An experimental study of electromigration in flip chip packages.

摘要

Flip chip technology, which offers advantages like smaller size, lesser weight, good I/O connection flexibility, high reliability and low cost is a well researched topic. The growing demand for smaller, lighter and more powerful components along with the ban on the use of lead in electronic assemblies has forced researchers to develop fine pitched lead-free packages. Some of the key concerns with fine pitched lead free flip chip packages are material compatibility issues regarding solder joint properties, new underfill materials properties like modulus, coefficient of thermal expansion, viscosity, surface tension, and filler content.;Electromigration occurring in solder bumps is one of the most important reliability concerns for these products. Previously, due to larger sized bumps and lower power applications, electromigration was not a reliability concern in flip chip products. The demand for increasing power in the chip along with reduced operational voltages ultimately results in an increase in the current density in the solder joint. For solder joints in a flip chip package, current densities in the range of 104 A/cm2 can cause failures due to electromigration.;The objective of this research endeavor was to determine the activation energy and current density exponent values for lead free solders by measuring the actual bump temperature rise due to Joule heating effects. A direct comparison in the lifetime of the lead free solder bumps using three different UBM structures was studied in detail.;The activation energy and current density exponent values for lead free solders using thick Cu UBM were determined. This was accomplished by measuring the actual bump temperature rise due to Joule heating effects. In this research temperature sensors were strategically placed right above the top of the solder bump to determine the actual bump temperature when current is passed through the bumps. A direct comparison in the lifetime of the lead free solder bumps using three different UBM structures was studied in detail. It was found that UBM with Cu/Ni cap UBM performed better than Cu UBM and Ni UBM during high stress conditions.