Radio Frequency Integrated Circuit Design - Webs

138 Radio Frequency Integrated Circuit Design
Figure 5.41 Simple amplifier with bond pad models shown.
For example, with four parallel bond pads, the impedance is j4.2�. However,
it must be noted that with n bond wires in parallel and close together, mutual
inductance between them can increase the inductance so that inductance is not
decreased by a factor of n, but by something less.
The input source and the load are referenced to the PCB ground. Multiple
pads are required for the on-chip ground to have low impedance to PCB ground.
Here, the emitter is at on-chip ground. The bond pads have capacitance to
substrate, as do any on-chip elements, as previously shown in Figure 5.6,
including capacitors, inductors, transistors, and tracks. The substrate has substrate
resistance, and substrate contacts are placed all over the chip and are here
shown connected to several bond pads. The pads are then connected through
bond wires and the package to the PCB, where they could be connected to
PCB ground. While bringing the substrate connection out to the printed circuit
board is common for mixed-signal designs, for RF circuits, the substrate is
usually connected to the on-chip ground.
The lead and foot of the package are over the PCB and so have capacitance
to the PCB ground. Note: PCBs usually have a ground plane except where
there are tracks.
5.25.1 Other Packaging Techniques
Other packages are available that have lower parasitics. Examples are flip-chip
and chip-on-board.
When using flip-chip packaging, a solder ball (or other conducting material)
is placed on a board with a matching pattern, and the circuit is connected