Best Practices for Two-resistor Components in SOLIDWORKS Flow Simulation

The Two-Resistor Component model provides a simplified intuitive method to represent the thermal behavior of small electronics packages like integrated circuits (IC’s) in the context of larger electronic systems. The model is based on the fairly well accepted Joint Electron Device Engineering Council (JEDEC) standard which provides increased accuracy over the classic single-resistor model, and the provides the added benefit of having a full library of the necessary component data readily available. The Two-Resistor Component is only available with the Electronic Cooling module.

Two-Resistor Component Definition

The Two-Resistor component simplifies the heat transfer inside the chip down into three temperature nodes: Junction, Board, and Case, connected by two user specified thermal resistances.

• Junction – considered to be a high conductivity solid plate with heat-insulated side walls. Heat can only flow to the Case or Board
• Board – considered to be in direct thermal contact with component the chip is attached to. Normally the Printed Circuit Board (PCB).
• Case – considered to be in direct thermal contact with environment directly above the component, air or heat sink.

The three nodes are connected together by two user specified thermal resistors, Junction-to-Board (θ_JB) and Junction-to-Case (θ_JC). The thermal resistances are used to calculate the heat conduction through the chip.

The complete definition for the Two-Resistor Component is represented by:

• A solid body of specific dimensions (optionally two bodies if using legacy model).
• Input power dissipation
• Thermal resistance – Junction-to-Case (θ_JC)
• Thermal resistance – Junction-to-Board (θ_JB)

Two-Resistor Component Geometry

There are two acceptable methods to represent the Two-Resistor component in SOLIDWORKS Flow Simulation.

Method 1 (Legacy method) uses two identical rectangular cubic solid bodies, one on top of the other, and mounted on another solid body representing the printed circuit board. This is the method that was the only method supported prior to version 2016. This method is still supported in the newer versions of SOLIDWORKS Flow Simulation. 

Two-Resistor Component using two identical bodies (Legacy approach)

Method 2 uses a single rectangular cubic solid body. 

Two-Resistor Component Model Single Body

With either method, the dimensions of the solid model should closely match the overall dimensions of the actual chip it’s representing. The dimensional data can be found on the component datasheet along with the Junction-to-Board (θ_JB) resistance and Junction-to-Case (θ_JC) resistance in many cases. There is also a large library of components built into SOLIDWORKS Flow Simulation.

Two-Resistor Mesh Requirements

The mesh requirements for Two-Resistor components are highly dependent on the size of the component and the amount of heat the component is dissipating. The developers recommend using the grid convergence method to determine if the mesh is sufficient. Start with a coarse mesh and run the analysis. Check component temperatures to establish a baseline. Then increase the mesh refinement, re-run the study, and check the component temperatures. If the temperature results change significantly from the baseline, then the increased mesh refinement becomes the new baseline and additional mesh refinement steps are needed. Continue refining the mesh, re-running the study, and checking the component temperatures until the component temperature results don’t change significantly.

Two-Resistor Component Tips

• The temperature at the Junction is the highest temperature of the IC. The maximum junction temperature is typically specified in the datasheet of the IC and is used when calculating the required case-to-ambient thermal resistance for a given power dissipation. This in turn is used to select an appropriate heat sink or fan if required.
• It is not necessary to apply a material to the body defined as a Two-Resistor component. The body will be treated as a high-conductivity body automatically. Any material applied will be ignored.
• The sides of the component are insulated (adiabatic). The heat only spreads through the top and bottom faces.
  

  Two-Resistor Component Heat Path

• The bottom face (board side) must be touching the PCB component. It cannot be exposed to fluid. The top face (case side) can be exposed to fluid or can be touching a heat sink.
• The body used for the Two-Resistor component should be a simple rectangular cube with dimensions that closely match the dimensions of the package model geometry (case and junction components). The top and bottom faces must be parallel to each other.
• The top and bottom faces should remain as one unbroken face each. Do not apply Split Lines or other features to modify the top or bottom faces of the rectangular cube.
• There should not be any other Boundary Conditions applied to the faces of the Two-Resistor Component. If a thermal resistance needs to be applied to the interface at the top or bottom, it must be applied to the PCB if on the bottom or the heat sink if on the top.
• Computing the individual node temperatures for the Two-Resistor component:
  • Case temperature: Measure the surface temperature of the upper face.
  • Board temperature: Measure the surface temperature of the lower face.
  • Junction temperature: Measure the volume temperature of the component.
• Most common issues with Two-Resistor Components are:
  • Dimensions of component not matching dimensions of package model geometry
  • Comparison of improper result quantities.
  • Insufficiently refined mesh