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10 Layout

10.1 Layout Guidelines

Note

To achieve a high precision temperature reading for a rigid PCB, do not solder down the thermal pad.

For a flexible PCB, the user can solder the thermal pad to increase board level reliability. If thermal

pad is soldered it should be connected to the ground or left floating.

For more information on board layout, refer to the related Precise temperature measurements with TMP116 and

TMP117 (SNOA986) and Wearable temperature-sensing layout considerations optimized for thermal response

(SNIA021) application reports on ti.com.

Place the power-supply bypass capacitor as close as possible to the supply and ground pins. The recommended

value of this bypass capacitor is 0.1 μF. In some cases, the pullup resistor can be the heat source, therefore,

maintain some distance between the resistor and the device.

Mount the TMP117 on the PCB pad to provide the minimum thermal resistance to the measured object surface

or to the surrounding air. The recommended PCB layout minimizes the device self-heating effect, reduces the

time delay as temperature changes, and minimizes the temperature offset between the device and the object.

1. Soldering the TMP117 thermal pad to the PCB minimizes the thermal resistance to the PCB, reduces

the response time as temperature changes and minimizes the temperature offset between the device and

measured object. Simultaneously the soldering of the thermal pad will, however, introduce mechanical stress

that can be a source of additional measurement error. For cases when system calibration is not planned,

TI recommends not soldering the thermal pad to the PCB. Due to the small thermal mass of the device,

not soldering the thermal pad will have a minimal impact on the described characteristics. Manual device

soldering to the PCB creates additional mechanical stress on the package, therefore to prevent precision

degradation a standard PCB reflow oven process is highly recommended.

2. If the device is used to measure solid surface temperature:

• Use PCB with minimal thickness.

• Prevent PCB bending which can create a mechanical stress to package.

• Cover bottom of the PCB with copper plane.

• Remove bottom solder mask and cover exposed copper with gold layer if possible.

• Use thermal conductive paste between PCB and object surface.

• If PCB has unused internal layers, extend these layers under the sensor.

• Minimize amount of copper wires on top of the board.

• To minimize temperature “leakage” to surrounding air locate sensor in place with minimal air movement.

Horizontal surfaces are preferable.

• To minimize temperature offset due to “leakage” to surrounding air cover sensor with thermo isolating

foam, tape or at least cover with a stain.

3. If the device is used to measure moving air temperature:

• Because moving air temperature usually has a lot of fluctuations the PCB increased thermal mass

reduces measurement noise.

• Design PCB soldering pads bigger than usual, especially package corner pads.

• Use a PCB with thicker copper layers if possible.

• Cover both side of unused board space with copper layer.

• Place PCB vertically along air flow.

4. If the device is used to measure still air temperature:

• Miniaturize the board to reduce thermal mass. Smaller thermal mass results in faster thermal response.

• Place two copper planes of equal size to the top and bottom of the exposed pad.

• Remove the top solder mask.

• To prevent oxidation, cover any exposed copper with solder paste.

• Thermal isolation is required to avoid thermal coupling from heat source components through the PCB.

• Avoid running the copper plane underneath the temperature sensor.

TMP117

SNOSD82D – JUNE 2018 – REVISED SEPTEMBER 2022

www.ti.com

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