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4 / 12 page Datasheet SHT21 www.sensirion.com Version 1.0 – January 2010 4/12 the package. The die pad aperture should cover about 70 – 90% of the pad area – say up to 1.4mm x 2.3mm 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 7 Soldering profile according to JEDEC standard. TP <= 260°C and tP < 40sec 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-020D with peak temperatures at 260°C during up to 40sec for Pb-free assembly in IR/Convection reflow ovens (see Figure 7). For manual soldering contact time must be limited to 5 seconds at up to 350°C12. 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 the soldering pads shall be sealed to prevent loose contacts or short cuts. 2.2 Storage Conditions and Handling Instructions Moisture Sensitivity Level (MSL) is 2; hence storage time is limited to one year. 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). 12 260°C = 500°F, 350°C = 662°F It is of great importance to understand that a humidity sensor is not a normal electronic component and needs to be handled with care. Chemical vapors at high concentration in combination with long exposure times may offset the sensor reading. For this reason it is recommended to store the sensors in original packaging including the sealed ESD bag at following conditions: Temperature shall be in the range of 10°C – 50°C and humidity at 20 – 60%RH (sensors that are not stored in ESD bags). For sensors that have been removed from the original packaging we recommend to store them in ESD bags made of PE-HD13. In manufacturing and transport the sensors shall be prevented of high concentration of chemical solvents and long exposure times. Out-gassing of glues, adhesive tapes and stickers or out-gassing packaging material such as bubble foils, foams, etc. shall be avoided. Manufacturing area shall be well ventilated. For more detailed information please consult the document “Handling Instructions” or contact Sensirion. 2.3 Reconditioning Procedure As stated above extreme conditions or exposure to solvent vapors may offset the sensor. The following reconditioning procedure may bring the sensor back to calibration state: Baking: 100 – 105°C at < 5%RH for 10h Re-Hydration: 20 – 30°C at ~ 75%RH for 12h 14. 2.4 Temperature Effects Relative humidity reading strongly depends on temperature. Therefore, it is essential to keep humidity sensors at the same temperature as the air of which the relative humidity is to be measured. In case of testing or qualification the reference sensor and test sensor must show equal temperature to allow for comparing humidity readings. If the sensor shares a PCB with electronic components that produce heat it should be mounted in a way that prevents heat transfer or keeps it as low as possible. Measures to reduce heat transfer can be ventilation, reduction of copper layers between the sensor and the rest of the PCB or milling a slit into the PCB around the sensor – see Figure 8. Furthermore, there are self-heating effects in case the measurement frequency is too high. To keep self heating below 0.1°C, SHT2x should not be active for more than 10% of the time – e.g. maximum two measurements per second at 12bit accuracy shall be made. 13 For example, 3M antistatic bag, product “1910” with zipper. 14 75%RH can conveniently be generated with saturated NaCl solution. 100 – 105°C correspond to 212 – 221°F, 20 – 30°C correspond to 68 – 86°F Time tP TP TL TS (max) tL preheating critical zone |
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