Evolution of the Stress-Strain and Creep Behavior of Underfill Encapsulants With Aging

2009 ◽  
Author(s):  
Chang Lin ◽  
Jeffrey C. Suhling ◽  
Pradeep Lall
Keyword(s):  
Creep Rate ◽  
Thermal Cycling ◽  
Aging Time ◽  
Aging Temperature ◽  
Isothermal Aging ◽  
Flip Chip ◽  
Material Behavior ◽  
Initial Slope ◽  
Aging Effects ◽  
Stress Strain

Microelectronic encapsulants exhibit evolving properties that change significantly with environmental exposures such as isothermal aging and thermal cycling. Such aging effects are exacerbated at higher temperatures typical of thermal cycling qualification tests for harsh environment electronic packaging. In this work, measurements of material behavior changes occurring in flip chip underfill encapsulants exposed to isothermal aging have been performed. A novel method has been developed to fabricate freestanding underfill uniaxial test specimens so that they accurately reflect the encapsulant layer present in flip chip assemblies. Using the developed specimen preparation procedure, isothermal aging effects have been characterized at several elevated temperatures (+ 80, +100, + 125, and +150 °C). Samples have been aged at the four temperatures for periods up to 6 months. Stress-strain and creep tests have been performed on non-aged and aged samples, and the changes in mechanical behavior have been recorded for the various aging temperatures and durations of isothermal exposure. Empirical models have been developed to predict the evolution of the material properties (modulus, strength) and the creep strain rate as a function of temperature, aging time, and aging temperature. The evaluated underfill illustrated softening behavior at temperatures exceeding 100 °C, although the documented Tg ranged from 130–150 °C. The obtained results showed an obvious enhancement of the underfill mechanical properties as a function of the aging temperature and aging time. Both the effective elastic modulus (initial slope) and ultimate tensile strength (highest stress before failure) increase monotonically with the amount of isothermal aging or aging temperature, regardless of whether the aging temperature is below, at, or above the Tg of the material. From the creep results, it was seen that at a given time, the creep strains were much lower for the aged samples relative to the non-aged samples. Thermal aging has a significant effect on the secondary creep rate, which decreases with both the aging temperature and the aging time. Up to a 100X reduction in the creep rate was observed, and the major changes occurred during the first 50 days of the isothermal aging.

FEA Based Reliability Predictions for PBGA Packages Subjected to Isothermal Aging Prior to Thermal Cycling

2015 ◽  
Author(s):  
Munshi Basit ◽  
Mohammad Motalab ◽  
Jeffrey C. Suhling ◽  
John L. Evans ◽  
Pradeep Lall
Keyword(s):  
Finite Element ◽  
Solder Joint ◽  
Thermal Cycling ◽  
Isothermal Aging ◽  
Material Behavior ◽  
Lead Free ◽  
Stress Strain ◽  
Solder Material ◽  
Free Solder ◽  
Temperature 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 on aging effects, we have demonstrated that the observed material behavior degradations of Sn-Ag-Cu (SAC) lead free solders during room temperature aging (25 C) and elevated temperature aging (50, 75, 100, 125, and 150 C) were unexpectedly large. 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. In this study, we have used both accelerated life testing and finite element modeling to explore how prior isothermal aging affects the overall reliability of PBGA packages subjected to thermal cycling. In the experimental work, an extensive test matrix of thermal cycling reliability testing has been performed using a test vehicle incorporating several sizes (5, 10, 15, 19 mm) of BGA daisy chain components with 0.4 and 0.8 mm solder joint pitches (SAC305). PCB test boards with 3 different surface finishes (ImAg, ENIG and ENEPIG) were utilized. In this paper, we concentrate on the reporting the results for a PBGA component with 15 mm body size. Before thermal cycling began, the assembled test boards were divided up into test groups that were subjected to several sets of aging conditions (preconditioning) including 0, 6, and 12 months aging at T = 125 °C. After aging, the assemblies were subjected to thermal cycling (−40 to +125 °C) until failure occurred. The Weibull data failure plots have demonstrated that the thermal cycling reliabilities of pre-aged assemblies were significantly less than those of non-aged assemblies. A three-dimensional finite element model of the tested 15 mm PBGA packages was also developed. The cross-sectional details of the solder ball and the internal structure of the BGA were examined by scanning electron microscopy (SEM) to capture the real geometry of the package. Simulations of thermal cycling from −40 to 125 C were performed. To include the effects of aging in the calculations, we have used a revised set of Anand viscoplastic stress-strain relations for the SAC305 Pb-free solder material that includes material parameters that evolve with the thermal history of the solder material. The accumulated plastic work (energy density dissipation) was used is the failure variable; and the Darveaux approach to predict crack initiation and crack growth was applied with aging dependent parameters to estimate the fatigue lives of the studied packages. We have obtained good correlation between our new reliability modeling procedure that includes aging and the measured solder joint reliability data. As expected from our prior studies on degradation of SAC material properties with aging, the reliability reductions were more severe for higher aging temperature and longer aging times.

