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1-100645-01 Datasheet(PDF) 4 Page - Sensitron

Part No. 1-100645-01
Description  Humidity and Temperature Sensor
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Maker  SENSITRON [Sensitron]
Homepage  http://www.sensitron.com
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Datasheet SHT21
www.sensirion.com
Version 1.0 – January 2010
4/12
the package. The die pad aperture should cover about 70
– 90% of the pad area – say up to 1.4mm x 2.3mm
centered on the thermal land area. It can also be split in
two openings.
Due to the low mounted height of the DFN, “no clean”
type 3 solder paste11 is recommended as well as Nitrogen
purge during reflow.
Figure 7 Soldering profile according to JEDEC standard. TP <=
260°C and tP < 40sec for Pb-free assembly. TL < 220°C and tL <
150sec. Ramp-up/down speeds shall be < 5°C/sec.
It is important to note that the diced edge or side faces of
the I/O pads may oxidise over time, therefore a solder fillet
may or may not form. Hence there is no guarantee for
solder joint fillet heights of any kind.
For soldering SHT2x, standard reflow soldering ovens may
be used. The sensor is qualified to withstand soldering
profile according to IPC/JEDEC J-STD-020D with peak
temperatures at 260°C during up to 40sec for Pb-free
assembly in IR/Convection reflow ovens (see Figure 7).
For manual soldering contact time must be limited to 5
seconds at up to 350°C12.
IMPORTANT: After soldering, the devices should be
stored at >75%RH for at least 12h to allow the sensor
element to re-hydrate. Otherwise the sensor may read an
offset that slowly disappears if exposed to ambient
conditions. Alternatively the re-hydration process may be
performed at ambient conditions (>40%RH) during more
than 5 days.
In no case, neither after manual nor reflow soldering, a
board wash shall be applied. Therefore, and as mentioned
above, it is strongly recommended to use “no-clean” solder
paste. In case of applications with exposure of the sensor
to corrosive gases the soldering pads shall be sealed to
prevent loose contacts or short cuts.
2.2
Storage Conditions and Handling Instructions
Moisture Sensitivity Level (MSL) is 2; hence storage time
is limited to one year.
11 Solder types are related to the solder particle size in the paste: Type 3 covers
the size range of 25 – 45 µm (powder type 42).
12 260°C = 500°F, 350°C = 662°F
It is of great importance to understand that a humidity
sensor is not a normal electronic component and needs to
be handled with care. Chemical vapors at high
concentration in combination with long exposure times
may offset the sensor reading.
For this reason it is recommended to store the sensors in
original packaging including the sealed ESD bag at
following conditions: Temperature shall be in the range of
10°C – 50°C and humidity at 20 – 60%RH (sensors that
are not stored in ESD bags). For sensors that have been
removed from the original packaging we recommend to
store them in ESD bags made of PE-HD13.
In manufacturing and transport the sensors shall be
prevented of high concentration of chemical solvents and
long exposure times. Out-gassing of glues, adhesive tapes
and stickers or out-gassing packaging material such as
bubble foils, foams, etc. shall be avoided. Manufacturing
area shall be well ventilated.
For more detailed information please consult the
document “Handling Instructions” or contact Sensirion.
2.3
Reconditioning Procedure
As stated above extreme conditions or exposure to solvent
vapors may offset the sensor. The following reconditioning
procedure may bring the sensor back to calibration state:
Baking:
100 – 105°C at < 5%RH for 10h
Re-Hydration:
20 – 30°C at ~ 75%RH for 12h 14.
2.4
Temperature Effects
Relative
humidity
reading
strongly
depends
on
temperature. Therefore, it is essential to keep humidity
sensors at the same temperature as the air of which the
relative humidity is to be measured. In case of testing or
qualification the reference sensor and test sensor must
show equal temperature to allow for comparing humidity
readings.
If the sensor shares a PCB with electronic components
that produce heat it should be mounted in a way that
prevents heat transfer or keeps it as low as possible.
Measures to reduce heat transfer can be ventilation,
reduction of copper layers between the sensor and the
rest of the PCB or milling a slit into the PCB around the
sensor – see Figure 8.
Furthermore, there are self-heating effects in case the
measurement frequency is too high. To keep self heating
below 0.1°C, SHT2x should not be active for more than
10% of the time – e.g. maximum two measurements per
second at 12bit accuracy shall be made.
13 For example, 3M antistatic bag, product “1910” with zipper.
14 75%RH can conveniently be generated with saturated NaCl solution.
100 – 105°C correspond to 212 – 221°F, 20 – 30°C correspond to 68 – 86°F
Time
tP
TP
TL
TS (max)
tL
preheating
critical zone




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