Hohyung Lee

Optimal Material Properties of Molding Compounds for MEMS Package

In this paper, an optimization study of molding compounds for a microelectromechanical systems (MEMS) sensor package has been performed. A comprehensive finite element analysis model was established for the MEMS sensor package to assess the stresses and deformations when the package was subjected to temperature loading. A series of stress relaxation tests were performed to characterize the viscoelastic material properties of a molding compound over temperature with dynamic mechanical analysis. A master curve for the molding compound was constructed by a proper shift function and the Prony pairs were obtained by curve fitting to be implemented in the simulation. To validate the simulation result, the thermal behavior of the MEMS package was measured. The digital image correlation technique was employed to observe the real-time deformation of the package exposed to temperature loading. The out-of-plane deformation of the package was compared with the simulation result. With the validated simulation model, the optimization study was conducted. By the process simulation, it has been shown that most of the thermal stress on the MEMS sensor chip was generated during the cooling process. Thus, a detailed cooling profile was developed by the transient heat analysis and applied to the parametric study. The modulus, coefficient of thermal expansion (CTE), and glass transition temperature (Tg) of molding compound were investigated. The result shows that a low modulus, low CTE, and low Tg molding compound can minimize the thermal stress on MEMS sensor die.

Investigation of Stress in MEMS Sensor Device Due to Hygroscopic and Viscoelastic Behavior of Molding Compound

The stresses due to moisture saturation on microelectromechanical systems (MEMS) sensor devices after exposure to temperature cycling have been addressed. Moisture-, temperature-, and time-dependent material property of molding compounds for the MEMS devices were characterized. To determine the coefficient of hygroscopic swelling of a molding compound and diffusivity (D) of water in the molding compound, dimensional change and weight loss of moisture saturated samples at various temperatures were monitored by the digital image correlation method combined with a weight scale. To obtain the viscoelastic property of the molding compound, a series of stress relaxation tests was performed using dynamic mechanical analysis (DMA). To explain the moisture-induced viscoelastic behavior, a simple assumption was introduced based on the temperature of glass transition point (Tg) shift from the DMA result. The experimental data were utilized in numerical simulations to estimate the temperature- and moisture-induced stress on MEMS sensor devices subjected to temperature cycles.

DYNAMIC ANALYSIS OF THIN GLASS UNDER BALL DROP IMPACT WITH NEW METRICS

Response of brittle plate to impact loads has been the subject of many research studies [1–7]. Specifically, glass presents a wide variety of applications in daily life, and helps to protect the displays of smartphones, tablets, PCs, and TVs from everyday wear and tear. Therefore, the necessity of glass to resist scratches, drop impacts, and bumps from everyday use leads to the importance of investigation of the glass response under dynamic impact loading. The ball drop test has been applied in the past, specifying an energy threshold as a prediction metric. Use of energy as the key parameter in impact testing is limited, since it does not account for the time spent in contact during the impact event. This study attempts to establish a reliable metric for impact testing based on a momentum change threshold. The deformation and the strain of the glass will be obtained by the Digital Image Correlation (DIC) system, while the rebound velocity will be measured with the high speed cameras. The global and local measurements are conducted to verify the accuracy of the experimental results. Finally, the FEA model is developed using ANSYS/LS-DYNA to provide a comprehensive understanding of the dynamic response of the glass. Excellent correlation in deflection is obtained between the measurements and predictions.Copyright

A new in-situ warpage measurement of a wafer with speckle -free digital image correlation (DIC) method

During microelectronic manufacturing, a wafer undergoes many fabrication processes. Such processes include exposing a wafer to high temperature. Significant residual stresses are built up in a wafer and they are manifested as warpage of a wafer. Understanding the wafer behavior, especially warpage, during reflow process becomes one of the most important things in developing assembly process. Three Dimensional (3-D) digital image correlation (DIC), as a non-contact optical deformation measurement method, documents both in-plane and out-of plane deformation. The in-situ measurement capability allows to understand a wafer (or a wafer with multiple chips) behavior during reflow (or assembly) process. However, an object such as a monotonic surface or a mirror like wafer needs artificial speckles on the sample surface to be discernable and measured by DIC technique. To avoid contamination on wafer for surface treatment, a speckle-free 3D DIC method is proposed and its effectiveness is investigated. This method introduces an optical pattern projection method and subsequent processes in documenting topography of a wafer placed in an environmental chamber to mimic a reflow process. To validate the new method, warpage measurement result is compared with the measurement from optical profiler, a white light interferometer, and its accuracy and sensitivity are assessed quantitatively.

Stress relaxation test of molding compound for MEMS packaging

Polymer based materials are widely used in electronic packaging. The molding compound, in particular, comprises a significant portion of the package with the purpose of protecting the chips from the environment. Material characterization of molding compounds, therefore, has been a critical issue in predicting the thermo-mechanical behavior and reliability of electronic packaging. One of the distinctive features of polymers is viscoelasticity, which refers to an intermediate behavior between a solid and a liquid. To characterize time and temperature dependent characteristics of polymers, various test methods have been utilized. Among those methods, the stress relaxation test using dynamic mechanical analysis (DMA) is widely used. However, there are no standards or guidelines for performing stress relaxation test on molding compounds with DMA. In this study, DMA stress relaxation tests have been performed with the molding compound. The initial value of relaxation modulus from DMA was compared with the Youngs modulus from tensile test. The temperature effect on the stress relaxation test was studied to determine the appropriate temperature profile. The sample thickness and strain dependency were also investigated. Finally, recommendations for proper future testing are proposed.