Die Stress Variation During Thermal Cycling Reliability Tests

2007 ◽  
Author(s):  
M. Kaysar Rahim ◽  
Jordan Roberts ◽  
Jeffrey C. Suhling ◽  
Richard C. Jaeger ◽  
Pradeep Lall
Keyword(s):  
Thermal Cycling ◽  
Electronic Packaging ◽  
Room Temperature ◽  
Flip Chip ◽  
Constitutive Relations ◽  
Temperature Cycling ◽  
Material Behavior ◽  
Aging Effects ◽  
Test Chips

Thermal cycling accelerated life testing is an established technique for thermo-mechanical evaluation and qualification of electronic packages. Finite element life predictions for thermal cycling configurations are challenging due to several reasons including the complicated temperature/time dependent constitutive relations and failure criteria needed for solders, encapsulants and their interfaces; aging/evolving material behavior for the packaging materials (e.g. solders); difficulties in modeling plating finishes; the complicated geometries of typical electronic assemblies; etc. In addition, in-situ measurements of stresses and strains in assemblies subjected to temperature cycling are difficult because of the extreme environmental conditions and the fact that the primary materials/interfaces of interest (e.g. solder joints, die device surface, wire bonds, etc.) are embedded within the assembly (not at the surface). For these reasons, little is known about the evolution of the stresses, strains, and deformations occurring within sophisticated electronic packaging geometries during thermal cycling. In this work, we have used test chips containing piezoresistive stress sensors to characterize the in-situ die surface stress during long-term thermal cycling of electronic packaging assemblies. Using (111) silicon test chips, the complete three-dimensional stress state (all 6 stress components) was measured at each rosette site by monitoring the resistance changes occurring in the sensors. The packaging configuration studied in this work was flip chip on laminate where 5 × 5 mm perimeter bumped die were assembled on FR-406 substrates. Three different thermal cycling temperature profiles were considered. In each case, the die stresses were initially measured at room temperature after packaging. The packaged assemblies were then subjected to thermal cycling and measurements were made either incrementally or continuously during the environmental exposures. In the incremental measurements, the packages were removed from the chamber after various durations of thermal cycling (e.g. 250, 500, 750, 1000 cycles, etc.), and the sensor resistances were measured at room temperature. In the continuous measurements, the sensor resistances at critical locations on the die device surface (e.g. die center and die corners) were recorded continuously during the thermal cycling exposure. From the resistance data, the stresses at each site were calculated and plotted versus time. The experimental observations show cycle-to-cycle evolution in the stress magnitudes due to material aging effects, stress relaxation and creep phenomena, and development of interfacial damage.

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

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

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

Aging Effects on the Mechanical Behavior and Reliability of SAC Alloys

2009 ◽  
Author(s):  
Yifei Zhang ◽  
Zijie Cai ◽  
Jeffrey C. Suhling ◽  
Pradeep Lall
Keyword(s):  
Thermal Cycling ◽  
Mechanical Behavior ◽  
Room Temperature ◽  
Material Behavior ◽  
Lead Free ◽  
Lead Free Solder ◽  
Aging Effects ◽  
Free Solder ◽  
Effects Of Aging ◽  
Temperature 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 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. 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°C), and several elevated temperatures (50, 75, 100, and 125°C). Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. 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. In this paper, we report on the creep results. 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). In order to reduce the aging induced degradation of the material behavior of the SAC alloys, we are testing several doped SAC alloys in our ongoing work. These materials include SAC0307-X, SAC105-X, and SAC305-X; where the standard SAC alloys 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, etc.) has become widespread to enhance shock/drop reliability, and we have extended this approach to examine the ability of dopants to reduce the effects of aging and extend thermal cycling reliability.

Isothermal Aging Effects on the Microstructure, IMC and Strength of SnAgCu/Cu Solder Joint

2007 ◽  
Vol 353-358 ◽  
pp. 2928-2931 ◽  
Author(s):  
Xiao Yan Li ◽  
Xiao Hua Yang ◽  
Wei Zhen Dui ◽  
Ben Sheng Wu
Keyword(s):  
Tensile Strength ◽  
Solder Joint ◽  
Aging Time ◽  
Isothermal Aging ◽  
Solder Matrix ◽  
Aging Effects ◽  
Imcs Layer ◽  
X Ray ◽  
Formation And Evolution ◽  
Free Solder

