5

I have to design a electronics device that is specified for environmental operating conditions of 0°C to 60°C with 5% to 95% relative humidity (non condensing).

I think the temperature part should be covered with commercial component selection (0°-70° operating temp.), but what about the humidity requirement?

  • Is this humidity specification hard to achieve?
  • Do I need to protect the PCBA with conformal coating?
  • What are the general recommendations for different humidity specifications?

Of course I will need to perform some tests in the climate chamber. But I think there should exist some general guidelines for the design regarding humidity?

Stefan Wyss
  • 6,471
  • 1
  • 11
  • 22
  • 1
    What do you mean by non condensing? I don't think conformal coating would be adequate. – K H Feb 08 '21 at 07:50
  • 3
    I have often seen this "non condensing" addition to relative humidity specifications. I think it means that when you perform tests in the climate chamber, you need to carefully control the climate in order not to reach the dew point. – Stefan Wyss Feb 08 '21 at 07:54
  • Well, you can easily find a cheapy commercial humidity sensor chip, and copycat its spec. Eg, (1) HDS1080 [0% to 100% relative humidity (Fig 2, 14 bit, 2~4% accuracy )] from TI:https://www.ti.com/lit/ds/symlink/hdc1080.pdf?ts=1612770926478&ref_url=https%253A%252F%252Fwww.google.com%252F – tlfong01 Feb 08 '21 at 08:02
  • 1
    If you have high impedance analogue and sensitive circuits, at around 0 degC, water will collect on PCB surfaces if there's humidity. So, I'd be tempted to ensure it works down to -10 degC and see what happens when you gradually warm it up through 0 degC. – Andy aka Feb 08 '21 at 08:53
  • 1
    @Andyaka Thanks for your comment. I have no high impedance analogue and sensitive circuits. It is mainly a digital device. Why do you think that 0°C will be special in term of humidity? When I look at a psychrometric chart, I see no differences to other other temp. ranges. – Stefan Wyss Feb 08 '21 at 09:13
  • I don't understand the difference between condensing and non condensing, but condensing modules are dirt cheap. (1) HR202 Sensor HDS10 Condensation Sensor Humidity Sensitive Module High Humidity Sensitive US$0.30 https://nl.aliexpress.com/item/32951082181.html?spm=a2g0o.search0302.0.0.4fad1d7aM8SYWL&algo_pvid=ecc2d194-d005-4ffd-8e53-3c0fddeed9f0&algo_expid=ecc2d194-d005-4ffd-8e53-3c0fddeed9f0-38&btsid=0bb0624216127753344355513e2689&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_ / ton continue, ... – tlfong01 Feb 08 '21 at 09:17
  • / continue, ... (2) Humidity Sensor Module HR202 Humidity Module Humidity Detection - US$0.5 https://nl.aliexpress.com/item/4000804824633.html?spm=a2g0o.search0302.0.0.4fad1d7aM8SYWL&algo_pvid=ecc2d194-d005-4ffd-8e53-3c0fddeed9f0&algo_expid=ecc2d194-d005-4ffd-8e53-3c0fddeed9f0-31&btsid=0bb0624216127753344355513e2689&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_ – tlfong01 Feb 08 '21 at 09:17
  • Actually I even forgot the difference between H and RH. Of course Google is my friend: (3) Choosing a Humidity Sensor: A Review of Three Technologies - Denes K. Roveti 2001jul01 https://www.fierceelectronics.com/components/choosing-a-humidity-sensor-a-review-three-technologies. Now I understand everything, including that the US$0.5 toy understands the First and Second Law of Thermodynamics and has a built in EEPROM with equations to do the RH calculations. – tlfong01 Feb 08 '21 at 09:36
  • 1
    @StefanWyss - ice that forms on PCB surfaces at below 0 degC starts to melt and becomes more conductive as temperature passes positively through 0 degC and I've had problems with that before that have been cured by varnish/lacquer. – Andy aka Feb 08 '21 at 09:48
  • @tlfong01 I do not understand why you are proposing a humidity sensor when I‘m asking for advice on humidity design requirements? – Stefan Wyss Feb 08 '21 at 11:58
  • @Stefan Wyss, Ah, there is some misunderstanding. I know you are asking (1) If the 5% ~ 95% relative humidity specification is hard to achieve, and (2) If there are general recommendations for different humiudity specifications. My first thought is that your specification is too linent. So I am recommending two example cheap humidity sensors for you to compare their much more strict user requirements. / to continue, ... – tlfong01 Feb 08 '21 at 13:20
  • / cont'd ... These two example, commercially available cheap sensors should also meet the general user requirements and can be practically implemented using 16 bit ADC, and EEPROM with humidity calculations etc. My apologies for misleading you that I am recommending sensors for you to purchase. Have a great project. Cheers. – tlfong01 Feb 08 '21 at 13:22
  • 1
    ZERO degrees is hard to handle, usually you specify from 5°C up. Also 60°C in the environment can quickly become unbearable if you have power devices heating the enclosure. The "non condensing" clause is because otherwise you're substantially required to conformal coat to handle moisture. Otherwise you'll need to dig in the specific environment: for example dust can become conductive and so on. Do a search for 'pollution degree rating' for more info on the humidity issue – Lorenzo Marcantonio Feb 08 '21 at 20:47

2 Answers2

2

For an electronic circuit, that specification is almost completely "bog standard". You likely don't have to do anything special to meet it.

Examples of things that are "non-standard" that you might have to take into account:

  1. something that measures a field, capacitive proximity sensing, for instance, since humidity changes the permittivity of the air

  2. something that has a "diffusion interface"--lots of biochemical sensors have a layer that the active species needs to cross and that can be sensitive to humidity

  3. something that has a weird, non-standard package that doesn't hermetically seal the chip

These are unusual unless you're in a specific domain, and you generally know if you are.

Normally where people get burned is that their boards get put in a "condensing" environment even though everything is specified as "non-condensing". But that's not a design problem so much as a specifications issue.

tl;dr Don't sweat it and just do your design.

Good luck.

Andrew Lentvorski
  • 1,091
  • 4
  • 12
0

Spray application of an acrylic conformal coating would be sufficient for your needs however if you truly wanted to ensure the parts are protected and completely unaffected by its working environment, use Parylene C.

Jacob Thomas
  • 11
  • 1