Yaofeng Sun

Finite element formulation for a digital image correlation method

A finite element formulation for a digital image correlation method is presented that will determine directly the complete, two-dimensional displacement field during the image correlation process on digital images. The entire interested image area is discretized into finite elements that are involved in the common image correlation process by use of our algorithms. This image correlation method with finite element formulation has an advantage over subset-based image correlation methods because it satisfies the requirements of displacement continuity and derivative continuity among elements on images. Numerical studies and a real experiment are used to verify the proposed formulation. Results have shown that the image correlation with the finite element formulation is computationally efficient, accurate, and robust.

Drop reliability study of PBGA assemblies with SAC305, SAC105 and SAC105-Ni solder ball on Cu-OSP and ENIG surface finish

Lead free SnAgCu solder joints used in surface mount packages like Ball Grid Array (BGA) have a great impact on the reliability of the end product. The mechanical properties of the solder are important factors. By changing the concentration of silver and copper, or by doping a very low portion of the fourth element (e.g. Ni), the strength of solder can be optimized. In this work, the board-level drop test reliability performance of different PBGA soldered assemblies with three different solder ball alloys (SAC305, SAC105 and SAC 105-Ni200 ppm ) and two surface finishes (Cu-OSP and NiAu) were studied. We found that SAC105-Ni showed the best impact reliability performance among the 3 types of materials, followed by SAC105 and SAC305 for either OSP or NiAu surface finish. OSP surface finish showed better drop lifetime than NiAu surface finish regardless of the type of solder used in the assemblies.

Digital image correlation for solder joint fatigue reliability in microelectronics packages

Digital image correlation (DIC) measurement of solder joint strain resulting from thermal cycling fatigue loading is reported in this study. The experimental measurements are compared to finite element analysis modeling of solder joint strain in a plastic ball grid array (PBGA) soldered assembly. The PBGA test sample was cross-sectioned and mounted in a thermal cycling chamber and subject to in-situ temperature cycling between 25 °C and 100 °C. The local solder joint strain range accumulated during temperature cycling was measured by the DIC technique. The in-plane inelastic strains in the solder joint accumulated after the 7th and the 14th thermal cycle were measured directly by the DIC system and computed by an element-based DIC analysis software developed by the author. Finite element analysis was conducted to simulate the loading used in the experiments and comparison was made to the experimental results. The measured and predicted displacement field patterns show satisfactory correlation and agreement.

Geometry dependence of gate oxide breakdown evolution [MOSFET]

The effects of geometrical arrangement of MOSFETs on breakdown (BD) evolution in ultrathin gate oxide have been studied. Specific attention was paid to the impact of heat confinement in narrow MOSFETs on the BD evolution from soft BD to hard BD. It is found that, based on a numerical simulation, the thermal effect, which is the main driving force of catastrophic BD, is more severe in narrow MOSFETs than wide MOSFETs, which is in agreement with the degradation rate measured from their respective BD transients.

Nanoscale Deformation and Strain Analysis by AFM/DIC Technique

Digital image correlation (DIC) technique with the aid of scanning probe microscopes has become a very promising tool for deformation analysis of micro- and nanoscale components. The scanner drift of the atomic force microscope (AFM) is a great disadvantage to the application of digital image correlation to micro/nanoscale deformation measurements. This chapter has addressed the image distortion induced by the scanner drifts and developed a method to reconstruct AFM images for the successful use of AFM image correlation. The proposed AFM/DIC method is to generate a corrected image from two correlated AFM images scanned at the angle of 0° and 90°, respectively. The method has been validated by two simple AFM/DIC experiments. The application of this AFM/DIC technique was demonstrated by the deformation measurement on the micro-interconnection in a micro thermoelectric cooler. AFM images of the scan region of interest were obtained separately when the microelectronic device was before and after operating at both its cooling and heating stages. The AFM images were then used to obtain the in-plane deformation fields in the observed region of the micro-assembly. AFM image correlation is performed for nanoscale deformation analysis using the authors’ AFM–DIC program. The results show that the observed region was subjected to cyclic strains when the device worked between its cooling and heating stages, and cyclic strain in the vertical direction was found to be significant deformation mode. The thermally induced deformation behavior of the micro-assembly device was modeled by finite element analysis (FEA). Both thermal-electric analysis and thermal stress analysis were conducted on a 3D finite element model of the device. It is shown that the experimental results were able to validate the finite element analysis results.

Micro- and nano-DIC deformation analysis for electronic packaging applications

This paper presents an automated digital image correlation (DIC) system for in-plane micro-deformation analysis. The system features microscale resolution, realtime thermal ramp and cycling loading, image auto-positioning and autofocus for image acquisition, and novel element-based DIC algorithm. The DIC tool is demonstrated to a micro-deformation measurement application of a solder joint in a plastic ball grid array (BGA) package. The solder joint deformation has been investigated under two loading cases, temperature ramp and temperature cycling. Finite element analysis was conducted and the U and V displacements, and calculated strain distribution in the solder joint is compared with the DIC measured result. The measured and predicted displacement fields under temperature ramp give good agreement. Research on nano-scale DIC measurements must employ an atomic force microscope (AFM) for in-situ digital imaging of in-plane deformation. It was discovered through calibration that AFM scanner drift is a intrinsic error and thus a correction methodology is required for DIC application. The application the AFM/DIC technique is used for deformation characterization of solder interconnection in a micro-thermoelectric cooler system subject to a small temperature gradient of +25C to -25C

Reliability analysis of sandwich microdisplay packaging using an integrated interferometry system

This article presents the reliability study of sandwich microdisplay packaging using an integrated interferometry system. In a microdisplay die with a gaseous cavity, the hermeticity of the microdisplay die can be tested non-destructively and the leak rate quantitatively checked to 10-7 ccatm/s by creating a pressure difference between the cavity and an external vacuum chamber. The warpage changes in the face and base plates resulting from the differential pressure was measured real-time simultaneously by the Twyman-Green and Newton Rings laser interferometry methods. Finite element analysis predicted warpage and stress distributions in the wafer package, as well as identified the key effects of the properties of components used in post wafer level packaging. The effect of elastic modulus of die attach materials on the reduction of stress and warpage build-up was studied. Failure mechanisms resulting from warpage stress were also discussed. The hermeticity and warpage analysis for sandwich microdisplay packaging contributes to the design, material selection and reliability testing of the microdisplay packages.