Electronic – 1-wire humidity sensor with ID

The primary concern is that the plastic packaging around the chips absorbs water. When you go to reflow that part on a board, that water boils and expands. With that expansion, bubbles form inside the plastic - this can cause the package to deform and even damage the internal connections. The visible external effects are called "popcorning".

This sensitivity to moisture is classified as Moisture Sensitivity Levels (MSL). Every part can be rated for how quickly it absorbs moisture. Higher numbers indicate higher sensitivity, with MSL 6 parts always requiring a bake before use. Most parts that I've seen are MSL 5/5a, in which a 48-24 hour exposure period before requiring a bake. Best practices would be to open the part bag on a moisture sensitive part just before assembly; and then reseal the bag after the part is removed. Look up Moisture Sensitivity Levels for more information.

My personal concern about MSL is proportional to the number of boards I'm making as well as the cost of the part. However, for one-off boards, it's simple enough to just open the part bag when you're ready to use it. Production lines need to keep track of the hours a part bag is open, and should bake the part as needed. Popcorning is most likely to show up in a reflow process, and high temperature reflow processes in particular (e.g. lead-free solder).

Since the moisture sensitivity is only related to the manufacturing aspect, you do not need to worry about it once the moisture sensitive part is attached to the PCB. The one exception is in the event that you want to remove the moisture sensitive part from the board after it has been in the field; and you want the part to be in good condition afterwards. In that case, you may need to bake the board before desoldering the part.

Page 3 of this paper has images of popcorning effects as well as a table of the different MSL requirements.

The MLX90614 actually can work over large distances. Since it is an infrared sensor, it detects the radiation from the object you want to measure, and this does not really change with the distance. The limiting factor for the distance is the angle which the sensor covers. When you look at the data sheet you see that the sensor comes in different variants, from 90 degree down to 10 degree sensor angle. The sensor is most accurate when the object to measure fully covers its field-of-view. For the 10 degree version, the object should be larger than 17cm (at a distance of 1m). If it is smaller, the sensor will report the average temperature of what it sees.

Another option would be the TMP006 from TI, which has an I2C interface. But since it has a larger FOV (nearly 180 degree), at a distance of 1m the object should have a diameter of about 4m. Nonetheless, the TMP006 userguide makes a good read, since it explains the basic of infrared measurements, and all the calculation much better than the Melexis data sheet.

One thing to note, though, is that IR sensors will measure the skin temperature, which can vary significantly from the body temperature.