EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: RogerThat on January 17, 2019, 10:59:27 am
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Hi,
Just wanted to share my recent finding and frustration. After searching the web there are very little first hand experience of ESD killing IC so I thought I would share my story. I've been a believer that ESD so seldomly damage ICs that for hobby stuff it's no point caring too much about it. Until now, I've never had this issue. :-BROKE
I'm building a PCB which holds a 24 bit ADC. The interface to the card is a 2.54 pitch row of straight connector (same way Arduino connects to shields). During my prototyping time I've just soldered cut off resistor legs to these places so that I could easily connect the board to my protobord, these lead directly to the chips SPI interface without any protection. Very smart I thought.
Ok, so in my first design the ADC never wanted to communicate. I concluded that I probably killed it during soldering (I bought two and non of them ever worked). They were TSSOP-24 parts, first time I used them. I switch my design to thru-hole to avoid this problem, putting the ADC in a socket instead of soldering.
Two months later with the new design, the socketed ADC is dead after one week. It did survive long enough for me to do some testing of the system, maybe because is used 200ohm in line resistors on the SPI interface.
So, where does the ESD come from?
My soldering station is in the basement and my measuring /computer is indoor in the heated house. Everytime I solder I walk to the garage and then back, good way to build up static electricity. My comfy sheep skin loafers not helping much. When I get back I push the connectors (with my fingers) in to the protoboard, effectivly discharging my static electricity to the SPI interface. During no time I have feelt/heard/seen a spark working like this in my house. I've also found that my computer(Macbook pro) was connected to a non-grounded outlet and thus effectively charging me up.
So how does an ESD damage IC behave?
It becomes unpredictable and the signals doesn't switch properly. I've attached a screen shot from my scope showing SCLK(yellow) and MISO(blue) signal. As you can see the blue signal (in the beginning and the end) doesn't switch properly between 0v to 5v, but kind of 1/5 way. This video show that transistors don't die in a digital way (work or don't work) but in a gradual way: https://www.youtube.com/watch?v=GshgZE5ANQ0 (https://www.youtube.com/watch?v=GshgZE5ANQ0)
I hope someone finds this useful.
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I've killed a few parts with sparks I could actually noticed, and have had other misbehave in similar ways without seeing sparks. Our animal brain doesn't seem to be very well equipped to handle things it can't directly relate, but that doesn't mean things aren't connected.
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Until now, you've never realised you had this issue.
ESD is a thing, it's fairly simple to avoid but so many people don't because they just don't link cause and effect and there's a mythology grown up around it saying that ESD is fiction.
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If I lived somewhere where static discharge was a thing, like anywhere it gets cold, and, after heating, the relative humidity is near 0%, I would be worried. I noticed this on a couple of trips to Minneapolis, Minnesota, where we could draw an inch long spark from a file cabinet just by walking across the carpet. Fun for a couple of minutes, a PITA by the end of the work day.
OTOH, I live in the Central Valley of California and the humidity is never much less than 50%. Today it is raining and the RH is about 83%. There is no way I can get a static discharge in this environment.
I do have an anti-static mat on my bench but I put it there when I was working on a motherboard with a CPU chip costing multiple hundreds of dollars. For my ordinary projects, there doesn't seem to be a problem. Things that worked a long time ago are still working.
I haven't experienced ESD damage but I have no doubt it can occur.
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It might not even be ESD, if you're supplying power through that header it's likely that some power and signal pins made contact before the ground pins did. Even more likely if you don't have tons of ground pins in the header.
Merely hot-plugging a connector can develop large voltages (2-3 times supply) at low impedance. Ample current to fry ICs, especially when that current goes through signal pins rather than the intended supply/ground path!
Hot-plugging can be protected with a TVS across the supply, and series resistors between connector and IC.
If you do this, then still get occasional failures from handling, it most likely is ESD. You would then add TVS or diode clamps to the connector pins, which absorb most of the ESD pulse, and clamp it to a modest voltage (10s volts), again at low impedance (~10A available during the spike!). The series resistor, between TVS and IC, again limits this current, protecting the IC.
