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Corrosion and Contaminant Diffusion Multi-Physics Model
Corrosion and Contaminant Diffusion Multi-Physics Model
Models for copper interconnect degradation are needed for life prediction modeling to ensure 10-year, 100,000 mile reliability for automotive applications.
Analysis Lab

Authored By:
Pradeep Lall and Yihua Luo
Auburn University
NSF-CAVE3 Electronics Research Center
Department of Mechanical Engineering
Auburn, AL, USA

Luu Nguyen
Texas Instruments, Inc,
Santa Clara, CA, USA
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Summary
Copper aluminum interconnects are being used in automotive applications for deployment underhood, onengine and on-transmission. Electronics is widely used for enabling safety function including lane departure warning systems, collision avoidance systems, antilock braking systems, and vehicle stability systems. Models for copper interconnect degradation are needed for life prediction modeling to ensure 10-year, 100,000 mile reliability for electronics in automotive applications.

Small concentrations of chloride ions may diffuse towards the bond pad interface under temperature, humidity, and electrical bias. The chloride ions may act as a catalyst breaking down the passivation layer of aluminum pad and accelerate the micro-galvanic corrosion at the copperaluminum leading to the failure of the wirebond. Models for prediction of the diffusion of the chloride ions and the corrosion of the copper-aluminum interface have been difficult to develop, because of the small scale of the interface and the lack of appropriate electro-chemical properties for the Cu-Al system and the Electronic Molding Compounds under conditions relevant to operation.

In this effort, a multiphysics model for galvanic corrosion in the presence of chloride has been presented. The contaminant diffusion along with the corrosion kinetics has been modeled. In addition, contaminated samples with known concentration of KCl contaminant have been subjected to the temperature humidity conditions of 130 degrees C/100RH. The resistance of the Cu-Al interconnects in the PARR test have been monitored periodically using resistance spectroscopy.

The diffusion coefficients of chloride ion has been measured in the electronic molding compound at various temperatures using two methods including diffusion cell and inductively coupled plasma (ICPMS). Moisture ingress into the EMC has been quantified through measurements of the weight gain in the EMC as a function of time. Tafel parameters including the open circuit potential and the slope of the polarization curve has been measured for both copper, aluminum under different concentrations of the ionic species and pH values in the EMC.

The measurements have been incorporated into the COMSOL model to predict the corrosion current at the Cu-Al bond pad. The model predictions have been correlated with experimental data.
Conclusions
In this paper a multiphysics model for galvanic corrosion in the presence of chloride has been developed. The diffusion coefficients of chloride ion has been measured in the electronic molding compound at various temperatures using two methods including diffusion cell and inductively coupled plasma (ICPMS). Moisture ingress into the EMC has been quantified through measurements of the weight gain in the EMC as a function of time.

Tafel parameters including the open circuit potential and the slope of the polarization curve has been measured for both copper, aluminum under different concentrations of the ionic species and pH values in the EMC. Electrochemical polarization tests on aluminum and copper indicates the galvanic corrosion between copper and aluminum is more likely to happen in the alkaline condition than in acidic condition.

SEM/EDS analysis shows that the ionic diffusion in EMCs is due to interfacial diffusion and degradation of EMCs under high temperature results in the loss of binding materials. The contaminant diffusion along with the corrosion kinetics has been modeled. The measurements have been incorporated into the COMSOL model to predict the corrosion current at the Cu-Al bond pad. The model uses moving boundary to keep track of the development of corrosion as time proceeds.

The model also show the gradual local alkalization at bond pad interface as the galvanic corrosion develops. The model predictions have been correlated with experimental data. In addition, contaminated samples with known concentration of KCl contaminant have been subjected to the temperature humidity conditions of 130 degrees C/100RH. The resistance of the Cu-Al interconnects in the PARR test have been monitored periodically using resistance spectroscopy. Model predictions indicate that the pH values in the vicinity of the Cu-Al wirebond continue to evolve as a function of time.
Initially Published in the SMTA Proceedings
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