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4.2. Hysteresis

The moisture absorbent film (polymeric dielectric) of the humidity sensor will carry a memory of its exposure

history, particularly its recent or extreme exposure history. A sensor exposed to relatively low humidity will carry a

negative offset relative to the factory calibration, and a sensor exposed to relatively high humidity will carry a

positive offset relative to the factory calibration. This factor causes a hysteresis effect illustrated by the solid trace

in Figure 6. The hysteresis value is the difference in %RH between the maximum absolute error on the decreasing

humidity ramp and the maximum absolute error on the increasing humidity ramp at a single relative humidity

setpoint and is expressed as a bipolar quantity relative to the average error (dashed trace). In the example of

Figure 6, the measurement uncertainty due to the hysteresis effect is ±1.0%RH.

4.3. Prolonged Exposure to High Humidity

Prolonged exposure to high humidity will result in a gradual upward drift of the RH reading. The shift in sensor

reading resulting from this drift will generally disappear slowly under normal ambient conditions. The amount of

shift is proportional to the magnitude of relative humidity and the length of exposure. In the case of lengthy

exposure to high humidity, some of the resulting shift may persist indefinitely under typical conditions. It is generally

possible to substantially reverse this affect by baking the device (see section “4.6. Bake/Hydrate Procedure” ).

4.4. PCB Assembly

4.4.1. Soldering

Like most ICs, TH06 devices are shipped from the factory vacuum-packed with an enclosed desiccant to avoid

any RH accuracy drift during storage and to prevent any moisture-related issues during solder reflow. The following

guidelines should be observed during PCB assembly:

TH06 devices are compatible with standard board assembly processes. Devices should be soldered

using reflow per the recommended card reflow profile. See Section “10. PCB Land Pattern and Solder

Mask Design” for the recommended card reflow profile.

A "no clean" solder process is recommended to minimize the need for water or solvent rinses after

soldering. Cleaning after soldering is possible, but must be done carefully to avoid impacting the

performance of the sensor.

It is essential that the exposed polymer sensing film be kept clean and undamaged. This can be

accomplished by careful handling and a clean, well-controlled assembly process. When in doubt or for

extra protection, a heat-resistant, protective cover such as Kapton™ KPPD-1/8 polyimide tape can be

installed during PCB assembly.

TH06 may be ordered with a factory-fitted, solder-resistant protective cover. This cover provides protection

during PCB assembly or rework but without the time and effort required to install and remove the Kapton tape. It

can be left in place for the lifetime of the product, preventing liquids, dust or other contaminants from coming into

contact with the polymer sensor film. See Section “8. Ordering Guide” for a list of ordering part numbers that

include the cover.

4.4.2. Rehydration

The measured humidity value will generally shift slightly after solder reflow. A portion of this shift is permanent and

is accounted for in the accuracy specifications in Table 4. After soldering, an TH06 should be allowed to

equilibrate under controlled RH conditions (room temperature, 45–55%RH) for at least 48 hours to eliminate the

remainder of the shift and return the device to its specified accuracy performance.