Corrosion and Contaminant Diffusion Multi-physics Model for Copper-aluminum Wirebonds in High Temperature High Humidity Environments

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°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. INTRODUCTION Copper wire has the advantages of the lower cost, lower thermal resistivity, lower electrical resistivity, higher mechanical strength and higher deformation stability over the gold wire. In spite of the upside above, the Cu-Al wire bond is susceptible to the galvanic corrosion and the reliability of Cu-Al wire bond is of great concern. Typical electronic molding compounds are hydrophilic and absorb moisture when exposed to humid environmental conditions. EMC may contain ionic contaminants including chloride ions as a result of the chemical synthesis of the subcomponents of the resin, etching of metallization, the decomposition of the die-attach, epichlorohydrin in the resin as a flame retardant. The presence of moisture in the operating environment of semiconductor package makes the ion more mobile in the EMC. The migration of chloride ions to the Cu-Al interface may induce galvanic corrosion inside the package causing degradation of the bond interface resulting in eventual failure. The rate at which the corrosion happens in the microelectronic packages is dependent upon the rate at which the ions transport through the EMC in addition to the reaction rate at the interface. Past research has shown that formation of excessive intermetallics at Cu-Al interface is a major contributor to failure of the wire bond accompanied by resistance increase and fracture or bond lifts [Boettcher 2011; England 2011]. Past researchers have studied the IMC growth under isothermal aging. IMC has been studied using various techniques, including the crystal structure, effect of annealing time and effect of temperature on IMC growth [Na 2011]. Composition of IMC has been studied using TEM and XRD, and reported to be CuAl2 and Cu9Al4 [Wieczorek-Ciurowa 2005; Laik 2008; Lee 2005]. Previous Proceedings of SMTA International, Sep. 27 Oct. 1, 2015, Rosemont, IL Page 14 As originally published in the SMTA Proceedings.