Galvanic Aluminum Bond Pad Corrosion in Copper Wire Bonded Device Assembly – Mechanism and Prevention

Abstract

The explosion of microelectronics use in automobiles has made the microelectronic corrosion control more critical over the past decade. Wire bonded devices form an integral part of the microelectronic systems used in automobiles. With the reliability requirements of automobile microelectronics pushing towards ppb levels of failure, halide induced corrosion issues have to be controlled to achieve such high reliability goals. The corrosion of Aluminum bond pads in slightly acidic chloride solutions (ppm level) has been one of the main modes of wire-bonding failure in wire bonded devices. Earlier researches pointed out the possibility of intermetallic compound formation as the main reason for this corrosion. However, our investigation of the electrochemical nature of the corrosion led to the discovery that bimetallic contact of the aluminum bond pads with copper wires is the main reason for this type of corrosion. Copper and Aluminum being apart in the galvanic series can act as a galvanic corrosion cell under suitable conditions. Furthermore, the presence of halides causes de-passivation of Aluminum oxide layer making the Aluminum bond pad more susceptible to corrosion. In this report we introduce corrosion screening as a mimicking platform for studying wire-bond device corrosion. For the first time we reported that Hydrogen evolution is the cathodic reaction and is responsible for the explosive nature of this bond pad corrosion. By selective surface treatment of the Cu wires (cathodic part), we were able to prevent the corrosion by blocking the cathodic reaction and thereby stopping the electron flow required for the corrosion cycle to continue. The prevention treatment was then applied to commercial wire-bonded device and excellent corrosion prevention was observed. The corrosion inhibition coating exhibited high thermal stability up to 260 O C. We also report a novel testing method for corrosion testing of molded wire-bonded device with internal chloride ion contamination. Electrical continuity and delamination analysis showed that the chosen method of inhibition treatment was able to prevent corrosion even in cases of high chloride contamination and severe delamination. The results of the Pressure Cooker Test showed that the inhibitor coating is suitable for prevention of corrosion in even 100% Relative Humidity environments. Work is now in progress to apply these treatments to Palladium coated Copper wire bonded devices to check corrosion prevention ability of the reported corrosion prevention treatment.