradient did not develop a Cu3Sn intermetallic compound (IMC) layer, and we observed disintegration of Cu6Sn5 IMC. On the other hand, samples subjected to isothermal annealing exhibited IMC growth. In samples subjected to thermomigration, near the cold side the Cu concentration is significantly higher compared with hot side. Extensive surface hardness testing showed an increase in hardness from the hot to cold sides, which possibly indicates that Sn grain coarsening is in the same direction.