DETAILED DESIGN PHASE
Once the architectural design is complete, overall system design is set. This leads to two main thrusts to the thermal design. One is refining the model of the enclosure, including any fans and vents. The other is to refine the model of the PCBs and components. This continued refinement of the models serves to provide greater confidence in the validity of the thermal simulation.
Thermal performance varies considerably and is designed to exploit different heat-flow paths, either down to the board or up to a heatsink. Package selection is largely based on cost and electronic performance. But the least expensive package can be the most expensive overall when you include the cost of the thermal solution. Consider thermal performance during package selection.
The block models described earlier are crude, providing “indicative” case temperatures. In conjunction with package selection, the component thermal model should be updated to at least a tworesistor compact thermal model.2 This will capture the package’s thermal behavior as two resistances: junction-to-case and junction- to-board, or preferably a Delphi compact thermal model.3
Alas, the availability of Delphi models from suppliers is limited. Fortunately, one Web-based tool allows system builders to create both two-resistor and Delphi compact thermal models for a wide range of chip packages using defaults based on the package outline.4 Knowledge of the die size further improves model accuracy in the absence of other vendor-supplied data. Detailed models that directly represent the thermally important package internals can also be created.
Heatsink design should be finalized before PCB layout is complete. If a heatsink with a base that’s larger than the component is needed, space on the PCB will need to be allocated for mounting. Heatsinks partially block the flow, so components in the wake behind a heatsink are likely to become hotter as they get less airflow. As a result, the layout could be affected.
Heatsink cost correlates well with weight. Thus, cheap also means light, which is desirable from a reliability perspective. Heavy heatsinks place more stress on the package interconnect and require attachment to the PCB. Some EC-specific CFD tools with robust meshing allow you to automatically optimize the design of a heatsink for a particular duty, minimizing the weight for a specified target temperature.5 If surface area increases minimally, maybe a heatsink made from a thermally conducting plastic will suffice.
Before PCB layout closure, the thermal design should have evolved to where there’s full confidence that the packages can be cooled. Further, for any packages requiring heatsinks, a working design of the heatsink should be available. Usually, there’s some expectation about the required PCB stackup well before routing.
The thermal representation of the PCB can be improved by estimating the percentage coverage of copper for each layer, (e.g., 20% for signal planes and 90% for power and ground planes) and including these discretely in the model. Once the boards are routed, the thermal design can be further refined by importing the trace details from the EDA system. This will account for the local variation of copper in each layer and any thermal and electrical vias.
PROTOTYPING THE SYSTEM
Prior to the first physical prototype, it’s common to perform a single, highly detailed thermal verification simulation (Fig. 3). If thermal issues were considered from early in the design cycle, final verification should be a formality before physical prototyping starts. Early consideration of thermal issues is perhaps the hallmark of a mature thermal design process.
Companies with sophisticated thermal design processes have thermal sign-offs throughout the project and consider thermal before committing to the concept. They also learn from experience. By applying the insights gained into the product’s thermal performance and measurements on prototypes, they can improve their thermal modeling and seek ways to push thermal design higher up the design flow.
REFERENCES
1. “Sense and nonsense of heat transfer correlations applied to electronics cooling,” Lasance, C.J.M., Proceedings of the 6th EuroSimE Conference, 18-20 April 2005, pp. 8-16
2. “Two-Resistor Compact Thermal Model Guideline,” www.jedec.org/download/search/JESD15-3.pdf
3. “DELPHI Compact Thermal Model Guideline,” www.jedec.org/download/search/JESD15-4.pdf
4. www.mentor.com/products/mechanical/products/flotherm-pack
5. “Simulation-based design optimization methodologies applied to CFD,” Parry, J., Bornoff, R., Stehouwer, P., Driessen, L., Stinstra, E., Proceedings of 19th SEMI-THERM Symposium, 11-13 March 2003, pp. 8-13