This article on semiconductor packaging will be the first in a series of articles geared toward those who are not technical specialists in the semiconductor processing field. Adding a semiconductor package is one of the final steps in semiconductor processing. A package is designed to provide four basic functions. It must provide a substantial lead system, physical protection to shield the chip against impact or damage, environmental protection to shield the chip against dust or other contaminants, and lastly thermal management.
[i] The lead system contained in the package holds the contact pins or leads that are used to connect the external circuits to the device. This connection can be made through either wire bonding or clip bonding. The distinction between these two techniques is described below.
Wire Bonding in Semiconductor Packaging
Wire bonding is a method of making connections between an integrated circuit (IC) or other semiconductor device and its packaging during the semiconductor fabrication process. The electrical connection between the silicon chip and the external leads of the semiconductor device are made using very fine bonding wires, often made of gold or aluminum. The earliest means of bonding wires to produce silicon semiconductor devices involved small solder balls that literally soldered the wire to the chip.[ii] These bonds were not reliable however, and sometimes the joint was mechanically weak. The solder joint would sometimes form a diode junction with the semiconductor materials and therefore degrade the performance of the semiconductor. Wire bonding has made many improvements and is generally considered a cost-effective and a flexible interconnect technology used to assemble a large majority of semiconductor packages. Two common wire bonding processes are gold ball bonding and aluminum wedge bonding.
During gold ball wire bonding, melting the end of the wire with an electronic flame-off creates the gold ball. The ball is then brought into contact with the bond pad. Pressure, heat, and ultrasonic forces are used to form the initial metallurgical weld between the ball and the bonding pad. The wire is then run to the corresponding finger of the package to form a loop between the bond pad and the package lead. Pressure and ultrasonic forces are once again applied to the wire to form the second bond, known as a wedge bond, stitch bond, or fishtail bond. The wire is then broken in preparation for the next bonding cycle. Aluminum wedge wire bonding is a very similar process, using aluminum wire and an aluminum bond pad.[iii]
Chip Bonding in Semiconductor Packaging
The demand for end products to have increasing power levels and power density requirements led high-power semiconductor modules to be assembled using the clip bonding technique. High-power packages such as metal oxide semiconductor field effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs), and switched output differential structures (SODs) use solder paste to connect the die to the basic substrate.[iv] Clip bonding allows for a wide, solid copper bridge between the die and lead. This leads to better thermal performance, ultra-fast switching performance, and unique package resistance. Another benefit of clip bonding is a decrease in noise. While clip bonding may have a higher initial cost then wire bonding, it does provide improved performance.
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[i] Zant, P. Microchip fabrication, a practical guide to semiconductor processing. (4 ed.). New York, NY: McGraw-Hill.