Analysis on interfacial failures of ultra-fine pitch wire with low inter-metallic coverage

Ultra-fine pitch Au wire has become inevitable in the electronic packaging industry due to decreasing electronic package dimension and increasing cost of gold. However, fine pitch Au wire with diameter less than 0.5 mil. (12 ¿m), bonded on Al pad, showed low inter-metallic coverage, and it caused distinctive interfacial failures: the oxidation of Au₄Al and overgrowth of inter-metallic compound (IMC). In order to enhance the interfacial failure, addition of Pd was proposed due to its known effect as the diffusion barrier. In this study, 0 ppm, 3000 ppm, 6000 ppm, and 10000 ppm of Pd was added to 0.5 mil. Au wire as alloying element and subsequently aged under high temperature storage test (HTST) in air and vacuum condition at 175°C. By the observation of Au/Al interface using scanning electron microscope (SEM) and reliability test using ball pull test (BPT), it was confirmed that major failure mode was the oxidation of Au₄Al for air-aging and overgrowth of IMC for vacuum-aging; however, it was observed that the interfacial failures were affected by the addition of Pd. For air-aging, the oxidation rate was in order of 10000 ppm (fastest) > 0 ppm = 6000 ppm > 3000 ppm (slowest) of Pd showing that the interfacial failure due to the oxidation was affected by Pd contents. For vacuum-aging, higher Pd contents decreased overgrowth of IMC and number of ball-lift. Detailed effects of Pd for the oxidation rate were analyzed using transmission electron microscope (TEM). Au wire with 2500 ppm of Pd was analyzed as most oxide-stable Pd composition, and was observed to form segregation of Pd mass at the grain boundary of Au₄Al, and the segregation protected Au₄Al from the oxidation. However, the sample with 10000 ppm of Pd made separate Pd-rich grain, and it didn´t prevent the oxidation and even caused quicker oxidation than the sample without Pd.