Datasheet SHT20 (Sensirion) - 4

link to page 4 link to page 4 Datasheet SHT20
2 Application Information
– 90% of the pad area – say up to 1.4 mm x 2.3 mm centered on the thermal land area. It can also be split in
2.1 Soldering Instructions
two openings. The DFN’s die pad (centre pad) and perimeter I/O pads Due to the low mounted height of the DFN, “no clean” are fabricated from a planar copper lead-frame by over- type 3 solder paste11 is recommended as well as Nitrogen molding leaving the die pad and I/O pads exposed for purge during reflow. 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 TP tP of the sensor pads is plated with Ni/Pd/Au.
ruat
TL tL On the PCB the I/O lands9 should be 0.2 mm longer than
erp
TS (max) the package I/O pads. Inward corners may be rounded to
em
match the I/O pad shape. The I/O land width should match
T e
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. preheatin critical zone
Time
g The solder mask10 design for the land pattern preferably is
Figure 9
Soldering profile according to JEDEC standard. TP <= of type Non-Solder Mask Defined (NSMD) with solder 260°C and tP < 30 sec for Pb-free assembly. TL < 220°C and tL mask openings larger than metal pads. For NSMD pads, < 150 sec. Ramp-up/down speeds shall be < 5°C/sec. the solder mask opening should be about 120 μm to It is important to note that the diced edge or side faces of 150 μm larger than the pad size, providing a 60 μm to the I/O pads may oxidise over time, therefore a solder fillet 75 μm design clearance between the copper pad and may or may not form. Hence there is no guarantee for solder mask. Rounded portions of package pads should solder joint fillet heights of any kind. have a matching rounded solder mask-opening shape to minimize the risk of solder bridging. For the actual pad For soldering SHT2x, standard reflow soldering ovens may dimensions, each pad on the PCB should have its own be used. The sensor is qualified to withstand soldering solder mask opening with a web of solder mask between profile according to IPC/JEDEC J-STD-020 with peak adjacent pads. temperatures at 260°C during up to 30 sec for Pb-free assembly in IR/Convection reflow ovens (see Figure 9). 0.4 0.3 2 .0 For manual soldering contact time must be limited to 4 . 5 seconds at up to 350°C. 7 . 0 0 Immediately after the exposure to high temperatures the 5 sensor may temporarily read a negative humidity offset .1 2.4 (typ. -1 to -2%RH after reflow soldering). This offset slowly disappears again by itself when the sensor is exposed to 2 .0 ambient conditions (typ. within 1-3 days). If RH testing is performed immediately after reflow soldering, this offset 1.0 1.0 should be considered when defining the test limits.
Figure 8
Recommended metal land pattern for SHT2x. Values In no case, neither after manual nor reflow soldering, a in mm. Die pad (centre pad) may be left floating or be connected board wash shall be applied. Therefore, and as mentioned to ground, NC pads shall be left floating. The outer dotted line represents the outer dimension of the DFN package. above, it is strongly recommended to use “no-clean” solder paste. In case of applications with exposure of the sensor For solder paste printing a laser-cut, stainless steel stencil to corrosive gases or condensed water (i.e. environments with electro-polished trapezoidal walls and with 0.125 mm with high relative humidity) the soldering pads shall be stencil thickness is recommended. For the I/O pads the sealed (e.g. conformal coating) to prevent loose contacts stencil apertures should be 0.1 mm longer than PCB pads or short cuts. and positioned with 0.1 mm offset away from the centre of the package. The die pad aperture should cover about 70 9 The land pat ern 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 11 Solder types are related to the solder particle size in the paste: Type 3 covers covering the connecting lines. the size range of 25 – 45 µm (powder type 42). www.sensirion.com Version 5 – October 2022 4/14