Influence Of Intermetallic Thickness And Elastic Modulus On Passivation Thermal Stress
Authors: Raj Sekar Sethu and How Ung Ha, Ph.D. and Kok Heng Soon, Ph.D. Company: X-FAB Semiconductor Foundries AG and Swinburne University of Technology Sarawak Date Published: 2/6/2017
Pan Pacific Symposium
Abstract: Semiconductor bond pad lifting precipitated by cracking of the passivation nitride during thermal reflow cooling of solder balls was first investigated using Scanning Electron Microscopy (SEM), Focused Ion Beam (FIB) and Energy Dispersive X-Ray spectroscopy (EDX) failure analysis techniques. It was determined that Intermetallic Compound (IMC) thickness varied between 400 to 1000 nm; and from literature reviews, the IMC’s elastic modulus ranges from 100 to 180 GPa. Constant monitoring of these characteristics is challenging. Therefore, a finite element analysis (FEA) model was built to predict the passivation nitride’s resultant stress. The Tresca failure criterion based on the maximum principal stress was used to predict the brittle fracture failure mode experienced by the passivation nitride. The thermal stress boundary condition is the cooling of solder ball from peak reflow temperature to room temperature. The FEA simulation mapping of the first principal stress distribution successfully matched the stress concentration area with actual crack location from FIB-SEM images.
The effect of IMC thickness and elastic modulus was investigated using factorial design of experiments (DOE) technique. Through simulation, it was determined that increase in both IMC thickness and elastic modulus increases the resultant first principal stress of the passivation nitride during reflow cool down.