Two-layer heat spreading approximations revisited

When confronted with system level thermal analysis, the designer needs at least a first guess about the thermal behavior of the whole system, even if she¹ is only responsible for a single part, such as the PCB or the heat sink. A useful strategy is to check globally whether some claims of vendors make sense or not. For example, some vendors of metal-core PCBs state that their enhanced (and hence more expensive) thermal-conductive dielectrics are required for reliable performance. A spreadsheet-based Calculator is discussed that allows the designer to assess the dominant thermal resistances in a series network comprising the thermal path from junction to ambient for a two-layer case such as an LED on a metal core PCB (MCPCB). An earlier paper has shown that it is possible to adapt the approximate one-layer heat spreading equations to accommodate two layers. However, while building the Calculator, it was found that these equations did not allow for a correct estimation of the spreading resistances only, which is required to get insight in the importance of this element in the total chain. The main purpose of this paper is to share this acquired knowledge.