Nusrat J. Chhanda

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

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.Copyright

Impelementatoin of a viscoelastic model for the temperature dependent material behavior of underfill encapsulants

In this work, the viscoelastic mechanical response of a typical underfill encapsulant has been characterized via rate dependent stress-strain testing over a wide temperature range, and via creep testing for a large range of applied stress levels and temperatures. The 60 × 3 × 0.5 mm test specimens were 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. The manufactured test specimens were used to evaluate the stress-strain and creep behavior of the underfill material as a function of temperature through testing in a microscale tension-torsion testing machine. Stress-strain curves have been measured at 5 temperatures (25, 50, 75, 100 and 125 C), and strain rates spanning over 4 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, a three-dimensional linear viscoelastic model based on Prony series response functions has been applied to fit the stress-strain and creep data, and excellent correlation has been obtained. The viscoelastic model for underfill has also been implemented in finite element analysis. A quarter model of a flip chip on laminate assembly has been developed for the analysis, and the underfill was modeled as both an elastic-plastic material and as a viscoelastic material. The time dependent variations of the stresses in the underfill and silicon die obtained with the viscoelastic model have been compared to the time-independent results from the conventional elastic-plastic material model.

Effects of Moisture Exposure on the Mechanical Behavior of Polymer Encapsulants in Microelectronic Packaging

Polymer encapsulants exhibit evolving properties that change significantly with environmental exposures such as moisture uptake, isothermal aging and thermal cycling. In this study, the effects of moisture adsorption on the stress-strain behavior of a polymer encapsulant were evaluated experimentally. The uniaxial test specimens were exposed in an adjustable thermal and humidity chamber to combined hygrothermal exposures at 85 °C/85% RH for various durations. After moisture preconditioning, a microscale tension-torsion testing machine was used to evaluate the complete stress-strain behavior of the material at several temperatures. It was found that moisture exposure caused plasticization and strongly reduced the mechanical properties of the encapsulant including the initial elastic modulus and ultimate tensile stress. Reversibility tests were also conducted to evaluate whether the degradations in the mechanical properties were recoverable. Upon fully redrying, the polymer was found to recover most but not all of its original mechanical properties. As revealed by FTIR, some of the adsorbed water had been hydrolyzed in the organic structure of the epoxy-based adhesive, causing permanent changes to the mechanical behavior.Copyright

The influence of aging on the creep behavior of underfill encapsulants

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, the material behavior changes occurring in flip chip underfill encapsulants (silica filled epoxies) have been characterized for isothermal aging at four different temperatures that are below, near, and above the Tg of the material. A microscale tension-torsion testing machine has been used to evaluate the creep behavior of the underfill material at several temperatures, after various durations of environmental exposure. A novel method has been developed to fabricate underfill uniaxial test specimens so that they accurately reflect the encapsulant layer present in flip chip assemblies. Using the developed specimen preparation procedure, samples were prepared and isothermally aged for up to 10 months at 80, 100, 125, and 150 °C. Creep tests were then performed on both non-aged and aged samples at three different elevated temperatures where creep is significant (80, 100, and 125 °C). The changes in mechanical behavior were recorded for the various aging temperatures and durations of isothermal exposure. Empirical models have been developed to predict the evolution of the creep strain rate as a function of temperature, aging time, and aging temperature.

Experimental Characterization of Underfill Materials Exposed to Moisture Including Preconditioning

Microelectronic encapsulants exhibit evolving properties that change significantly with environmental exposures such as isothermal aging and high humidity conditions. In this work, the material behavior changes occurring in underfill materials subjected to moisture exposures in an humidity chamber have been characterized using 60 × 3 × 0.5 mm uniaxial test specimens which were cured with production equipment using the same conditions as those used in actual flip chip assembly. After curing, the samples were divided into two groups and subjected to different preconditioning: (1) no preconditioning, (2) prebaking at 85 C for 24 hours. The fabricated and preconditioned uniaxial test specimens were then exposed in an adjustable thermal and humidity chamber to combined hygrothermal exposures at 85 C and 85% RH for various durations (0, 1, 3, 10, 30, 60 days). After the moisture exposures, a microscale tension-torsion testing machine was used to evaluate the complete stress-strain behavior of the material at room temperature (25 C). In addition, the viscoelastic mechanical response of the underfill encapsulant has also been characterized via creep testing at room temperature for several applied stress levels after the moisture exposures. From the recorded results, it was found that the moisture exposures strongly degrade the mechanical properties of the tested underfill including the initial elastic modulus, ultimate tensile stress, and tensile creep rate. Prebaking was found to increase the initial material properties, but the degradations due to subsequent moisture exposures occurred in a similar manner.Copyright

