Transport of Corrosive Constituents in Epoxy Moulding Compounds

Epoxy moulding compounds are the leading encapsulating material in today´s microelectronic packaging industry. These compounds are hydrophilic and absorb moisture when exposed to humid environments. As a result of the absorbed moisture, ions in the material will become more mobile. This, in combination with high electrical field strengths due to the continuously decreasing feature sizes of ICs, will result in a large flux of ions toward charged interfaces, such as aluminium bond pads. Hence, ion-related failure mechanisms, such as corrosion, might become more prominent. In this paper a method for obtaining the diffusion coefficients of the different ions at elevated temperatures in saturated moulding compounds is reported. This method is based on determining the total ionic content by ion chromatography after immersing a presaturated sample in a water bath containing the ions and fitting the data to a Fickian diffusion model. The measured diffusion coefficients of NaCl in a commercially available moulding compound at 30degC and 60degC are 2.9 x 10¹³ m²/s and 9.5 x 10¹³ m²/s, respectively. These coefficients are used to compare experimental data for bond pad corrosion as function of humidity, time and bias with a multi-physics finite element model.