This Is AuburnElectronic Theses and Dissertations

Characterization of Moisture and Thermally Induced Die Stresses in Microelectronic Packages




Nguyen, Quang

Type of Degree

PhD Dissertation


Mechanical Engineering


Moisture has been one of the major concerns for package designers and reliability researchers. It is well-known that high humidity combined with high temperature can cause a number of failure modes to electronic devices, such as popcorn cracking, delamination, or electrochemical migration. While the fundamental knowledge of moisture effects on electronic packages has been extensively explored, it is still a challenge for researchers to fully understand the failure mechanisms associated with moisture or to numerically predict those failure modes due to the complexity of the moisture effects. In fact, it is believed that the moisture failure mechanism is the combination of material properties degradation, interfacial adhesion strength degradation, vapor pressure, and hygroscopic swelling. In this work, a study on hygroscopic properties of polymeric materials was conducted where diffusivity (D), saturated concentration (Csat) and coefficient of moisture expansion (CME) of underfill, BT substrate and molding compound were characterized. A novel methodology to measure CME was developed and successfully implemented using nanoindentation technology. A study on moisture desorption in polymeric materials was also completed. On-chip piezoresistive sensors were used to measure moisture-induced device side die stresses in Flip Chip on Laminate, Quad Flat Package (QFP) and Plastic Ball Grid Array Package (PBGA) under high humidity and high temperature conditions. The die stresses were also monitored during the subsequent drying to evaluate the reversibility of the moisture effects. After the initial 10 days of moisture exposure, moisture was found to have significant effects on the package, generating tensile die normal stresses of up to 30, 120, and 35 MPa for Flip Chip, QFP and PBGA package respectively under 85 %RH, 85 °C condition. Shear stresses however were found to be quite small relative to normal stresses. Upon the subsequent drying, it was seen in flip chip packages and PBGA package that the moisture-induced stress changes were almost fully recoverable while permanent changes in die stresses were found in QFP packages. In addition to the measurements of the moisture-induced die stresses, FEM moisture diffusion simulations were also performed to validate the experimental results. The hygroscopic properties obtained earlier were used for the FEM modeling. The numerical predictions were finally correlated with the experimental results. They were found to be in great agreement. The effects of temperature and humidity level on hygroscopic properties of polymeric materials were characterized. Diffusivities and saturated concentrations of three polymeric materials were determined at various moisture and temperature conditions ranging from 45 to 95 °C and 45 to 95% RH. Valuable observations were made on the effects of temperature and humidity level on the hygroscopic properties of these three polymeric materials. Finally, an FEM parametric study was performed to characterize the dependence of moisture induced die stresses on three hygroscopic properties of polymeric materials in the packages. Insight into how moisture induced die stresses vary with each property was provided and this can be a great tool to predict the moisture induced die stresses and therefore offer the material selection to enhance the reliability of electronic packages