Reliability Assessment of Preloaded Solder Joint Under Thermal Cycling

The ever increasing power density in modern semiconductor devices requires heat dissi-pation solution such as heat sink to remove heat away from the device. A compressiveloading is usually applied to reduce the interfacial thermal resistance between packageand heat sink. In this paper, both experimental approaches and numerical modeling wereemployed to study the effect of compressive loading on the interconnect reliability underthermal cycling conditions. A special loading fixture which simulated the heat sink wasdesigned to apply compressive loading to the package. The JEDEC standard thermalcycle tests were performed and the resistance of daisy chained circuits was in situ meas-ured. The time to crack initiation and time to permanent failure were identified separatelybased on in situ resistance measurement results. Failure analysis has been performed toidentify the failure modes of solder joint with and without the presence of compressiveloading. A finite element based thermal-fatigue life prediction model for SAC305 solderjoint under compressive loading was also developed to understand the thermal-fatiguecrack behaviors of solder joint and successfully validated with the experimental results.[DOI: 10.1115/1.4007674]

Creep Strain Measurement of an Actual Leadfree Solder Interconnect Using Digital Image Correlation

Creep behavior of the solder joint is one of the critical concerns in reliability of the electronic package, especially above half of the homologous temperatures. Considering melting point of the lead free solder alloys, creep deformation can be dominantly occur even at room temperature.Among the three phase of the creep curve, the steady state strain rate in secondary phase is the most important design parameter to predict life time of the solder joint. To measure the strain rate of the solder during the creep deformation, several studies conducted creep test using bulk solder samples. However, mechanical properties of the bulk solder are different from actual solder joint due to its microstructure and intermetallic layer. Thus, creep behavior of the actual solder ball need to be measured to obtain the realistic creep data. In this study, the constant creep rate of the actual solder ball in ball grid array (BGA) packages is measured using 2D Digital Image Correlation (DIC) measurement technique.Three lead free BGA packages, SAC105, SAC305 and Sn/Cu/Ni alloy (SCN) are used for the creep test. While 9.78 N, 19.56 N, and 29.34 N of the compressive loads are applied at 20°C, 40°C, 60°C and 90°C to each type of the specimen, a microscopic images of the cross sectioned surface of the solder ball are captured. The series of images are sequentially taken during the predetermined time interval and transferred to DIC software to generate full-filled surface displacement data. From the displacement data, constant creep rate of the actual solder ball is acquired.Copyright

Moisture Diffusion and Hygroscopic Swelling of Adhesives in Electronics Packaging

This paper presents a continuation of the previous hygroscopic characterization work on epoxy molding compounds, which aims to accurately measure the diffusivity and coefficient of hygroswelling (CHS) of a die-attach adhesive in MEMS packages. We obtained the CHS of the adhesives from 25 °C to 150 °C through modified DIC quick scanning approach [1-2], which includes strain measurement by digital image correlation method and correction of weight loss through the Finite Element (FE) analysis. The CHS shows a dependence on temperature above the water evaporation temperature and ranges from 0.4 to 0.55 (strain/percentage weight). The test accuracy of CHS was validated through a comparison with the correlation method [3] of in situ hygroswelling and weight loss. From moisture absorption and desorption curves of the adhesive samples, we obtained the diffusivities at different temperatures. A notable finding is that bulk samples follow the Ficks diffusion model while the thin-film samples demonstrate a non-Fickian behavior. By adding a concentration-dependent coefficient to the diffusivitys expression, the non-Fickian model can also be implemented in the FE models. As an outcome, a thorough guideline for measurement procedure is provided for hygroscopic characterization in electronics packaging.

MEMS Die Warpage During Curing of Die Attach Material

Microelectromechanical system (MEMS) packages are vulnerable to stresses due to its functional structure. During the assembly process of the package, stresses stemming out of CTE mismatches of the structural elements and curing of the die attach material can cause warpage of the MEMS die [1]. Even though die attach material takes relatively small volumetric portion of the package, it plays a critical role in warpage of the die due to its location and sensitivity of a MEMS sensor.Most of virgin die attach adhesives are in a state of viscous liquid and, as it is cured the material properties such as modulus and CTE change. Accordingly, residual strain is cumulated on MEMS die after curing process and signal trim process is required. Therefore, the material properties changes depending on the curing profile is valuable information for assembly process of the MEMS package.To monitor the material properties changes and shrinkage during curing process, strain and modulus of a die attach material are measured in each curing step. Also, to investigate the material property change depending on the curing profile, two different curing profiles are used.Experimental data show that die attach materials are gradually cured after each thermal cycling, which cause the increment of the modulus and glass transition temperature (Tg) with shrinkage at elevated temperature. Using the measurement data, FEA model is built to predict the warpage of the MEMS die. In the FEA model, residual strain on MEMS die is calculated by inputting material properties of die attach in each curing step. Also, die warpage of the package during the curing process is monitored using an optical profiler for the validation of the simulation results.Copyright

An Experimental and Numerical Study of the Dynamic Fracture of Glass

The needs of glass to resist the scratches, drops impact, and bump from everyday use lead to the importance of investigation of the glass fracture under dynamic impact loading. The strength of the glass under dynamic fracture conditions is significantly larger than that under quasi-static loading. There are several theoretic models. In this study, an accumulated damage model is implemented. The relation among the stress, loading rate, contact time and the fracture is investigated. The effect of impact area, impact energy and impact momentum on the glass fracture has been proved to further improve the dynamic fracture criterion of glass. For the experimental studies, the Digital Image Correlation (DIC) method enables one to obtain the first principal strain of the glass during the impact process. Moreover, the FEA model is developed in ANSYS/LS-DYNA™.