What’s old is new again : how often have we heard this saying or observed how technology can recreate itself and follow this saying? Copper pillars have been used for many years in flip chip or high-density semiconductors as a way to create higher-density I/O interconnect from the chip to the lead frame or even to the printed circuit board (PCB), in some cases.
The process is fairly simple. Grow a “pillar” of copper on top of the chip with the assistance of photo resist and properly controlled process techniques. Two benefits of using copper pillars are:
1. Better thermal resistance from the die to the lead frame vs traditional bond wires.
2. Lower interconnect resistance from the die to the lead frame vs traditional bond wires
Both of these offer big advantages when used in analog circuits, especially those that need to conduct or control power.
Figure 1 illustrates the difference in using traditional bond wires and copper pillars when connecting Semiconductors die to a lead frame.
Figure 1. Die to lead frame interconnect, bond wires (left) vs copper pillar (right).
Figure 2 shows the implementation of copper pillar on an analog integrated circuit, its resulting bond to the leadframe and the bottom view of a final package.
Figure 2. Implementation of copper pillar on an analog IC.
A traditional wire-bonded lead frame requires additional area for the bond wires to mount from the die to the lead frame. Therefore, what was previously possible only in a 1.6mm × 1.6mm package can now be produced in a 1mm × 1mm package. The wasted area of the bond wire ‘loop’ is not required since the copper pillar is both the electrical/thermal contact and the mechanical structure which mounts the die to the lead frame.
Products such as high-side switches used to enable power to sections of circuitry are an example of how copper pillars can help