Leon Stiborek

Effect of Sn grain structure on electromigration reliability of Pb-free solders

The anisotropic properties of the Sn crystal play an important role in the electromigration (EM) reliability of Sn-based Pb-free solder interconnects in flip chip packages. This study investigates the effect of the Sn grain structure on the EM lifetime and statistics of Pb-free solder bumps. The electron backscattering diffraction (EBSD) technique was applied in this study to characterize the Sn grain size and the grain orientation of the Pb-free solders. Results of failure analyses on Sn2.5Ag solders after EM tests showed that the Sn structure is important in controlling in the kinetics of the intermetallic compound (IMC) growth and void formation under EM. Further microstructural analysis revealed that the grain sizes and orientations of Sn2.5Ag solders with multiple solder reflows were statistically different from those with a single solder reflow and resulted in an improved EM lifetime and statistics. This demonstrates that the EM reliability of Pb-free solders can be improved by optimization of the Sn grain structure.

Effect of compatibility between fluxes and cyanate ester underfill on lead free flip chip assembly and reliability

The quality and reliability of flip-chip assembly is severely impacted by the compatibility between various materials used in the package. Currently, no-clean fluxes are widely used for the assembly of flip chips. Poor compatibility between the flux residue and the underfill can lead to the formation of voids, and consequently, reliability problems. Therefore, a major concern for flip-chip assembly is the compatibility between the flux residues and the underfill. The principal objective of this research was to develop a flux for lead-free packaging, which would be compatible with high performance moisture resistant cyanate ester-based underfills. During this study, commercially available fluxes, along with tailor made epoxy-based flux, were tested for their compatibility with the underfill. The results indicated that the assembly process window for the rosin-based fluxes was much wider than the epoxy-based fluxes. Synthetic flux had poor soldering performance and relatively poor compatibility with the cya...

Microwave Materials Dielectric Measurements

Texas Instruments Defense Systems and Electronics Group develops radars, missile systems, infra-red systems and electronic systems for the Defense Department. Materials having special microwave properties are needed for radar systems, missile systems that home on microwave emissions, microwave emitter locator systems, and jamming systems. The microwave materials appear in the form of antennas, lenses, radomes, windows, dielectric loading materials, potting materials, dielectric transmission lines, microwave absorbing materials, adhesives, sealants, coatings, and foams. Naturally, the complex permittivity and permeability of these materials need to be known to enhance their application and control their properties in manufacturing. Texas Instruments has a microwave materials test laboratory that includes permittivity and permeability measurements as well as physical and thermal properties. The samples come in many different forms, measurement at different frequencies are needed and the electrical properties cover a wide span (conductors to dielectrics and foams to ferrites). For this reason, many different methods of dielectric and magnetic measurement are used. These various methods will be discussed below.