Datasheet SHT21

www.sensirion.com / D1

Version 5 – October 2022

5/14

2.2

Storage Conditions and Handling Instructions

Moisture Sensitivity Level (MSL) is 1, according to

IPC/JEDEC J-STD-020. At the same time, it is

recommended to further process the sensors within 1 year

after date of delivery.

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 metal-in PE-HD12.

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” [1] or contact Sensirion.

2.3

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 10.

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 12 bit accuracy shall be made.

12 For example, 3M antistatic bag, product “1910” with zipper.

Figure 10 Top view of example of mounted SHT2x with slits

milled into PCB to minimize heat transfer.

2.4

Light

The SHT2x is not light sensitive. Prolonged direct

exposure to sunshine or strong UV radiation may age the

sensor.

2.5

Materials Used for Sealing / Mounting

Many materials absorb humidity and will act as a buffer

increasing response times and hysteresis. Materials in the

vicinity of the sensor must therefore be carefully chosen.

Recommended materials are: Any metals, LCP, POM

(Delrin), PTFE (Teflon), PEEK, PP, PB, PPS, PSU, PVDF,

PVF.

For sealing and gluing (use sparingly): Use high filled

epoxy for electronic packaging (e.g. glob top, underfill),

and Silicone. Out-gassing of these materials may also

contaminate the sensor (see Section 2.2). Therefore try to

add the sensor as a last manufacturing step to the

assembly, store the assembly well ventilated after

manufacturing or bake at >50°C for 24 h to outgas

contaminants before packing.

2.6

Wiring Considerations and Signal Integrity

Carrying the SCL and SDA signal parallel and in close

proximity (e.g. in wires) for more than 10 cm may result in

cross talk and loss of communication. This may be

resolved by routing VDD and/or VSS between the two

SDA signals and/or using shielded cables. Furthermore,

slowing down SCL frequency will possibly improve signal

integrity. Power supply pins (VDD, VSS) must be

decoupled with a 100 nF capacitor – see Section 3.