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Thermal Relief Simulation

About thermal relief

The term "thermal relief" describes circuit board modifications made to impede heat flow from copper pads during soldering. As shown in Figure 1, thermal relief involves creating a break in the copper, sometimes referred to as a "thermal moat" or a "thermal dam". The thermal relief makes it easier to heat a pad for reflow soldering since there is less heat loss to adjacent pads. However, there is an impact on components, such as SOT-223 package devices, which use the circuit board to dissipate heat.

thermal relief configuration

Figure 1: Typical thermal relief configuration for SOT-223 package

Using Sauna to evaluate thermal relief configurations

The Sauna program provides the necessary tools to quickly and accurately model a thermal relief configuration. To model the geometry shown in Figure 1, the first step is to define the pads. Since the SOT-223 is one of the many standard packages in Sauna's library, these pads can be quickly created. As part of this process, you would specify the size of the heatsinking pad (10 mm x 10 mm in the picture) and the trace width and length.

Next, you would use Sauna's tools for editing copper traces to create the "moat" and "drawbridge" shown in the picture. This is a straightforward process.

When the copper pattern has been defined, you would add the SOT-223 device. The device is represented with an "enhanced heat source" in Sauna. The enhanced heat source includes a thermal connection between the junction and each lead pad. There is also heat transfer from the top of the component body to the room, and another thermal path down into the circuit board. With an enhanced source, the simulation will incorporate heat flow from all surfaces of the package. If the analysis only focuses on heat flow through the tab, the negative impact of the thermal relief will be overstated.

After adding the component, you would complete the model by adding float resistors to the room environment. You can calculate temperatures for either natural or forced air cooling. The entire model shown in Figure 1 can be created and analyzed in minutes.

A multi-configuration study for the SOT-223 device

With Sauna you can quickly evaluate a series of design alternatives. For a SOT-223 device, Sauna models were created for a series of heatsinking pad sizes: 3.5 mm x 2 mm (7 mm²), 10 x 10 (100 mm²), 17.3 x 17.3 (300 mm²) and 25 x 25 (625 mm²/1 inch²). The models were first analyzed without thermal relief. Then thermal relief was added and temperatures recalculated. The results of these analyses are shown in Figure 2:

click to enlarge

Figure 2: Impact of adding thermal relief

The graph shows interesting results. For the small pad of 100 mm² (0.16 in²), the impact of thermal relief is fairly modest. The junction-to-ambient thermal resistance (Rja) does increase by 23.8% relative to the no thermal relief case. However, the 100 mm² pad still provides a clear benefit when compared with the minimal 7 mm² pad, as Rja decreases from 188.1°C/W to 141.7°C/W. For larger pads, the negative impact is significantly greater. With the 300 mm² (0.47 in²) pad, thermal relief causes an Rja increase of 43.2%. For a 625 mm² (1 in²); pad, the increase is 56.6%.

Also, the graph shows that thermal relief greatly limits the benefit of using a large heatsinking pad. When thermal relief is present, an increase in pad size from 100 mm² to 625 mm² only reduces Rja by 16.2% (reduced from 141.7°C/W to 118.7°C/W). The impact is even more dramatic when comparing with and without thermal relief. The Rja for a 100 mm² pad without thermal relief (114.5°C/W) is largely identical to the Rja for a 625 mm² pad with thermal relief included (118.7°C/W). Thermal relief can have a substantial negative impact for some situations.

To read the full study, see Impact Of Thermal Relief On The Cooling Of A SOT-223 Device.

The next step

The best way to learn about the Sauna program is to try the free evaluation package. You can request the evaluation package here.

If you want additional information on Sauna features, try one of these links:


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