2 Bonding Wire Package Introduction

There are 500 to 1,200 chips, which are also called dies, being attached to each sheet of wafers that have completed the front-end process. A dicing process will divide the chips into individual and use wires to connect the chip with the outside so that each chip can be used for the required field. Bonding wire is a method of bonding thin metal wires to a pad, as a technology that connects the internal die and the outside.

As shown in Figure 2-1, the chip fastens on the middle of the lead frame with glue. Wires act as a bridge between the bonding pad of the chip (first bond) and the pad of the carrier (second bond). The material of the wire is copper or gold, the length is from several hundred μm to several mm and the diameter is typically 15 to 35 μm. The total R_DSon is the R_DSon of the die plus the bonding wire’s resistor. For a high-current converter, the resistor of the bonding wire significantly increases the total R_DSon of the device, meaning to keep the total R_DSon smaller, you must increase the die size to decrease the die R_DSon. The bonding wire will also increase the parasitic inductance between Vin, SW pin to Vin, and the SW pad on the die. It will decrease the SW ringing slew rate because of higher parasitic inductance, which is bad for efficiency.

Figure 2-1 Simplified Structure of Bonding Wire Package.

To improve the thermal performance, the Power PAD can be added so that the leadframe die pad (or thermal pad) is exposed on the bottom of the IC. This provides an extremely low thermal resistance path between the die and the exterior of the package. The thermal pad on the bottom of the IC can then be soldered directly to the printed circuit board (PCB), using the PCB as a heatsink. In addition, through the use of thermal vias, the thermal pad can be directly connected to a ground plane or special heat sink structure designed into the PCB.