Challenges In RF Design

Designing highly integrated components for radio frequency applications poses special challenges for system engineers, designers and the commissioning engineers. The boundary between chip, package and board is increasingly vanishing on modern components. It is growing more common for parts of the functionality to be moved to the package or even the board. In some cases, the requirements have become so expansive that the functionality can only be guaranteed with perfect interaction between chip, package and board.  To ensure a robust and reliable design for such components, a number of physical effects must be taken into account with special tests and their influence must be evaluated. This is in addition to the usual effects observed in other components and the associated testing, such as timing, voltage drop, etc.

In RF design, an increasing number of components are also being moved from the circuit to the package, either because this results in better performance or because it is cheaper to manufacture. Such repositioning is only possible with intensive co-design of the chip, package, and board. Appropriate tool flows must be created to support this throughout the entire design process. This begins with early exploration of variants at the functional system level, continues with very early floor planning and co-design of the individual components and finishes with co-verification of the entire system, taking into account parasitic interconnect and substrate effects.

On one hand, the functional properties of the components must be realized and verified. In the area of RF design, this means for example gain values, noise factors, crosstalk attenuation, or spectral efficiency.

On the other hand, the non-functional properties also call for special attention. Among other aspects, this involves ensuring long-term reliability in the chip, package, and board and at the chip-package and package-board interfaces as well as complying with thermal constraints.

Given their applications (automotive, industrial, etc.), RF components must frequently handle a large temperature range. Furthermore, they typically utilize large-signal operation. When it comes to verifying reliability, this requires additional tests to capture the aging behavior in the extended range as well (temperature, amplitude, etc.). The large voltage steps result in a number of stress mechanisms for RF circuits, such as bias temperature instability (BTI), hot carrier injection (HCI), non-conducting HCI (nHCI) and on/off state time dependent dielectric breakdown (TDDB).

Simulation of the expected aging behavior requires not only very good DC models for simulating the RF figures of merit (FOM), it is also necessary to model the transistor capacities and how they change over lifetime as accurately as possible. It has also been shown that quasistatic approximation is no longer applicable for mapping of DC aging onto time-variable aging for higher frequencies.

Alongside the voltage level, power also plays an important role in the characterization and modeling of RF components. The input power is another stress variable that influences the output power and the power added efficiency (PAE). For RF power amplifier devices, the important drain voltage can be over twice as high as the operating voltage. This is why HCI and nHCI are the most important aging mechanisms to be considered.

The processing of higher powers in tiny RF devices also places special demands on the thermal design of the components. Local heating has a negative impact on the long-term reliability. Uneven warming of a component with large surface area can also lead to unexpected power losses. These thermal influences must always be taken into account for designing the layout, positioning thermal vias, and designing the packaging solution.

It is therefore necessary to pursue new design approaches for the development of RF systems and components due to their special properties. A number of specific conditions must also be considered to ensure correct function and long-term reliability in the intended application.