Assessment of XRF Technique as a Method to Measure Percent Ag in SnAg Solders for Flip Chip Applications

Abstract

Pb-free SnAg solder has become the industry standard for fabricating flip chip interconnects utilizing C4 (controlled collapse chip connection) technology. One area of interest for manufacturability of Pb-free solders is the ability to control and measure the %Ag composition and its variation from wafer to wafer, chip to chip, and C4 to C4. There are various ways to measure solder composition. These are divided into two categories which are invasive and non-invasive referring to whether solder must be removed from the wafer in order to conduct the measurement. There are a variety of invasive methods including Atomic Absorption (AA), Differential Scanning Calorimetry (DSC), Inductively Coupled Plasma (ICP) and Electron Probe Micro-Analyzer (EPMA) used with cross sections. Non-invasive methods are limited, making the development of the non-invasive X-Ray Fluorescence (XRF) method an important technique to determine both the thickness and composition of C4s on wafers without modifying the wafer. There are many factors which can affect the accuracy of the XRF measurements. These include bump geometry, composition, UBM (under bump metallurgy) stack, bump spatial density, underlying chip wiring, tool vibration and tool parameters, such as collimator size, power levels, scan time, etc. This paper will address the implementation issues in utilizing XRF for Pb-free solder SnAg systems. The paper will describe:(1) Experimental bumping variables,(2) XRF configuration, calibration, optimized measuring methodology and the importance of having known standards with the same dimensions of the bumps being measured(3) Measuring accuracy and correlation with ICP and DSC,(4) Ag distribution study in the die and wafer level