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At what is humidity the risk of electrostatic damage to electronics significant? Clearly 0% would be most dangerous and 100% would be less dangerous, but what are the risks in between?

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    \$\begingroup\$ That depends what is anyone's definition of significant, how much the electronics already have protection in them, and the environment the electronics are in, such as inside a metal chassis, or if you have a carpet and clothes that allows a lot of charge to build up or allows charge to equalize quickly. Can you be more specific about the enviromment? \$\endgroup\$
    – Justme
    Oct 18, 2023 at 17:29
  • \$\begingroup\$ @Justme Hard to be specific, but no carpets and wool clothes involved. Yet MOSFETs are magically dying every day, which never happened before. And I can't see any reasonable explanation. \$\endgroup\$
    – TQQQ
    Oct 18, 2023 at 18:19
  • \$\begingroup\$ How are the FETs being stored, handled, or used? Some of them have ESD protection. Which part number? \$\endgroup\$
    – Justme
    Oct 18, 2023 at 18:24
  • \$\begingroup\$ @Justme these guys are irf200p222. Storage is good, usage - slightly less "by the book", but again, things used to work, now mortality is very high. I am looking to un-voodoo the reasons. \$\endgroup\$
    – TQQQ
    Oct 18, 2023 at 19:03
  • \$\begingroup\$ If they are getting damaged in a circuit, it's likely not ESD then as they are not supposed to be getting ESD under use, unless used incorrectly. Can you post schematics? \$\endgroup\$
    – Justme
    Oct 18, 2023 at 19:09

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Table IV of Department of Defense Handbook MIL-HDBK-263B "Electrostatic Discharge Control Handbook for Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices)" lists typical electrostatic voltages generated in a facility as a function of humidity levels:

https://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=53974 enter image description here

Clearly, less voltage is generated under humid conditions. This being said, humidity is not an indication of the risk of electrostatic damage to electronics, as stated in a note below the table "Caution should be exercised when attempting to correlate the above, or actual measured voltages with the potential to damage ESDS items." Note that ESDS is defined as "Electrical and electronic parts which have been determined to be ESD sensitive".

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  • \$\begingroup\$ Interesting source material +1. One potential compensating factor is that humans tend to sweat more under high temperature and humidity conditions so the effective series resistance from a human finger touch may be quite a bit lower if the finger is wet with salty sweat. \$\endgroup\$ Oct 18, 2023 at 18:29
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According to the this web site, and others with similar wording:

The relative humidity must be below 40 percent in order for a static charge to be produced. 40 to 60 percent will still enable for build-up, but at a significantly reduced level due to the fact that the static leaks to the ground through the air. Conditions above 55 percent RH will never experience static build-up,

A table from this web site which is excerpt from this article: "Exploding the Humidity Half-Truth and Other Dangerous Myths", Moss, R., EOS/ESD Technology Magazine, page 10, April 1987. This table dispels the above article's statement that "above 55% RH will never experience static build-up."

                                   STATIC VOLTAGES
ACTIVITY (@ 70°F, 21°C)            20 % RH  80% RH
================================   =======  ======
Walking across vinyl floor         12 kV    250 V
Walking across synthetic carpet    35 kV    1.5 kV
Arising from foam cushion          18 kV    1.5 kV
Picking up polyethylene bag        20 kV    600 V
Sliding styrene box on carpet      18 kV    1.5 kV
Removing Mylar tape from PC board  12 kV    1.5 kV
Shrinkable film on PC board        16 kV    3 kV
Triggering vacuum solder remover   8 kV     1 kV
Aerosol circuit freeze spray       15 kV    5 kV

The risks of damaging electronic equipment is dependent on the design of the equipment regarding ESD.

For products our company designs, we have ESD protection on any user accessible electrical contacts where it matters. This is tested at an independent lab.
Our workers that handle electronic assemblies wear grounding wrist and/or conductive shoe straps. I go barefoot.

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If this is a relatively new issue then carefully review all your assembly and handling stages. Has anything changed recently that might coincide with the increased failure rates? Did you add a new assembly station, a new process step, a cleaning operation, packing operation, storage procedure, was a process system replaced, moved, or serviced?

Place humidity data loggers in your facilities. Check if the defect rate follows an inverted humidity rate. That along with spot checking can help confirm if the failure issue is being caused locally or externally.

At any of your assembly stations could there have been a ground strap removed or broken? If you added a cleaning step are you using non-static generating cleaning materials? Did your facilities recently switch on their heating system (producing dryer air)?

If you have simple test equipment to perform spot checks try checking assemblies at the midpoint of your assemble process. If you find defects perform additional testing towards the start of the process, if there are no high defect rates at the center move down towards the end of your process.