The formation and evolution of the intermetallic compound (IMCs) between SnAgCu lead-free solder and Cu substrate, after isothermal aging at 150°C for 24, 48, 120, 240 and 480 hours, were studied. Scanning electron microscope (SEM) was used to observe the microstructure evolution of solder joint during aging. The IMC phases were identified by energy dispersive X-ray (EDX). The results showed that IMCs layer of Cu6Sn5 was formed at the interface of solder and Cu during reflowing. With the increase of aging time, the grain size of the interfacial Cu6Sn5 increased and the morphology of the interfacial Cu6Sn5 column was changed from scallop-like to needle-like and then to rod-like and finally to particles. At the same time, the rod-like Ag3Sn phase formed at the interface of solder and the IMCs layer of Cu6Sn5 with the aging time increased. In addition, large Cu6Sn5 formed in the solder with the aging time increased. The tensile strength was measured for the solder joints. The results showed that the tensile strength increases slightly at beginning and then decreases with the aging time. SEM was used to observe the fracture surface and it showed that the fracture position moved from solder matrix to the interfacial Cu6Sn5 IMCs layer with the aging time increased. The weakening of the solder matrix is caused by the coarsening of the eutectic solder structures. The weakening of the interfacial IMCs layer is caused by the evolution of morphology and size of the interface Cu6Sn5 layer.

Thermal Cycling Guidelines for Automotive, Computer, Portable, and Implantable Medical Device Applications

2000 ◽  
Author(s):  
Rajiv Raghunathan ◽  
Raghuram V. Pucha ◽  
Suresh K. Sitaraman
Keyword(s):  
Thermal Cycling ◽  
Flip Chip ◽  
Inelastic Strain ◽  
Computer Applications ◽  
Material Behavior ◽  
Implantable Medical Device ◽  
Implantable Medical Devices ◽  
Device Applications ◽  
Component Assembly ◽  
Virtual Qualification

Abstract The objective of this work is to develop qualification guidelines for Flip-Chip on Board (FCOB) and Flip Chip Chip-Scale Packages (FCCSP) used in implantable medical devices, automotive applications, computer applications and portables, taking into consideration the thermal history associated with the field conditions. The accumulated equivalent inelastic strain per cycle and the maximum strain energy density have been used as damage parameters to correlate solder fatigue damage during field use and thermal cycling. The component assembly process mechanics, the time and temperature-dependent material behavior, and the critical geometric features of the assembly have been taken into consideration for developing the comprehensive virtual qualification methodology.

A Model for Hyper-Elastic Material Behavior Under Thermal Aging With an Application to Natural Rubber

2018 ◽  
Author(s):  
Ahmed G. Korba ◽  
Abhishek Kumar ◽  
Mark E. Barkey
Keyword(s):  
Natural Rubber ◽  
Aging Time ◽  
Elastic Material ◽  
Thermal Aging ◽  
Least Square ◽  
Material Behavior ◽  
Model Parameters ◽  
Stress Strain ◽  
Strain Behavior ◽  
Hyper Elastic

Numerous hyper-elastic theoretical material models have been proposed over the past 60 years to capture the stress-strain behavior of large deformation incompressible isotropic materials. Among them, however, only few models have considered the thermal aging effect on model parameters. Having a simple, closed-form equation that includes the effect of aging temperature and time in describing the stress-strain behavior could facilitate fatigue analysis and life time prediction of rubber-like materials. In this vein, this paper defines a new and simple Weight Function Based (WFB) model that describes hyper-elastic materials’ behavior as a function of aging time and temperature variations. More than 130 natural rubber specimens were thermally aged in an oven and tested under uni-axial loading to observe their stress-strain behavior at various temperatures and aging times. The temperature ranged from 76.7 °C to 115.5 °C, and the aging time from zero to 600 hours. The proposed WFB model is based on the Yeoh model and basic continuum mechanics assumptions, and it was applied to the tested natural rubber materials. Moreover, it was verified against Treloar’s historic tensile test data for uni-axial tension of vulcanized natural rubber material, and also compared to the Ogden and the Yeoh models. A non-linear least square optimization tool in Matlab was used to determine all hyper-elastic material model parameters and all other fitting purposes. The proposed model has better accuracy in fitting Treloar’s data compared to the Ogden and the Yeoh models using the same fitting tool under the same initial numerical conditions.

Evolution of Lead Free Solder Material Behavior During Isothermal Aging

2006 ◽  
Author(s):  
Hongtao Ma ◽  
Jeffrey C. Suhling ◽  
Pradeep Lall ◽  
Michael J. Bozack
Keyword(s):  
Isothermal Aging ◽  
Room Temperature ◽  
Material Behavior ◽  
Lead Free ◽  
Aging Effects ◽  
Uniaxial Test ◽  
Room Temperature Aging ◽  
Free Solder ◽  
Lead Free Solders ◽  
Temperature Aging

Solder materials demonstrate evolving microstructure and mechanical behavior that changes significantly with environmental exposures such as isothermal aging and thermal cycling. These 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 95.5Sn4.0Ag-0.5Cu (SAC405) and 96.5Sn-3.0Ag-0.5Cu (SAC305) 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, samples have been solidified with both reflowed and water quenching temperature profiles, and isothermal aging has been performed at room temperature (25 °C) and elevated temperatures (100 °C, 125 °C and 150 °C). In this paper, we have concentrated on reporting the results of the room 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.

