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Gold Ball Wire Bonding Process

Thermosonic tailless ball and stitch bonding is the most widely used assembly technique in the semiconductors to interconnect the internal circuitry of the die in the external world. This method is commonly called, Wire Bonding. It uses force, power, time, temperature, and ultrasonic energy (sometimes referred to as bonding parameters) to form both the ball and stitch bonds. Typically for the ball bond, the metallurgical interface is between gold (Au), and aluminum (Al) bond pad (typically with 1% silicon (Si) and 0.5% copper (Cu). As for the stitch bond, it is bonded to a copper alloy with thin silver (Ag) plating for lead frame. For BGA substrate, the stitch bond is bonded to copper/nickel base material with gold (Au) plating.

Ball bond formation

Stitch bond formation

Ball bond formation

Stitch bond formation

The ultrasonic transducer (typically for new generation of wire bonders, the piezoelectric element is >100KHz), which converts the electrical energy into mechanical energy, transmits this resonant energy at the tip of the bonding capillary. The capillary that is clamped perpendicularly to the axis of the transducer-tapered horn is usually driven in a y-axis direction vibration mode.  Bonding capillaries are made of high-density Alumina ceramic material, Al2O3, typically 1/16" (.0625" / 1.587mm) in diameter and .437" (11.10mm) in length. The final capillary design depends upon the package/ device application and wire diameter to be used. To determine the correct capillary design in general, bond pad pitch (BPP), bond pad opening (BPO), target mashed ball diameter (MBD) are the essentials.

Variation characteristic of the capillary

Variation characteristic of the capillary

















A fine gold wire made of soft, face-centered-cubic metal (FCC), usually ranging from 18µm to 33µm in diameter (depending upon the device/ package application) is fed down through the capillary. It is usually characterized by its elongation (shear strain), and tensile strength (breaking load). Selection of the appropriate wire type to be used for a given application would be dependent on the specification of these elongation, and tensile strength. In general, the higher elongation (or higher strain), it means that the wire is more ductile. This is a good choice for low-loop, and short wire type of wire bonding application. If the requirement is for higher pull strength readings, a harder wire type having a higher tensile strength has to be considered.

The small incursions of ultrasonic energy at the tip of the capillary are transmitted to the Au ball and down to the Al bond pad to form the ball bond. After which, the capillary lifts up and form the looping profile, and then comes down to form the stitch bond. This cycle is repeated until the next unit is bonded.

The bonding cycle

 

 

 

 



















An intermetallic compound, Au-Al, is formed when the Au is bonded thermosonically to the Al bond pad metallization. The metallurgical interface of void free Au-Al formation has a significant increase in the shear strength readings of the ball bonds tested- provided that there are no impurities present in the bond interface even if it has been exposed to high temperatures. However, if the impurities are in the interface are welded poorly, the ball shear strength produces a significant degradation in its readings.

Formation of intermetallic compound between wire and bond pad
Formation of intermetallic compound

 

 



 

 
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