Did you change supply vendors, or did your vendor's shipping containers change? Do you perform incoming inspection? Do the failed components have a common date code?

If you still cannot find the reasons for the failures you might ask your supply vendor (or the component manufacturer) if they can perform failure analysis on some defective parts. That should be able to verify if you have a static damage issue or something else. At the same time you could inquire with your vendor on the recommended humidity levels for proper handling of the components. Maybe they could send a representative to look over you facility. You might even hint that you hope to increase your production and likely increase component purchases, but the current failure rate is hampering those plans.

Most of the above is just to get you started, there can can many other production issues that can create a static prone environment.

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    \$\begingroup\$ Thank you for the suggestions. The one thing that has really changed is weather - twice a year, in fall and spring, I get a lot of static. So this is what I am trying to figure out. Other than that, it might also be a design mistake, which would be embarrassing 😁 \$\endgroup\$
    – TQQQ
    Oct 19, 2023 at 5:36
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    \$\begingroup\$ Dry hot air heating in the cooler months, then humidity reducing AC air in the warmer months. Perhaps your facility could benefit from an add-on humidity control system. \$\endgroup\$
    – Nedd
    Oct 21, 2023 at 1:18
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This will very much depend on the type of electronics you are working with, i. e. how ESD sensitive they are.

The popular standard DIN EN 61340-5-1 doesn't provide a limit for humidity. It just states that products designed to avoid ESD (ESD floors, wrist straps, ...) need to be qualified at 12 % humidity. So everything below 12% should be considered very dangerous as even an electrostatic protected area (EPA) might not achieve its rated performance.

For general purpose electronics with ESD ratings > 100 V (like most assembled PCBs) the humidity should not drop below 20 %. At least that's the threshold we chose in our company's EPA.

Outside of an EPA all bets are off anyway, if you aren't close to 100 %, which is bad for other reasons.

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  • \$\begingroup\$ So 50% to 70% can be considered as relatively safe? \$\endgroup\$
    – TQQQ
    Oct 18, 2023 at 18:17
  • \$\begingroup\$ @TQQQ Yes, but emphasis on "relatively" ;) Like I said: Outside of an EPA all bets are off. \$\endgroup\$
    – feynman
    Oct 19, 2023 at 6:05
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At what is humidity the risk of electrostatic damage to electronics significant? Clearly 0% would be most dangerous and 100% would be less dangerous, but what are the risks in between?

The risk of ESD damage increases with decreasing humidity. Dry air lacks the water vapor that can help dissipate electrostatic charges, so in low humidity environments, it's easier for these charges to build up and then be discharged when they come into contact with another object, potentially damaging sensitive electronic components.

So:

  1. <20% Relative Humidity (RH): This is considered very low humidity, and the risk of ESD is very high. Electrostatic charges can build up rapidly, and the likelihood of ESD events is greatly increased.

  2. 20% - 40% RH: The risk is still significant in this range. While not as dangerous as <20% RH, precautions should still be taken.

  3. 40% - 60% RH: This is often considered the ideal range for handling most electronics. The risk of ESD is reduced, but it's still possible, especially if other factors (like the type of materials in the environment) contribute to ESD.

  4. 60% - 80% RH: The risk of ESD continues to decrease, but now you start to have concerns about other potential issues, like corrosion or mold growth, especially on the higher end of this range.

  5. >80% RH: ESD risk is minimal, but the high humidity can bring about other problems for electronics, such as condensation, corrosion, and other moisture-related issues.

It's important to note that while humidity plays a significant role in ESD risk, other factors also contribute, including the materials present (some materials are more prone to static build-up than others), the procedures used in handling the electronics, the presence (or lack) of ESD protection measures, and the sensitivity of the specific electronic components being handled.

For most electronics manufacturing and handling facilities, maintaining the environment in the 40% - 60% RH range is ideal. In this range, the risk of ESD is minimized while also avoiding the moisture-related problems of higher humidities. In addition to humidity control, proper grounding, ESD-safe workstations, and the use of ESD-safe tools and garments are essential for preventing ESD damage.

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    \$\begingroup\$ Can you include citation? \$\endgroup\$ Oct 19, 2023 at 11:36
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    \$\begingroup\$ Jerzy Przezdziecki - Hi, Analysis indicates that this answer has been sourced (mostly or completely) from elsewhere. To comply with this site rule, we need posts that copy or adapt material from elsewhere, to properly reference the original source. For online sources, that means adding the source name & webpage/PDF/video link to the answer (and show which part(s) of the post were copied/adapted from that source). For offline sources e.g. books, research papers etc, then a full citation is needed. Please edit your answer to add the required reference. Thanks. \$\endgroup\$
    – SamGibson
    Oct 19, 2023 at 11:50

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