Conversion of the under bump metallurgy into intermetallics: the impact on flip chip reliability

Abstract In high temperature applications, the conversion of the under bump metallurgy (UBM) into UBM-Sn intermetallics can ultimately limit the reliability of flip chip components. Here, an intermetallic growth model characterizing the rate of electrical failure to the rate of UBM consumption is developed which derives a relationship between field reliability and accelerated temperature tests. For a flip chip structure employing eutectic Sn/Pb solder joined to Ni(V), bump shear and temperature storage were then used in concert with temperature cycling or high temperature operating life tests to evaluate the interaction of UBM loss with respect to electromigration and solder fatigue. In this way, field life was estimated for a given set of conditions: mechanical loading, temperature, temperature gradients, and applied current. While shear testing proved a poor indicator of bump reliability, a modified bump shear technique allowed a simple visual method for monitoring the amount of UBM consumption following accelerated testing. A seemingly unimportant variable, the surface finish of the substrate, was found to play a significant role in determining the rate of intermetallic growth, and hence, impacted the overall reliability. In particular, the incorporation of small amounts of gold into the solder joint during assembly reflow of gold finished substrates influenced the reaction of Ni into SnNi intermetallics. The role of gold, the impact on reliability, and potential corrective actions are discussed.