Shidong Li

Thermomigration induced degradation in solder alloys

Miniaturization of electronics to the nanoscale brings new challenges. Because of their small size and immense information and power processing capacity, large temperature gradients exist across nanoelectronics and power electronics solder joints. In this paper, a fully coupled thermomechanical-diffusion model is introduced to study the thermomigration induced strength degradation. A nonlinear viscoplastic material model with kinematic and isotropic hardening features is utilized. The model takes into account microstructural evolution of the material. A grain coarsening capability is built into the model to study its influence on thermomigration in solder alloys. The model is validated by comparing the simulation results with experimental data.

Lattice Strain Due to an Atomic Vacancy

Volumetric strain can be divided into two parts: strain due to bond distance change and strain due to vacancy sources and sinks. In this paper, efforts are focused on studying the atomic lattice strain due to a vacancy in an FCC metal lattice with molecular dynamics simulation (MDS). The result has been compared with that from a continuum mechanics method. It is shown that using a continuum mechanics approach yields constitutive results similar to the ones obtained based purely on molecular dynamics considerations.

Damage Mechanics of Low Temperature Electromigration and Thermomigration

Electromigration (EM) and thermomigration (TM) are processes of mass transport which are critical reliability issue for next generation nanoelectronics. The purpose of this project is to study the influence of low temperature on EM and TM interaction. In this paper, a model for EM and TM process is proposed and has been implemented in finite element method. The governing equations include mass conservation, force equilibrium, heat transfer, and electricity conduction equations. A damage evolution model based on thermodynamics is introduced to evaluate the degradation in solder joints subjected to high current densities and high temperature gradients. The results are compared with experimental data to validate the model.

Electromigration time to failure of SnAgCuNi solder joints

Electromigration time to failure and electrical resistivity of 95.5%Sn–1.5%Ag–0.5%Cu–0.03W%Ni microelectronics solder joints have been investigated experimentally. A Black-type electromigration time to failure equation is developed to describe the time to failure versus current density and temperature. It is observed that resistance of a solder joint is not just a function of the temperature but also a function of the current density. The activation energy over the range of 83–174°C is measured to be 0.77±0.12eV, and the current density exponent is found to be (8.60±1.65). It is also shown that the most commonly used Black’s electromigration time to failure equation cannot be used for solder joints.

Thermomigration in lead-free solder joints

In the next generation nanoelectronics and SiC based electronic packaging, current density and temperature gradient will be larger in orders of magnitude. Electromigration and thermomigration are considered to be major road blocks leading to realisation of nanoelectronics and SiC based high temperature power electronics. In this paper, damage mechanics of 95.5Sn4Ag0.5Cu (SAC405) lead-free solder joints under high temperature gradients have been studied. This paper presents observations on samples which were subjected to 1000°C/cm thermal gradient for two hours, 286 hours, 712 hours and 1156 hours. It was observed that samples subjected to thermal gradient did not develop a Cu3Sn intermetallic compound (IMC) layer at the hot side due to Cu migration to the cold side thus causing insufficient Cu mass concentration to form Cu3Sn. On the other hand, in samples subjected to isothermal annealing exhibited IMC growth. In samples subjected to thermomigration, near the cold side the Cu concentration is significantly higher, compared to hot side. Extensive surface hardness testing showed increase in hardness from the hot to cold sides, which indicates vacancy migration and Sn grain coarsening are in the opposing direction.

Mean time to failure of SnAgCuNi solder joints under DC

Electromigration time to failure and electrical resistivity of 95.5%Sn-1.5%Ag-0.5%Cu-0.03W%Ni (SACN) microelectronics solder joints have been investigated experimentally. A Blacks type electromigration time to failure equation is developed to describe the time to failure versus current density and temperature. The activation energy over the range of 83°C~174°C is measured to be 0.77±0.12eV, and the current density exponent is found to be (8.60±1.65). It is also shown that the most commonly used Blacks electromigration time to failure equation cannot be used for solder joints.

Thermomigration induced strain field simulation for microelectronic lead free solder joints

Many factors such as high intensity current, thermal load, shock load, vibration load and etc., can induce the failure in electronic equipment. It is common for electronic equipment to be subjected to a combination of the loads mentioned above simultaneously. In this paper, qualitative finite element simulations of thermomigration induced strain fields in lead free solders are conducted using a fully coupled displacement-diffusion model [1] with nonlinear mechanical material properties. The solutions are discussed and compared to experimental data as well as theoretical developments from literature.© 2006 ASME