There are more approaches, depending on pin characteristics (output pins probably can't afford much series resistance), range (analog inputs might need lower leakage than a BAT54S or SMAJ5.0A has), or voltage domain (you don't want to use clamp diodes on a shared bus, on a device where its supply voltage is different, or where its supply can be shut down to save power -- clamp diodes would parasite-power it from the bus itself!). But this is the basic outline, absorb the transient then deal with the residual.
Tim
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Thanks for the good feedback! :-+
I'm hard at work getting some ESD protection on the board.....and ESD safe my work place.
The input power to the card is going to be from a 4 cell lithium battery(or 12V Ni-Mh).
What spec would you recommend for the TVS?
For the signal lines I've found a nice 4ch SOT package.
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The problem here is that some years back a whole hocus-pocus industry sprang up around ESD paranoia. Whole industries made money from selling dubious ESD protection gadgets. Manufacturers also used ESD as an excuse for parts failing when the real reason was straightforward shoddy design work. It got to the stage where anything and everything electronic that went wrong was blamed on ESD.
As a consequence of people becoming aware of the scammy nature of such claims, many formed the opinion that ESD itself was nonsense.
The truth, as is often the case, lies somewhere between. ESD damage can occur, but it is mostly seen in specific circumstances.
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After reading some sccary articles on esd I opened my wallet and bought a static meter. No, not new, but something from ebay. Unbelieavable cheap actually, just $70 or so.
I'm not sure how to properly test it. I thought my lab is a bad place (in terms of esd), but my measurements show just 0.08kV of potential difference. Imho too good to believe. But when I put on rubber flip flops and wipe them on the mat under my chair, it shows 0.7-0.8kV. Does this mean my esd meter actually works? I don't really care about precision, but I want to know if it's above some hundreds of volts. That's bad for fets :)
In my lab I have wooden floor, a typical office chair, a plastic mat under it, a typical ikea table and a cutting mat from China. Not sure if I have proper grounding, but if I rub my clothes and then touch tip of my soldering iron I get an electric shock. Hope this means that the tip is grounded and I have a proper ground/earth in mains. Probably, a lot of factors influence esd, most noteably humidity and clothes. My observation is it's better to avoid synthetic clothing.
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No idea if the meter is reading correctly. 5 or 10 kV is absolutely reasonable for that situation. You've got the better part of a Van de Graaf generator there. Don't forget the voltage generated when your butt shifts or sits up from your chair, either!
If you don't have an ESD strap and all that, make a habit of always touching parts by their ground first (or anywhere else that ESD will spread out reasonably safely from), and only then, handling them freely, or plugging them together.
Good example, say, building a PC: touch the metal enclosure first, then while touching that, touch the expansion card's bracket, then pick up the card and plug it in.
Bad example: grabbing the expansion card in the middle, or by the edge connector or any other vulnerable area, then plugging it into the PC where potentially, your body's charge gets shunted to enclosure (and if the enclosure is grounded, to ground ultimately as well) when the bracket makes contact, or when the edge connector plugs in. If your body happens to be dangerously charged at this point, the resulting zap will probably rip through two or more IO pins on the board(s), likely damaging one or both.
Note that it doesn't matter if your body, or the enclosure, is Earth-ground. Voltage is just a difference. Earth-grounding is used commercially because it's less risky to get everyone and everything grounded to one potential, than it is to have them ensure their potentials are matched before interacting.
Tim
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After reading some sccary articles on esd I opened my wallet and bought a static meter. No, not new, but something from ebay. Unbelieavable cheap actually, just $70 or so.
I'm not sure how to properly test it. I thought my lab is a bad place (in terms of esd), but my measurements show just 0.08kV of potential difference. Imho too good to believe. But when I put on rubber flip flops and wipe them on the mat under my chair, it shows 0.7-0.8kV. Does this mean my esd meter actually works? I don't really care about precision, but I want to know if it's above some hundreds of volts. That's bad for fets :)
In my lab I have wooden floor, a typical office chair, a plastic mat under it, a typical ikea table and a cutting mat from China. Not sure if I have proper grounding, but if I rub my clothes and then touch tip of my soldering iron I get an electric shock. Hope this means that the tip is grounded and I have a proper ground/earth in mains. Probably, a lot of factors influence esd, most noteably humidity and clothes. My observation is it's better to avoid synthetic clothing.
