Datasheet STS21

www.sensirion.com

Version 2 – December 2011

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”

purge during reflow.

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 STS21, standard reflow soldering ovens may

be used. The sensor is qualified to withstand soldering

profile according to IPC/JEDEC J-STD-020 with peak

temperatures at 260°C during up to 30sec for Pb-free

assembly in IR/Convection reflow ovens (see Figure 6).

For manual soldering contact time must be limited to 5

seconds at up to 350°C.

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.

During storage, temperature shall be in the range of 10°C

– 50°C.

2.3

Temperature Effects

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 7.

the size range of 25 – 45 µm (powder type 42).

Furthermore, there are self-heating effects in case the

measurement frequency is too high. To keep self heating

below 0.1°C, STS21 should not be active for more than

10% of the time – e.g. maximum two measurements per

second at 14bit accuracy shall be made.

Figure 7 Top view of example of mounted STS21 with slits

milled into PCB to minimize heat transfer.

2.4

Light

The STS21 is not light sensitive. Prolonged direct

exposure to sunshine or strong UV radiation may age the

sensor.

2.5

Wiring Considerations and Signal Integrity

Carrying the SCL and SDA signal parallel and in close

proximity (e.g. in wires) for more than 10cm 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 100nF capacitor – see next Section.

3 Interface Specifications

Pin Name

Comment

1

SDA Serial Data, bidirectional

2

VSS Ground

5

VDD Supply Voltage

6

SCL Serial Clock, bidirectional

3,4

NC Not Connected

Table 2 STS21 pin assignment, NC remain floating (top view)

3.1

Power Pins (VDD, VSS)

The supply voltage of STS21 must be in the range of 2.1 –

3.6V, recommended supply voltage is 3.0V. Power supply

pins Supply Voltage (VDD) and Ground (VSS) must be

decoupled with a 100nF capacitor, that shall be placed as

close to the sensor as possible – see Figure 8.

3.2

Serial clock (SCL)

SCL is used to synchronize the communication between

microcontroller (MCU) and the sensor. Since the interface

Time

preheating

critical zone

1

6

5

2

4

3