Datasheet

www.sensirion.com Version 2 – December 2011 3/12
Users Guide STS21
1 Extended Specification
1.1 Electrical Specification
Current consumption as given in Table 1 is dependent on
temperature and supply voltage VDD. For estimations on
energy consumption of the sensor Figures 3 and 4 may be
consulted. Please note that values given in these Figures
are of typical nature and the variance is considerable.
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120
Temperature (°C)
Supply Current IDD (μA)
Figure 3 Typical dependency of supply current (sleep mode)
versus temperature at VDD = 3.0V. Please note that the
variance of these data can be above ±25% of displayed value.
6
8
10
12
14
16
18
20
2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5
Supply Voltage (VDD)
Supply Current IDD (nA)
Figure 4 Typical dependency of supply current (sleep mode)
versus supply voltage at 25°C. Please note that deviations may
be up to ±50% of displayed value. Values at 60°C scale with a
factor of about 15 (compare Table 1).
2 Application Information
2.1 Soldering Instructions
The DFN’s die pad (centre pad) and perimeter I/O pads
are fabricated from a planar copper lead-frame by over-
molding leaving the die pad and I/O pads exposed for
mechanical and electrical connection. Both the I/O pads
and die pad should be soldered to the PCB. In order to
prevent oxidation and optimize soldering, the bottom side
of the sensor pads is plated with Ni/Pd/Au.
On the PCB the I/O lands
5
should be 0.2mm longer than
the package I/O pads. Inward corners may be rounded to
match the I/O pad shape. The I/O land width should match
the DFN-package I/O-pads width 1:1 and the land for the
die pad should match 1:1 with the DFN package – see
Figure 5.
The solder mask
6
design for the land pattern preferably is
of type Non-Solder Mask Defined (NSMD) with solder
mask openings larger than metal pads. For NSMD pads,
the solder mask opening should be about 120μm to
150μm larger than the pad size, providing a 60μm to 75μm
design clearance between the copper pad and solder
mask. Rounded portions of package pads should have a
matching rounded solder mask-opening shape to minimize
the risk of solder bridging. For the actual pad dimensions,
each pad on the PCB should have its own solder mask
opening with a web of solder mask between adjacent
pads.
Figure 5 Recommended metal land pattern for STS21. Values
in mm. Die pad (centre pad) may be left floating or be connected
to ground, NC pads shall be left floating. The outer dotted line
represents the outer dimension of the DFN package.
For solder paste printing a laser-cut, stainless steel stencil
with electro-polished trapezoidal walls and with 0.125mm
stencil thickness is recommended. For the I/O pads the
stencil apertures should be 0.1mm longer than PCB pads
and positioned with 0.1mm offset away from the centre of
5
The land pattern is understood to be the metal layer on the PCB, onto which
the DFN pads are soldered to.
6
The solder mask is understood to be the insulating layer on top of the PCB
covering the connecting lines.
1.0
1.0
0.3
0.4
1.5
0.4
0.7
0.2
0.2
2.4