SI7015-A10 Datasheet, PDF (17/39 Pages) Silicon Laboratories – Excellent long term stability

Si7015-A10
4.7. Protecting the Sensor
Because the sensor operates on the principal of measuring a change in capacitance, any changes to the dielectric
constant of the polymer film will be detected as a change in relative humidity. Therefore, it is important to minimize
the probability of contaminants coming into contact with the sensor. Dust and other particles as well as liquids can
affect the RH reading. It is recommended that a filter cover is employed in the end system that blocks contaminants
but allows water vapor to pass through. Depending on the needs of the application, this can be as simple as plastic
or metallic gauze for basic protection against particulates or something more sophisticated such as a hydrophobic
membrane providing up to IP67 compliant protection.
Si7015s may be ordered with a factory fitted, solder-resistant cover, which can be left in place for the lifetime of the
product. It is very low-profile, hydrophobic and oleophobic, and excludes particulates down to 0.35 microns in size.
See Section “8. Ordering Guide” for a list of ordering part numbers that include the cover. A dimensioned drawing
of the IC with the cover is included in Section “9. Package Outline”. Other characteristics of the cover are listed in
Table 10. The sensor should be protected from direct sunlight to prevent heating effects as well as possible
material degradation.
Table 10. Specifications of Protective Cover
Parameter
Material
Water Entry Pressure
Pore Size
Operating Temperature
Maximum Reflow Temperature
Oleophobicity (AATCC 118 – 1992)
IP Rating (per IEC 529)
Value
ePTFE
2.7 bar
0.35µ
–40 to +125 °C
260 °C
7
IP67
4.8. Bake/Hydrate Procedure
After exposure to extremes of temperature and/or humidity for prolonged periods, the polymer sensor film can
become either very dry or very wet; in each case the result is either high or low relative humidity readings. Under
normal operating conditions, the induced error will diminish over time. From a very dry condition, such as after
shipment and soldering, the error will diminish over a few days at typical controlled ambient conditions, e.g., 48
hours of 45 ≤ %RH ≤ 55. However, from a very wet condition, recovery may take significantly longer. To accelerate
recovery from a wet condition, a bake and hydrate cycle can be implemented. This operation consists of the
following steps:
Baking the sensor at 125 °C for ≥ 12 hours
Hydration at 30 °C in 75 %RH for ≥ 10 hours
Following this cycle, the sensor will return to normal operation in typical ambient conditions after a few days.
4.9. Long Term Drift/Aging
Over long periods of time, the sensor readings may drift due to aging of the device. Standard accelerated life
testing of the Si7015 has resulted in the specifications for long-term drift shown in Table 4 and Table 5. This
contribution to the overall sensor accuracy accounts only for the long-term aging of the device in an otherwise
benign operating environment and does not include the affects of damage, contamination, or exposure to extreme
environmental conditions.
Rev. 1.0
17