SYMMIC Users Manual CapeSym

Mesa Resistor Template

The Mesa Resistor Template is a simple device template example. This template models a uniform heat source on the top of the substrate and could represent a resistor fabricated from semiconductor epi-layers, a resistor fabricated by deposition, or some other uniform heat source. The figure below shows the device and its components and parameters.


Layers

Backside Adhesive. The adhesive layer attaching shim to heat sink

Metal shim. The metal shim to which the device is soldered

Solder. The solder attaching the substrate to the metal shim

Substrate. The electrically insulating substrate

Mesa. The resistive layer on top of the substrate

Dimensions

Boundary Size. The distance (same on all sides) from the edge of the mesa to the device boundary

Mesa Length. The size of the mesa in the x-direction

Mesa Width. The size of the mesa in the y-direction

Boundary Conditions

The Backside Film Temperature and the Backside Film Coefficient (a heat transfer coefficient for film, often called “h”,) set the film boundary condition at the bottom of the device. The heat transfer coefficient and temperature are intended to be used to capture the thermal resistance from the backside of the chip to the reference (e.g. cold plate) temperature. A very large value for the heat transfer coefficient (e.g. 1 W/um^2.°C) can be used to hold the backside temperature to a constant temperature. These parameters are modified in the Heat Generation parameters list.

Heating Profile

The power parameters set the heat flux in the mesa. For the standard MesaResistor.xml template, heating is modeled as volume flux uniformly distributed throughout the mesa to give the specified total dissipated power. For the special case of MesaResistorS.xml, the volume flux is replaced with a surface flux at the top of the mesa.

Power Cycle Parameters

The total dissipated power (W) is divided temporally into two pieces, the ON Power and the OFF Power. In a steady-state solution, the ON Power is dissipated continuously for all time. For a solution through time, the ON Power is dissipated for the ON Duration and then the OFF Power is dissipated for the OFF Duration. This ON/OFF power cycle is repeated until the solution completes. The Cycle Start parameter determines when in the power cycle the simulation will begin. A Cycle Start of zero means the simulation begins at the onset of the ON power level. The figure below depicts power levels for a 30% duty cycle starting in the ON segment.



For accurate simulations, the simulation time step should be much smaller than the ON or OFF power duration, because only the average power is dissipated for a time step in which a power switch occurs. See the section on Solutions Through Time for more information on setting the time step size for a simulation.

Note: When restarting a simulation from a previous solution file, the user must determine where in the power cycle to begin in order to continue the power cycle correctly. For example, if a simulation was originally carried out with ON Duration=25µs, OFF Duration=50µs, and Cycle Start=0, to restart the duty cycle from t=90µs the Cycle Start should be set to start 15µs into the second power cycle.

Materials

The template includes a set of materials appropriate for microwave circuits. The materials and their data sources are described in detail in the Material Properties section of the Appendices.

Meshing

Mesh resolution is determined by the size of the mesa. The meshing parameters are designed to keep a minimum mesa size at the edges of the mesa to ensure reasonable accuracy and avoid spatial oscillations at switching transients of solutions through time. The template includes a Mesh Refinement parameter that allows a more refined mesh to be selected by setting Mesh Refinement = 2.

Layout Characteristics

The resistor device can be incorporated in a layout. The layers that are expanded to the boundaries of the layout are those of the Backside Adhesive, Metal Shim, Solder, and Substrate. The backside boundary conditions are also extended. When incorporated into a layout with multiple devices, the default settings ensure that the materials used for the components Backside Adhesive, Metal Shim, Solder and Substrate are the same for all devices. The default settings also equalize the following parameter values across devices:

Backside Adhesive Thickness

Mesh Refinement

Metal Shim Thickness

Cycle Start

Solder Thickness

OFF Duration

Substrate Thickness

ON Duration

Backside Film Coefficient

Backside Film Temperature



For more on why these parameters should remained equalized, see the Layout Templates section.

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