Reliability and Prognostics of Copper-Aluminum Wirebond System Subjected to Extreme Ambient and Operating Conditions

Abstract

Wire bonding is predominant first level interconnect used in semiconductor packaging industry. Gold (Au) is traditionally used for wirebonding since inception. Recent increase in the cost of Au has forced industry to seek an alternative. Copper (Cu) wire has emerged as the most promising material candidate to replace incumbent Au wires. Along with being very cheap, Copper has better thermal and electric conductivity, better mechanical strength and stiffness than gold. Cu when bonded onto Aluminum (Al) pad form Inter-metallic compounds (IMCs) at the bond-pad interface. IMC growth for Cu-Al system is much slower than Au-Al system even at higher temperatures. This makes Cu wirebonds more reliable than Au wirebonds. Higher reactivity of Cu is a major issue in the implementation of Cu wirebonding into commercial packaging. Optimal window for wirebonding process is very narrow and small deviations in the process parameters can result into the poor and unreliable bond. These issues have been tacked by past researchers by undertaking various optimization studies and by using inert atmosphere for the bonding process. Cu, being prone to corrosion gets easily affected by the mechanical and chemical properties of surrounding entities. Semiconductor devices are often molded with commercial epoxy mold compounds (EMCs). EMCs contain various levels of ionic contamination introduced in various manufacturing processes. The level of contamination can adversely affect the reliability of Cu wirebond and might result into premature failures. Further, EMCc are hydrophilic in nature. Under high humidity conditions, the moisture can easily penetrate the EMC and can corrode wirebond or pad resulting into open circuit failures. These reliability concerns have stalled the transition from Au to Cu wirebonding in critical automotive, aerospace and down-hole drilling applications. Past researchers have mainly focused on statistical quantification of the Cu wirebonded parts under various environments. This involves monitoring predetermined number of packages for certain time to check for failures. failed parts are then analyzed using various techniques to identify root cause of failure. This approach does not underline progression of damage as a function of time; which is critical in understanding response of wirebond junctions to different environments. Cu wire has different failure mode and degradation mechanism than Au wires. Current literature lacks the quantification needed for redefining various accelerated life testing standards for Cu wirebonds. In this work, various failure modes and mechanisms for Cu wirebonded devices under various environmental conditions is presented in details. Effect of different high temperature and effects of various high humidity levels were separately studied and acceleration factors (AF) were established. Acid based decapsulation technique was developed to dissolve EMC surrounding wirebond without affecting integrity of the wire. Neural Network (ANN) based model was developed to understand the sensitivity of the decapsulation process parameters. Change in electric response of the wirebond was correlated with the change in morphology of bond pad interface and reduction in ball shear strength. Effect of various EMC properties such as ionic contamination, pH value, external bias on time to failure of Cu wirebonded devices was studied and quantified. All reliability test data was then compiled to develop various life prediction models to predict life expectancy of the field extracted part based on current state of electric performance and properties of the EMC. Different techniques such as ANN, Levenberg-Marquardt algorithm (LM), Extended Kalman Filtering (EKF) were used for this purpose. Thermal cycling reliability of the Cu wirebonded parts was studied using FE analysis, performed on the model generated from the computed tomography (CT) database. Finally a brief study is presented which describes different failure modes and time to failure for Gold, Copper, Silver (Ag) and Palladium coated Copper (PCC) wirebonded parts subjected to high temperature and electromigration test.