Characterization of Die Stress Distributions in Area Array Flip Chip Packaging

2009 ◽  
Author(s):  
Jordan Roberts ◽  
M. Kaysar Rahim ◽  
Jeffrey C. Suhling ◽  
Richard C. Jaeger ◽  
Pradeep Lall ◽  
...  
Keyword(s):  
Thermal Cycling ◽  
Flip Chip ◽  
Thermal Power ◽  
Temperature Cycling ◽  
Aging Effects ◽  
Area Array ◽  
Power Cycling ◽  
Die Stress ◽  
On Chip ◽  
Test Chips

On-chip piezoresistive stress sensors represent a unique approach for characterizing stresses in silicon die embedded within complicated packaging architectures. In this work, we have used test chips containing such sensors to measure the stresses induced in microprocessor die after various steps of the assembly process, as well as to continuously characterize the in-situ die surface stress during thermal cycling and power cycling. The utilized (111) silicon sensor rosettes were able to measure the complete three-dimensional stress state (all 6 stress components) at each sensor site being monitored by the data acquisition hardware. The test chips had dimensions of 20 × 20 mm, and 3600 lead free solder interconnects (full area array) were used to connect the chips to high CTE ceramic chip carriers. Before packaging, the sensor resistances were measured by directly probing the test chip wafers. The chips were then diced, reflowed to the ceramic substrate, and then underfilled and cured. Finally, a metallic lid was attached to complete the ceramic LGA package. After every packaging step (solder reflow, underfill dispense and cure, lid attachment and adhesive cure), the sensor resistances were re-measured, so that the die stresses induced by each assembly operation could be characterized. The build-up of the die stresses was found to be monotonically increasing, and the relative severity of each assembly step was judged and compared. Such an approach also allows for various material sets (solders, underfills, TIM materials, lid metals, and lid adhesives) to be analyzed and rated for their contribution to the die stress level. After first level packaging of the chips on the ceramic chip carriers, initial experiments have been performed to analyze the effects of thermal cycling and power cycling on the die stresses. Thermal cycling of selected parts was performed from 0 to 100 C (40 minute cycle, 10 minute ramps and dwells). Power cycling of selected parts was performed by exciting the on-chip heaters on the test chips with power levels typical of microprocessor die. During the thermal/power cycling, sensor resistances at critical locations on the die device surface (e.g. die center and die corners) were recorded continuously. From the resistance data, the stresses at each site were calculated and plotted versus time. The experimental observations show some cycle-to-cycle evolution in the stress magnitudes due to material aging effects, stress relaxation and creep phenomena, and development of interfacial damage. The observed stress variations as a function of temperature cycling duration are currently being correlated with the delaminations occurring at the interfaces between the die and underfill and the die and lid adhesive. In addition, finite element models of the packages are being developed and correlated with the data.

Experimental Characterization and Viscoplastic Modeling of the Temperature Dependent Material Behavior of Underfill Encapsulants

2011 ◽  
Author(s):  
Nusrat J. Chhanda ◽  
Jeffrey C. Suhling ◽  
Pradeep Lall
Keyword(s):  
Flip Chip ◽  
Mechanical Response ◽  
Testing Machine ◽  
Production Equipment ◽  
Material Behavior ◽  
Specimen Preparation ◽  
Viscoplastic Material ◽  
Stress Strain ◽  
Release Agent ◽  
Stress Levels

In this work, the viscoplastic mechanical response of a typical underfill encapsulant has been characterized via rate dependent stress-strain testing over a wide temperature range, and creep testing for a large range of applied stress levels and temperatures. A specimen preparation procedure has been developed to manufacture 80 × 5 mm uniaxial tension test samples with a specified thickness of .5 mm. The test specimens are dispensed and cured with production equipment using the same conditions as those used in actual flip chip assembly, and no release agent is required to extract them from the mold. Using the manufactured test specimens, a microscale tension-torsion testing machine has been used to evaluate stress-strain and creep behavior of the underfill material as a function of temperature. Stress-strain curves have been measured at 5 temperatures (25, 50, 75, 100 and 125 C), and strain rates spanning over 5 orders of magnitude. In addition, creep curves have been evaluated for the same 5 temperatures and several stress levels. With the obtained mechanical property data, several viscoelastic and viscoplastic material models have been fit to the data, and optimum constitutive models for subsequent use in finite element simulations have been determined.

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