10.1 Layout Guidelines
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 (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 TMP116 on the PCB pad to provide the minimum thermal resistance to the measured object surface or to the surrounding air. The PCB layout should minimize the device self-heating effect, reduce the time delay as temperature changes, and minimize the temperature offset between the device and the measured object.
- Soldering the TMP116 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 PCB creates additional mechanical stress to 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 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.
- 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.
- 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.
- Maximize the air gap between the sensor and the surrounding copper areas (anti-etch), especially when close to the heat source.
- 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.