Understanding PCB Effective Thermal Conductivity

Key Takeaways

• First, we will define PCB effective thermal conductivity.

• Next, we will discuss the key factors that affect PCB effective thermal conductivity.

• Finally, we will consider the accuracy of effective thermal conductivity for thermal modelling.

If you’re serving a bowl of hot porridge to your hungry child, it can be tough to cool it down quickly enough before impatience gets the best of them and a spoonful of steaming porridge scalds their mouth (or if I’m honest, sometimes my own impatient mouth). The solution? Use a plate instead of a bowl and spread the porridge thinly over it to increase the surface area that can cool.

You’ll be amazed at how switching from a bowl to a plate drastically reduces the cooling time for the porridge. In electronics, understanding the thermal behavior of a printed circuit board (PCB) is more complicated than flipping between a bowl and a plate. Learning the ins and outs of a PCB’s effective thermal conductivity starts with thermal management planning in the design stage. 

What Is PCB Effective Thermal Conductivity?

PCB effective thermal conductivity reflects the heat transfer capacity of the PCB.

The term, “effective thermal conductivity,” refers to the ability of a material to conduct and transfer heat. When we’re talking specifically about a PCB’s effective thermal conductivity, we’re talking about how well the PCB can move the heat generated from its components to its surrounding area. Effective thermal conductivity is represented by the symbol, k_eff , and values are expressed in  W/m•K.

In PCB design, effective thermal conductivity is an important aspect used in thermal modelling and analysis because it allows engineers to predict how well a populated PCB would conduct heat based on specific assumptions and models. With electronic modules continuing to shrink in size, it is a parameter that deserves attention amongst designers. 

Factors That Affect Effective Thermal Conductivity for a PCB

Thermal via density influences PCB effective thermal conductivity.

A PCB consists of conductive material, insulators, and mounted components. Every material used in a PCB has a different thermal conductance. When deriving the effective thermal conductivity for a PCB, the varying thermal conductance is taken into consideration. 

Various studies have been conducted to analyze thermal conductivity for a PCB. Depending on the study, different types of models and assumptions are used. As far as designers are concerned, the effective thermal conductivity for a PCB is dependent on a few factors.

Component Size 

As components become smaller, so does the ability to dissipate heat naturally. For example, a MOSFET with a SOT-23 package will have less area for its heat pad than one in TO-220. Therefore, the size of components mounted on a PCB will influence its ability to propagate heat. 

Thermal Vias

Thermal vias are holes intentionally placed to dissipate heat off a component. It’s kind of like providing more opportunity for the steam to escape from your porridge instead of trapping it in a container. So, naturally, this means that the more thermal vias there are in an area, the more they will increase the effective thermal conductivity of a PCB. 

Internal Layers

The presence of internal copper layers can also change the rate and direction of heat dissipation. The thermal conductivity for copper is 355 W/m-K, while FR-4 is 0.25 m-K. With multiple inner-layers of copper, the effective thermal conductivity of the PCB would decrease. Of course, the presence of thermal vias could help to move the heat to the inner layers more efficiently.  

Trace Geometries

Now, if you have copper traces that run along one end to the other, you can expect a high effective thermal conductivity value on the PCB. However, the value may decrease if the traces are discontinued, which is often the case in an actual PCB. 

Is Effective Thermal Conductivity an Accurate Approach for PCB Thermal Modelling? 

Effective thermal conductivity is preferred for its ease of computation in an analysis. It is usually based on a homogeneous PCB model. However, PCBs are rarely homogenous, particularly multilayered ones. The component arrangement, traces, copper plane, vias, and pads can be different in each layer.

Thus, there will be a margin of error when deriving effective thermal conductivity from a conventional model. In order to achieve a more accurate estimation of the effective thermal conductivity, a PCB needs to be profiled in-depth for each layer. The surface is then pixelated and analyzed for better predictions. 

The accuracy of PCB effective thermal conductivity depends on the modelling, and modelling, regardless of the technique, is only as accurate as the software tool involved. If you’re using OrCAD, the Sigrity PowerDC will be a great help with thermal analysis for your design. You can also use InspectAR to interactively assess and improve your PCB design process using augmented reality. Inspecting, debugging, reworking, and assembling PCBs has never been simpler or more accurate. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.