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
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
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.
- 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.
- 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
- If PCB has unused internal layers, extend these layers
under the sensor.
- Minimize amount of copper wires on top of the
- To minimize temperature “leakage” to surrounding air
locate sensor in place with minimal air movement. Horizontal surfaces
- To minimize temperature offset due to “leakage” to
surrounding air cover sensor with thermo isolating foam, tape or at
least cover with a stain.
- 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
- 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
- Place PCB vertically along air flow.
- 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
- Thermal isolation is required to avoid thermal
coupling from heat source components through the PCB.
- Avoid running the copper plane underneath the
- Maximize the air gap between the sensor and the
surrounding copper areas (anti-etch), especially when close to the heat
- Create a PCB cutout between sensor and other circuits.
Leave a narrow channel away from heat source components as a routing
bridge into the island.
- If the heat source is top side, avoid running traces on
top; instead, route all signals on the bottom side.
- Place the board vertically to improve air flow and to
reduce dust collection.