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A project log for SHT21 Humidity sensor board

Using a Sensirion SHT21 sensor for environmental measurements. Equipped with filter cap, I2C connection.

TheotherMikeTheotherMike 05/15/2016 at 11:590 Comments

All parts arrived and today I tried my first smt soldering at all.

Here, just some photographs during the process....

At first I aligned the board in between the holders made by OSH:

Well, easy and I was glad to have had a small wooden board to which I fixed everything on.

With help of some scotch tape I placed the stencil over it:

And afterwards solder paste was applied. Sorry, no foto, but here´s the PCB with solder on it:

It seems, that there is a little less solder on the SHT21 landing pads....Nevertheless, I tried to move on and placed the few parts onto the pcb.

Well, for me, 0603-size was damn small !! I will need to buy much better tweezers, magnifying glass and perhaps a better solder paste.

I used this one and still don`t know if it was a good choice:

http://www.ebay.de/itm/SMD-BGA-Loeten-Loetpaste-mit-Silber-Sn95-75-Ag3-5-ECO-Solder-Paste-80g-/182146971621?

Ok, so here´s my "soldering setup":

It´s just a hotplate set to 250°C, some pliers and a small piece of aluminum to pick&place the pcb on it onto the hotplate or a small block of aluminum to cool it fastly.

I used a stopwatch and after about 1.5 minutes on the hotplate, the solder of the resistors melted, some seconds later the SHT21 was mooving slightly. Yeaahh, looks good. 2 minutes and 11 seconds later, the pcb was cooled down again.

Hopefully the sensor is OK with that, because when looking at the datasheet, we can find that soldering duration should be less than 30s at max. temperature:

So, just hook it up now...

Teensy 3.0 and Arduino 1.6.8 was used as well as the corresponding library ( https://github.com/elechouse/SHT21_Arduino ):

Et voila, it works !!! The sensor is incredible!!

Of course, I do not know how trustworthy the values are, but response time and small changes of rel. H. induced by breathing or hovering the finger over it is measureble !!


Conclusion:

  1. Great, it works !! :-))
  2. Four mounting holes look nice, but seem to be too many for this small pcb. Within next iteration might use two only.
  3. A little bit more solder paste on the sensor landing pads might be beneficial.
  4. I will do some more projects using the stencil & hotplate process. Hopefully I will get used to it further.

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