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)

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)

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.

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.

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

the DFN pads are soldered to.

covering the connecting lines.

1.0

1.0

0.3

0.4

2.4