Au-Al System

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.

12.5.1.2 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.

12.5.1.3 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.

12.5.1.4 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 [30].

12.5.2 Aluminum Systems

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: 1244