Datasheet

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”
type 3 solder paste
7
is recommended as well as Nitrogen
purge during reflow.
Figure 6 Soldering profile according to JEDEC standard. T
P
<=
260°C and t
P
< 30sec for Pb-free assembly. T
L
< 220°C and t
L
<
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.
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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).
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
Temperature
Time
t
P
T
P
T
L
T
S
(max)
t
L
preheating
critical zone
1
6
5
2
4
3