Solid-state reaction in an Au wire connection with an Al-Cu pad during aging

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)Au4, (Al,Cu)3Au8, and (Al,Cu)Au2, were found between the Au wire and the Al-Cu pad in the as-assembled wire bond. After 168 hr of aging, Al3(Au,Cu)8 formed between (Al,Cu)3Au8 and (Al,Cu)Au2 in the center of the wire bond. In fact, the Al-Cu pad, (Al,Cu)Au2, and Al3(Au,Cu)8 IMCs were completely reacted after 500 hr of aging. (Al,Cu)3)Au8 was thus transformed into (Al,Cu)Au4. Near the edge of the wire bonds, (Al,Cu)Au2 formed between the Al-Cu pad and (Al,Cu)3Au8 during 500 hr of aging. For aging longer than 1000 hr, Al3(Au,Cu)8 was detected between (Al,Cu)3Au8 and (Al,Cu)Au2. It was noted that the Al3(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)Au4 and (Al,Cu)3Au8 in the center of wire bonds were also evaluated and discussed.

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