Datasheet SHT25

www.sensirion.com

Version 0.91 – October 2010

4/12

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 8.

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 8 Recommended metal land pattern for SHT2x. Values

in mm. Die pad (centre pad) and NC pads may be left floating or

be connected to ground. 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 package. The die pad aperture should cover about 70

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

the DFN pads are soldered to.

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”

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 SHT2x, standard reflow soldering ovens may

be used. The sensor is qualified to withstand soldering

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

temperatures at 260°C during up to 40sec for Pb-free

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

For manual soldering contact time must be limited to 5

IMPORTANT: After soldering, the devices should be

stored at >75%RH for at least 12h to allow the sensor

element to re-hydrate. Otherwise the sensor may read an

offset that slowly disappears if exposed to ambient

conditions. Alternatively the re-hydration process may be

performed at ambient conditions (>40%RH) during more

than 5 days.

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.

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

Time

preheating

critical zone

1.0

1.0

0.3

0.4

2.4

≤2.3