NTC ThermistorsKey Points for NTC Thermistor Mounting Positions

• Six key points for NTC thermistor mounting positions
• Experiment 1. Temperature distribution of the heat source
• Experiment 2. Orientation relative to the heat source
• Experiment 3. Influence of thermal vias
• Experiment 4. Reduction of offset temperature by using a floating layer
• Information on Murata simulation software, Femtet™
• Basic Knowledge of NTC Thermistor
• Related Links

Six key points for NTC thermistor mounting positions

1. It is better to place the NTC thermistor as close to the heat source as possible.
2. By placing the NTC thermistor near the terminals of the heat source, the impact of temperature variance due to measurement position can be reduced, potentially reducing the offset temperature*.
3. It may be better to orient the NTC thermistor parallel to the heat source.
4. When placing the NTC thermistor on the surface without the heat source, it may be better to place it near a thermal via.
5. When placing the NTC thermistor on the surface with the heat source, it may be better not to place it near a thermal via.
6. When placement near the measurement target is not possible, it may be possible to reduce the offset temperature* by using a floating layer.

• *Offset temperature: The temperature difference between the heat source and the thermistor

Experiment 1. Temperature distribution of the heat source

It is better to place the NTC thermistor as close to the heat source as possible.

• In order to reduce the offset temperature, Key Point 1) it is better to place the NTC thermistor as close to the heat source as possible.
• We mounted a MOSFET (TPCA8028-H) on an original Murata board (see Figure 1), applied voltage, and measured the temperature distribution on the board using a thermo-viewer.
• See Figure 2 for the temperature distribution results.
• The temperature drops rapidly as the position moves away from the heat source.

Figure1: Condition of expt1

Figure2: Result1 of expt1

It may be better to place the NTC thermistor near the terminals of the heat source.

• Graph 1 shows the temperature slope of the X axis, which are the MOSFET sides without terminals.
• Graph 2 shows the temperature slope of the Y axis, which are the MOSFET sides with terminals.
• The temperature slope on the sides with terminals is more gradual, indicating that positional differences have less of an impact on temperature, resulting in a smaller offset temperature.
• This is because the heat from the MOSFET is first transmitted to the board via the terminals.

Figure3: Result2 of expt1

Experiment 2. Orientation relative to the heat source

It may be better to orient the NTC thermistor parallel to the heat source.

• We compared the cases with an NTC thermistor placed in parallel and orthogonally to a MOSFET.
• This experiment showed that the offset temperature is smaller when the NTC thermistor is placed in parallel to the heat source.

Figure4: Result of expt2

Experiment 3. Influence of thermal vias

When placing the NTC thermistor on the surface without the heat source, it may be better to place it near a thermal via.

When placing the NTC thermistor on the surface with the heat source, it may be better not to place it near a thermal via.

• On a board with thermal vias, we measured the surfaces with and without the heat sources using a thermo-viewer. Figure 5 shows the board condition and the measurement results.
• The heat from the surface with the heat sources is easily transmitted through the thermal vias to the surface without the heat sources.
• On the other hand, the board temperature around the thermal vias on the surface with the heat sources often becomes lower due to heat dissipation to the surface without the heat sources.
• If there is no space on the side with the heat sources, a floating layer can be used. Refer to Experiment 4 on the following page.

Figure5: Result of expt3

Experiment 4. Reduction of offset temperature by using a floating layer

When placement near the measurement target is not possible, it may be possible to reduce the offset temperature* by using a floating layer.

• We created two simulation models to observe the impact of heat on the internal electrode layer (floating layer).
• Model A does not have a floating layer, and is a model that adds a 2nd layer to the model in Experiment 2.
• The 2nd layer is located 300 um below the surface (1st) layer and covers the whole area of the board. It is conductive to the 1st layer by eight thermal vias.
• Model B has a floating layer, and is a model that adds a 3rd layer to model A. This 3rd layer is the floating layer, and is located 150 um below the 1st layer.
• The temperature at point 1 on the 1st layer of model B is 10 degC higher compared to model A.
• The temperature of model B is higher because heat is stored in the floating layer.

Figure6: Image of 1st, 2nd, and 3rd layer

Figure7: Result of the no floating model A

Figure8: Result of the floating model B

Information on Murata simulation software, Femtet™

• The effect of the floating layer shown in Key Point 6) was confirmed through simulation.
• The models on the previous page were created using Murata's simulation software Femtet™, and confirmed by comparison with experimental results.
• The heat generation of the MOSFET in the simulation approximately reproduced the experimental results.

Figure9: Compared the simulation and the experimental results

Information on Murata simulation software, Femtet™

Basic Knowledge of NTC Thermistor

Related Links

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