ABSTRACT
Quite obviously, the electrical design cannot stop at the circuit model for the silicon or GaAs die itself. Particularly at higher frequencies, such as those in the RF or microwave arena, the electromagnetic propagation due to all circuit elements create interactions, interference, and possibly circuit oscillations if these electrical effects are not accounted for and managed. Of course the customer’s initial requirement will be that a packaged device, component, or module have a specific impedance into and out of their system. Typically, this has been 50 ohms for many microwave systems. It can be achieved through properly dimensioned microstrip input and output leads, through coaxial feeds, or through stripline to microstrip connections that feed into the customer system. These are modeled using standard industry software such as that provided by Hewlett Packard or Ansoft. The next consideration for the package designer is that all of the circuit functions that require isolation are provided that isolation. This can be accomplished through the use of actual metal wall structures within the package. It can also be done by burying those circuit elements in cavities surrounded by ground planes or through the use of solid vias all around the functional elements. These are only some of the predictive means of providing isolation. The need for isolating circuit elements and functions is ascertained by using full wave electromagnetic solvers such as HFSS, Sonnet, or other full wave tools. The EM analysis of the packaged structure will output an S parameter block. From this block, an electrical equivalent circuit can then be extracted with circuit optimization software such as Libra, MDS, ADS, etc. An example of an equivalent circuit representation can be seen in Figure 12.5.
