Datasheet SHT21

www.sensirion.com / D1

Version 5 – October 2022

4/14

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 lands9 should be 0.2 mm 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 8.

The solder mask10 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 120 μm to

150 μm larger than the pad size, providing a 60 μm to

75 μm 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 8 Recommended metal land pattern for SHT2x. 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 0.125 mm

stencil thickness is recommended. For the I/O pads the

stencil apertures should be 0.1 mm longer than PCB pads

and positioned with 0.1 mm offset away from the centre of

the package. The die pad aperture should cover about

70 – 90% of the pad area – say up to 1.4 mm x 2.3 mm

9 The land pattern is understood to be the metal layer on the PCB, onto which

the DFN pads are soldered to.

10 The solder mask is understood to be the insulating layer on top of the PCB

covering the connecting lines.

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 paste11 is recommended as well as Nitrogen

purge during reflow.

Figure 9 Soldering profile according to JEDEC standard. TP <=

260°C and tP < 30 sec for Pb-free assembly. TL < 220°C and tL

< 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 SHT2x, 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 30 sec for Pb-free

assembly in IR/Convection reflow ovens (see Figure 9).

For manual soldering contact time must be limited to

5 seconds at up to 350°C.

Immediately after the exposure to high temperatures the

sensor may temporarily read a negative humidity offset

(typ. -1 to -2 %RH after reflow soldering). This offset

slowly disappears again by itself when the sensor is

exposed to ambient conditions (typ. within 1-3 days). If RH

testing is performed immediately after reflow soldering,

this offset should be considered when defining the test

limits.

In no case, neither after manual nor reflow soldering, a

board wash shall be applied. Therefore, and as mentioned

above, it is strongly recommended to use “no-clean” solder

paste. In case of applications with exposure of the sensor

to corrosive gases or condensed water (i.e. environments

with high relative humidity) the soldering pads shall be

sealed (e.g. conformal coating) to prevent loose contacts

or short cuts.

11 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).

1.0

1.0

0.3

0.4

2.4

Time

tP

TP

TL

TS (max)

tL

preheating

critical zone