Wirebonding Theory

As mentioned in the main page about wirebonding, bonding is achieved by interaction of these factors: ultrasonic force, pressure, temperature, and time.  Metallurgical bonding or cold welding is initiated between the wire and the bond pad by placing these two metals under intimate contact with each other.  In the case of gold ball bonding, for instance, the free-air ball at the end of the wire is brought into contact with the aluminum bond pad by a needle-like tool known as a capillary, squashing the ball against the bond pad.

Ultrasonic energy is then applied by the bonding tool to the bond which, in effect, scrubs the bond against the bond pad.  This scrubbing action cleans the bond pad of debris and oxides, exposing a fresh surface of the bond pad in the process.    

    

Fig 5. Two examples of wirebonders

The metallurgical bond or weld between the bond and the bond pad is further enhanced with the continued application of ultrasonic energy, resulting in plastic deformation of the bond and bond pad against each other.  Aside from the physical contact and deformation of the metals unto each other, interdiffusion of the bond and bond pad metal atoms also occurs to further enhance the cold weld. 

The process of bond and bond pad metal counterdiffusion to form intermetallic bonds is known as intermetallic formation (Fig. 6).  The area of intermetallic formation between the bond and the bond pad is known as intermetallic coverage (IMC).  In general, bond reliability increases with the IMC, as long as overbonding does not take place.  Many companies monitor the IMC as a means of detecting bonding problems on the line. Good bonds generally exhibit IMC's > 50% of the ball impression's area.

The most common reason for insufficient intermetallic formation aside from incorrect parameter settings is the presence of foreign materials or contaminants on the surface of the bond pad.  Frequently-encountered bond pad contaminants include unetched glass, silicon saw dust, and process residues.  Fig. 7 shows a photo of a bond pad with unetched glass on the surface, which resulted in less than 30% IMC and subsequent ball bond lifting.

If the bond and bond pad are composed of different metals, such as in the case of gold ball/aluminum bond pad bonding, thermal energy is required to 'soften' the harder metal (aluminum) to match their hardness. Au-Al ball bonding generally takes place at 200-240 deg C. In cases where the wire and bond pad have similar metallurgies, bonding may occur at ambient temperature, such as in the case of aluminum wedge/aluminum bond pad bonding.

Pressure is applied to the bonding tool to keep it in control as it scrubs the bond against the bond pad.  Without pressure, the bonding tool would bounce around the bond pad as it receives and transmits the ultrasonic energy.

The quality of a bond may be assessed using bond strength tests.

See also:  Effects of Grain Size Distribution on Wire Bonding

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Front-End Assembly Links:  Wafer Backgrind;  Die Preparation;  Die Attach;  Wirebonding;  Die Overcoat

Back-End Assembly Links:  Molding;  Sealing;  Marking;  DTFS;  Leadfinish          

See Also:  Wirebond Metallurgies;  Bonding Failures;  Bonding Wires;  Bonding Tools;

Bond Strength Tests;  Bond Lifting;  IC Manufacturing;  Assembly Equipment