FAQs
cancel
Turn on suggestions
Auto-suggest helps you quickly narrow down your search results by suggesting possible matches as you type.
Showing results for 
Search instead for 
Did you mean: 

HTS221 temperature stuck at 35C and unreliable humidity measurement

JISEA
Associate

‎2019-09-26 06:37 PM

Hi there,

I managed to solder about 20 HTS221 on my custom board using a small oven, some boards have a correct temperature and humidity measurement, but others are out of luck.

issue 1:

temperature stuck around 35C, in my room at 24c, changes on the temperature on the sensor surface e.g putting a finger above it, makes it change few decimals above or below 35c

issue 2:

I noticed a huge inconsistency with the humidity reads, I am almost a constant 55% humidity, and some boards, read 41%, 80%, 99%, 130%, 1460%!!, why this inconsistency? I know after soldering I have to re-hydrate the sensor, but it has been at 25c and 56% humidity for almost 2 weeks, and still the same inconsistency.

I am doing the soldering by myself using a small oven following the reflow profile below for a Clean Lead Free Low Temperature Solder Paste which melt around 141c

0690X000009jKQeQAM.png

in some custom boards, after I noticed the issue with the humidity/temperature reads, I change it manually using a hot air gun never exceeding 160c for my soldering profile, and the issue persist, I tried with some sensor replacing them 6 times in row presenting the same issue as well

0 Kudos
1 REPLY 1
Eleon BORLINI
ST Employee

‎2019-09-27 06:46 AM

Hi @JISEA​ , thank you for the detailed explanation of the issue. A few points to be clarified:

  1. Consider that the humidity sensor output is a 16-bit data with typical sensitivity 256 and a zero point around 30%fH. if you consider 2^15/2^7 is 2^8 max, meaning 128%+30%=158%rH.
  2. Samples appear to be completely uncalibrated or, at least, their data are wrongly interpreted. Can you please share me the regmap dumping of the registers 10h to 1Fh, 28h to 2Bh and 30h to 34Fh. They mostly refer to the calibration and factory values.
  3. Another point is that you can enable the test heater and check if the humidity and temperature changes after an on-off cycle, to check if the sensor is well connected.

About the soldering phase, especially if the temperature is not fully controlled, you need to absorb the residual humidity from the package. So you should perform a 1 or 2 days of re-hydration process

Regards

0 Kudos
Top