With power components coming in smaller and smaller surface mount packages it is important to come up with a coherent approach to mitigating the thermal dissipation demands of these components in a PCB design. While the development of an exact mathematical analysis of the thermal characteristics of a PCB design can be a complex process, it's possible to apply some simple rules to improve the thermal conduction of your design. Ultimately, properly controlling the dissipation of heat in your design will allow you to produce a more reliable and economical PCB design. What follows is a brief discussion of the standard thermal dissipation model and then some general rules for dealing with thermal dissipation in your designs.
First it is important to define the terminology that is going to be used through the rest of this entry. The next figure presents the different components to a power IC that we must consider when discussing thermal management. We will be discussing the temperature of the Junction, Top and Case of the component and their thermal resistances to the ambient environment throughout this article.
With these terms in hand, we will briefly look at the standard model used to simulate the thermal dissipation of a component. Thermal resistance is normally modeled as a resistor network. The standard model for a component is presented in the following figure:
In the presented figure TJ is defined as the temperature of the junction (the internal working portion of the component), TT is the temperature of the "top" of the package (typically the plastic enclosure of the component), TC is the temperature of the "case" (this is the temperature of the highly thermally conductive pads of the component and the attached PCB) and TA is ambient environment's temperature. The goal of the electronics designer is to then produce the lowest thermal resistance possible between the junction and the ambient environment. With the exception of θCA, the thermal resistances of the system (θJT, θTA and θJC) are defined by the properties of the component and can be pulled from the data sheet for said component. As a PCB designer we principally have influence over the value of θCA, which is dependent on our PCB design. As such, the primary challenge for the designer is the reduction of the thermal resistance of the IC's case to the ambient environment by reducing this resistance. How well we are able to lower this thermal resistance (θCA) will largely define the temperature differential (or lack thereof) that will develop between the ambient environment and the junction of the component.
Of note is that the other path for thermal conduction is the plastic case (or the "top") of the component. As the plastic packaging of most power components do not provide a good thermal path to the ambient environment the efficiency of thermal dissipation of the design is more heavily dependent on the design's ability to dissipate thermal energy to the surrounding environment through its case. The only exception is when the power IC in question is designed with a thermal pad located on the top of the component. In this case, the IC is designed for a heat sink to be attached directly to the top of the IC and the thermal dissipation of the component through its "top" becomes a much more important factor in the design.
The standard approach to translating heat away from power components is through thermally connecting the power components to adjacent copper planes by way of thermal vias. This is typically achieved by placing a number of vias in the foot print of the power IC. These vias provide a thermal connection to the copper layers below the IC, which then conduct heat away from the component.
Additionally, the more power copper planes connected to power IC by said thermal vias, the higher efficiency of thermal dissipation of the PCB. e.g. using a 4-layer design vs. a 2-layer design can increase power dissipation capacity of the PCB up to 30% when comparing the same area of those designs.
