Abstract
In integrated-circuit packages, wire-bonding techniques are the preferred methods for making electrical connections between the chip and the lead frame. The influence of aging at 150°C up to 3,000 hr on interfacial reactions of Au wire bonded with the Al-Cu pad was investigated herein. To observe various intermetallic compounds (IMCs) with field-emission scanning electron microscopy, polished samples were ion-milled through precision etching and coating techniques. Three IMCs, i.e., (Al,Cu)Au 4 , (Al,Cu) 3 Au 8 , and (Al,Cu)Au 2 , were found between the Au wire and the Al-Cu pad in the as-assembled wire bond. After 168 hr of aging, Al 3 (Au,Cu) 8 formed between (Al,Cu) 3 Au 8 and (Al,Cu)Au 2 in the center of the wire bond. In fact, the Al-Cu pad, (Al,Cu)Au 2 , and Al 3 (Au,Cu) 8 IMCs were completely reacted after 500 hr of aging. (Al,Cu) 3 Au 8 was thus transformed into (Al,Cu)Au 4 . Near the edge of the wire bonds, (Al,Cu)Au 2 formed between the Al-Cu pad and (Al,Cu) 3 Au 8 during 500 hr of aging. For aging longer than 1000 hr, Al 3 (Au,Cu) 8 was detected between (Al,Cu) 3 Au 8 and (Al,Cu)Au 2 . It was noted that the Al 3 (Au,Cu) 8 IMC gradually grew with aging. With the aid of microstructure evolution and quantitative analysis, the interfacial phase transformation between the Au wire and the Al-Cu pad could be probed. In addition, the growth kinetics of (Al,Cu)Au 4 and (Al,Cu) 3 Au 8 in the center of wire bonds were also evaluated and discussed.