The gold-aluminum (Au-Al) system is the most common in microelectronics. This method utilizes the metallization of aluminum to create a bond and does not require any additional processing steps. Au-Al thermosonic bonding requires elevated temperatures of about 125°C. At this temperature, the needed intermetallics form quickly between the Au ball and Al bond pad.
Although intermetallic formation is necessary to form the Au to Al weld, a life limiting factor can be excessive growth of these Au/Al intermetallic compounds. Over time, Kirkendall voids can form because of differences in diffusion rates between Al and Au; these voids along the Al-Au interface weaken the brittle inter-metallic layer, allowing fracturing to occur. As this is a diffusion related process, composition of the wire and bond pad can be varied to impede interdiffusion, but at the cost of making initial bonding more difficult - a clear production versus reliability tradeoff.
220.127.116.11 Au-Ag System
The gold-silver (Au-Ag) system is a bonding scheme that is primarily used for secondary bonds (i.e., off-die). Such bonding is found in most leadframe-based package technologies where a wedge bond is formed between gold wire and a silver-plated package lead. Bond formation may also be enhanced through thermosonic means at elevated temperatures of about 250°C. Thermosonic processes sweep aside surface oxide or sulfide films to raise the bondability of the silver pad.
18.104.22.168 Au-Au System
Au-Au bonding schemes are often found in MEMS packaging. Risks such as interface corrosion, intermetallic formation, and other mechanisms that degrade the bond strength are avoided by using this monometallic system. Au-Au bonding is usually performed at elevated temperature by thermocompression or thermosonic means, although cold ultrasonic Au-Au wire bonding can also be achieved.
22.214.171.124 Au-Cu System
Gold-copper (Au-Cu) bonding metallurgy is usually employed in bonding gold wires to bare copper lead frames. Gold wire-copper leadframe bonding produces three ductile intermetallic phases (Cu3Au, AuCu, and Au3Cu), that at high temperatures, tend to form voids which degrade the bond strength and lower its reliability. Cleanliness of the bonding surface is therefore imperative in gold-copper systems to ensure reliable bonding .
12.5.2 Aluminum Systems
Although not as common as gold systems, aluminum systems are used throughout the industry with Al-Al being the most common. Table 12.11 shows typical mechanical properties for aluminum wires .
For critical end item usage the monometallic bonding scheme of Al to Al is widely utilized. Al-Al systems are used primarily in hermetic packaging, for bonding aluminum wires onto the aluminum bond pads of the die. As this bonding scheme avoids intermetallic formation and corrosion, it is also a reliable wire bonding metallurgical system. Aluminum wire on aluminum bond pad is achieved ultrasonically at room temperature. Pure aluminum is too soft to be a fine wire and is often alloyed with 1% Si or 1% Mg to provide strength.
Date: 2015-02-28; view: 493