What did you buy? I've been looking for something reasonably priced.
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What did you buy? I've been looking for something reasonably priced.
"Monroe Electronics Model 282 Stat-Arc 2 Digital Static Meter". Unfortunately, delivery and customs almost doubled the price, I paid $118 total.
There are some cheap static meters on ali express (~$150 for old model, $300+ for newer models). But most new devices start from $800, even though they look quite cheap :(. It took me a while to find a good deal on ebay.
Most devices look very similar. I wonder if they are made by the same OEM.
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Hey @RogerThat,
any chance you tried to use a MCP3461 or MCP3561 (16/24 bit ADC). I killed a bunch of them too and was wondering why. However, in my case the SPI communication works fine, just the analog section / internal registers seem to be broken.
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Measure the potential between your soldering tip and earth, it might not be properly grounded.
Also, better to walk with naked feet :D
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Also, better to walk with naked feet :D
Ha-ha, so I'm not the only one doing that). I also put one of my feet on the wooden floor (out of the plastic mat). I thought wood is an insulator, but, according to my ESD meter, it helps. I'm also not sure where all the static goes to, the wooden floor is laied on top of concrete floor.
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haha, I am doing that too. I once had a bad experience wearing flip-flops. In my case the ICs worked after soldering but stopped doing so after some time. My assumption is that I touched the board at a sensitive place and then destroyed the ICs. If this is really the case, I should think of something to protect my board from this. It would be really impractical if the board is that sensitive.
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I killed a MOSFET during washing!
I assembled a board, and reworked it, making a right mess. Anyway, tested it and it was all fully functional. I then popped the board in a polypropylene bowl and washed it in aqueous detergent, before rinsing and air drying.
When I tested it again, I found that 1 of the BSS138 MOSFETs had failed short. Cue a lot of grumbling about buying the BSS138's instead of some equivalent with ESD diodes because they were 1p cheaper.
I guess the issue was the polypropylene bowl and the fact that the failed MOSFET had it's gate terminal brought out to a pin header, with only a small series resistor and weak-pulldown to protect it.
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From the symptoms of the failure the original poster reports I can't see how one can be sure it was ESD, nor can I see any way to be sure it wasn't. Taking some ESD precautions in general makes sense, but this failure could easily have had many other causes. Not everything mysterious is necessarily ESD.
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ESD is bad.. I had one very strange happening on PIC microcontroller where ESD damaged it so that it behave strangely. It would have been obvious if it just didn't work but these partially damaged are worst as you spend time looking for the error in software etc..
My setup is not ideal. I have the programmer on other table where my PC is and worktable where PSU and other electronic stuff are on the other side of the room. On worktable I have ESD wristband but I have to remove that every time I go to programming the MCU. On programming table I installed stainless steel plate which is wired to the workstation PE connected frame. It only needs one time getting up from the chair without touching the plate and ZAP! Game over. At wintertime humidity can go down to 16% so you can imagine the static.
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Its interesting about central California. I worked at a couple of places in San Diego and ocassionally there was a weather pattern where instead of a moderating sea breeze, hot winds would come from the dessert and turn us into Phoenix. If the humidity fell below 20% (IIRC), quality would shut down board work and give everyone the day off. We called these Santa Ana snow days and we would all got to the beach for the afternoon. These boss would give the engineers that afternoon off as a morale booster and to interact with the production people. It didn't happen very often, once a year?. ESD is indeed real but like most poorly understood phenomena has a lot of mysticism and BS surrounding it. Its often blamed for failures falsely.
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Tomstein, it was an AD7172-2 if a remember correctly.
Since I wrote the original post I have ESD mats on all my working tables and everything is properly grounded. Not had any issues since.