Corrosion and Protection of a Conductive Silver Paste

One of the possible uses for a conductive paste is as an adhesive in interconnect technology that could replace PbSn solder. The interconnections are expected to perform under a variety of environmental conditions, and with an applied voltage. Thus knowledge of their corrosion and dissolution resistance is of utmost importance. This is a study of the dissolution and protection of polymer/metal composite films, prepared with a high loading of silver or gold particles. Electrochemical tests were conducted in a droplet of triple-distilled water with or without benzotriazole (BTA) and BTA derivatives. Results indicate that, in spite of some protection obtained by the polymer, silver paste dissolution at high anedic potentials is rapid, reaching values of 10 -~ A/cm 2, which corresponds to a catastrophic silver removal rate of at least 35.6 nm/s. With a reservoir of azole in the corrosive environment, this rate can be reduced by up to five orders of magnitude. This azole effect greatly reduces the probabili ty of electrolytic silver migration, but the Ag dissolution rate is still higher than the anodic activity shown by Au paste under the same conditions. Introduction Insulating thermoset and thermoplastic polymers can be made conductive by loading them with metallic powder particles such as silver, copper, or gold. The resulting conductive paste has many potential applications. Such a material can be used for electrostatic discharge protection, electromagnetic interference shielding, and as adhesive for interconnect technology that could replace PbSn solder. The widespread use of Pb containing solder in electronics is an environmental concern, and regulations banning Pb are being considered. Whereas paste conductivity is the critical property in intercennection applications, the corrosion and dissolution resistance are also of the utmost importance. In order to be conductive, the paste has to have a high loading level, i.e., it should contain more than 50 weight percent (w/o) of the metal powder. Interconnections are expected to perform under a variety of environmental conditions, and with an applied voltage, where metal dissolution can result in electrolytic migration, dendrite formation, and eventually electrical shorts or opens. Thus, the susceptibility of the paste to dissolve under an applied potential indicates a potential problem for a device in operation. This is a study of the dissolution and protection of polymer/metal composite (PMC) films, prepared in our laboratory with a high loading of silver particles. Electrochemical tests were conducted in a droplet of triple-distilled water with or without benzotriazole (BTA) and BTA derivatives. Same of the patent literature, I as well as experience in our laboratory, has indicated that Ag forms a thin layer of AgBTA in a manner similar to Cu under the same conditions. Yet, the protective nature of Ag-BTA films is not well documented. Results are discussed in terms of observed protection offered by BTA and compared to the results obtained on a similar paste formulated with Au particles. Experimental The PMC samples were prepared on a glass substrate as 1.,0Q blanket films with dimensions of 1.9 • 3.2 cm and a thickI , ness range from 25 to 40 p~m. Ag or Au particles (with a ~_,oo typical loading factor of 88 w/o) were used in the preparation of the paste. Some of the variations used in the PMC ..... preparation include (i) additions of 1000 ppm BTA to the paste formulating mixture, (it) 10 min immersion into an ~ ..... aqueous solution of 10 2M BTA, and (iii) 3 min dip into an ~ 0 . 2 0 0 alcohol solution of BTA or 5CH3BTA in concentrations of 0.02 and 0.2M prior to electrochemical evaluation of the ~ -0.200 corrosion behavior in a selected electrolyte. The temperature of the alcohol-azole solution was controlled in the -o.~o0 range of room temperature to 60~ Plating tape was used to expose an area of 0.32 cm 2 for .... oo I I electrochemical testing. The electrochemical cell also contained a Pt mesh as a counterelectrode and a mercurous sulfate electrode (MSE, with saturated solution of K2SO4) as a reference electrode, that were separated by a filter * Electrochemical Society Active Member. J. Electrochem. Soc., Vol. 142, No. 8, Augus t 1995 9 The Electrochemical Society, Inc. paper. This cell design, described elsewhere, 2 was proven suitable for use with a small volume of an electrolyte, 0.01 ml or so, and with electrolytes that are normally difficult to handle or have low conductivity. Because of the closeness of the electrodes, the cell is suitable for tests in water without significant ohmic potential drop. The use of pure water as an electrolyte mimics the conditions that apply in processing (such as product rinsing) or in exposure to a humid atmosphere. The electrochemical tests consisted of corrosion potential measurement, which was interrupted in regular time intervals by measurement of linear polarization to determine the corrosion rate. After about 10 min, when the open potential reached a stable value (changing no more than 2 mV/min), a potentiodynamic sweep was applied. A sweep rate of 1 mV/s was used, starting 250 mV below the corrosion potential and reaching 1.3 V vs. MSE. In several tests, dissolution of Ag paste was examined by an application of a constant anodic potential and a continuous recording of the current, i.e., with potentiostatic control. The electrolyte was water, with or without BTA or an azole derivative. Most of the electrochemical tests were conducted within hours of the paste preparation. Some of the experiments were performed after months of sample storage in either open containers, or in closed containers containing a dry 5CHs-BTA reservoir. In a few experiments, using Ag foil, ellipsometry was used to determine the presence and thickness of Ag-azole films on the silver surface, and time-of-fl ight static secondary ion mass spectroscopy (SIMS) was employed to determine the film composition. Results and Discussion Anodic dissolution of si lver--Although noble, silver dissolves in water saturated with air at a corrosion rate of