HTS221 Temperature/Humidity sensor: What to do about conformal coating?

JSele.1 · ‎2021-06-10

We are in production of a product designed for continuous outdoor deployment. The enclosure is IP67 with a Gore vent to allow access of water vapor (for humidity measurement). Hence, the PCB must be conformal coated to protect against corrosion/oxidation of pads and leads. Obviously cannot conformal coat the HTS221, so we mask with solder mask goop. When "cured" with time, we pluck off the goop but find that humidity testing is subsequently compromised: Accuracy is wanting. Has anyone experience with conformal coating a PCB with HTS221. We suspect that there is a residue left behind when pulling off the solder mask. Experience and suggestions are greatly appreciated.

Jim S.

Pi-Lit

Eleon BORLINI · ‎2021-06-11

Hi @JSele.1 Jim S.,

it is certainly a delicate process, I agree with you.

My suggestion is to try to "calibrate" again the sensor, after the mounting on the application board, and for this you can try to activate for some second the Heater bit of the CTRL_REG2 (21h) register.

As reported in the datasheet, p.23:

The Heater bit is used to control an internal heating element, that can effectively be used to speed up the sensor recovery time in case of condensation. The heater can be operated only by an external controller, which means that it has to be switched on/off directly by FW.

Please let me know if this can at least mitigate the accuracy drift.

-Eleon

JSele.1 · ‎2021-07-01

Thank you Eleon, and my apologies for the delayed reply. To dig deeper into this issue, a few questions:

1) What is the mechanical mechanism of humidity sensing? Not looking for proprietary information, simply the mechanical construction of the component. Where is the vent for humidity access? This information will allow us to customize the conformal coating process, or to mask appropriately.

2) Is it your understanding that an inert single layer film left my a glob of solder mask when plucked off would disrupt the humidity measurement? I am not sure that we even leave a film, but suppose that this occurs. What is the mechanism that this film might interfere with measurement (this related back to question 1)?

3) On a slightly different topic, during production we program our microcontroller with a test FW that interrogates various sensors and registers. We measure temperature and humidity and compare that against a standard laboratory sensor connected to the PC by USB. If the HTS221 reads within 10% of the standard, we pass. This all takes place in a few seconds. We find that the humidity universally fails, even without conformal coating. The datasheet suggests that the humidity sensor requires 10 seconds to accurately report a "step" (15 seconds for temperature). Question: assuming thermal equilibrium has been established (no step - PCB at stable temperature for hours), how long from application of power to reading is required for accurate measurement?

Thank you.

Jim S.