Effects of moisture exposure on the mechanical behavior of flip chip underfills in microelectronic packaging

Reliable, consistent, and comprehensive material property data are needed for microelectronics encapsulants for the purpose of mechanical design, reliability assessment, and process optimization of electronic packages. Since the vast majority of contemporary underfills are epoxy based, they have the propensity to absorb moisture, which can lead to undesirable changes in their mechanical and adhesion behaviors. In this study, the effects of moisture adsorption on the stress-strain behavior of an underfill encapsulant were evaluated experimentally and theoretically. A novel specimen preparation procedure has been used to manufacture 60 × 3 mm uniaxial tension test samples, with a specified thickness of 0.5 mm. The test specimens were dispensed and cured with production equipment using the same conditions as those used in actual flip chip assembly, and no release agent was required to extract them from the mold. The fabricated uniaxial test specimens were then exposed in an adjustable thermal and humidity chamber to combined hygrothermal exposures at 85 C and 85% RH for various durations. After moisture preconditioning, a microscale tension-torsion testing machine was used to evaluate the complete stress-strain behavior of the material at several temperatures (T = 25, 50, 75, 100 and 125 C). The viscoelastic mechanical response of the underfill encapsulant has also been characterized via creep testing for a large range of applied stress levels and temperatures before moisture exposure. From the recorded results, it was found that the moisture exposures strongly affected the mechanical properties of the tested underfill including the initial elastic modulus and ultimate tensile stress. With the obtained mechanical property data, a three-dimensional linear viscoelastic model based on Prony series response functions has been applied to fit the stress-strain and creep data, and excellent correlation had been obtained for samples with and without moisture exposure. The effects of moisture were built into the model using the observed changes in the glass transition temperature within the WLF Shift Function.

Parametric Analysis of MEMSBased Heat Exchanger with Different Test Fluids

The latest generations of micro-scale systems are becoming more challenging to fit into designs. These chips are squeezing into smaller and smaller spaces with very little place for heat to escape. Therefore, MEMS heat exchanger has become popular in many practical applications although improvement of heat transfer characteristics is a key issue for the users as well as researchers. In the present study it is suggested that instead of using conventional working fluids, the micro sized hot structures can be cooled with an effective coolant which can be a good substitute of the conventional fluids. Ammonia has shown the highest outlet mean temperature during the study. The analysis is conducted using commercial finite element package to determine outlet mean temperature tha t is then used for further calculation of effectiveness, heat transfer coefficient and friction factor.

Effects of moisture exposure on the mechanical behavior of polycarbonate materials used in electronic packaging

The mechanical properties of polymer materials are often a key concern of the microelectronic packaging industry. The theoretical analysis of stress, strain, and deformation induced in electronic assemblies due to environmental exposures such as moisture adsorption, isothermal aging, and thermal cycling require the complete characterization of mechanical properties and constitutive behavior of the constituent materials. In this work, an experimental investigation has been performed on the effects of moisture adsorption on the stress-strain behavior of polycarbonate materials used in electronic packaging. Uniaxial test specimens were exposed in a controlled temperature and humidity chamber to combined hygrothermal exposures at 60 C and 90% RH, 60 C and 50% RH, and 40 C and 50% RH for various durations. After moisture preconditioning, a microscale tension-torsion testing machine was used to evaluate the complete stress-strain behavior of the material at several temperatures (T = 20 C, 40 C, and 60 C). It was found that moisture exposure strongly affected the mechanical properties of the tested polycarbonate, especially ultimate strain limit. Reversibility tests were also conducted to evaluate whether the degradations in the mechanical properties were recoverable. Upon fully redrying, the material was found to recover most of its original mechanical properties. In addition, optical microscopy was utilized to examine the fracture surfaces of the failed specimens, and observe the influence of moisture exposure.