Is it 'ok' to have electrostatic sensitive components on an end-user, exposed PCB design?

Question

I'm currently working on an electronics project. This project is intended to be portable and battery powered. I'm curious about whether or not I should use certain components on my PCB do being too ESD sensitive.

For example, I'm currently trying to use this clock on my design. After ordering it, I notice that it came packaged with a warning to only open/handle at an electrostatic-free workstation.

Now obviously I could oblige to this, however, my end design is an exposed PCB in which a user may accidentally touch. The end user (colleagues of mine, not external customers) will be handling this device to press pushbuttons, look at lights, etc. Furthermore, my design only covers the electronic portion thus I can't create an enclosure to protect during operation. That's the end user's duty.

So I'm somewhat concerned as to whether or not I should proceed with using the previously referenced IC. I'm also not sure as to what kind of ESD protection is used on portable electronics that may allow me to use the chip.

So my questions:

• Is there ever a point in which a certain component shouldn't be selected on an open PCB design because it's too ESD sensitive? Or can you just bypass this with appropriate ESD protection?
  • What kind of ESD proection is used for portable electronics?

Answer 1

There will always be a certain ESD level which can damage your product. The question is, how much protection do you need so that it is "good enough" ?

For (portable) consumer products the sensitive points are all the conductive / metal contacts which are exposed. So for a mobile phone, you would use TVS diodes or such to implement extra protection on the charging port and headphone socket for example. That is not to say that there would be no protection without these extra diodes ! There is always ESD protection in any modern IC but it will be a lower level protection, it can handle ESD but not too much. For example: 500 V HBM (Human Body Model).

This level of protection (500 V HBM for example) would be enough for handling that crystal oscillator you ordered in a somewhat ESD safe environment. With this I mean, precautions have been taken to keep ESD pulses at a low level. For example: a conductive and grounded ESD mat and the user grounded by wearing an ESD wrist band.

Don't draw too much of a conclusion about ESD sensitivity by looking at the packaging or warnings companies like mouser use. If you would order resistors I bet they would also come in ESD safe packaging and might also come with a warning although resistors are not ESD sensitive.

Regarding your PCB, since it is not a consumer product and you can expect the user to know that (s)he should not be poking his/her finger everywhere and generate ESD discharges. In my experience the build-in ESD protection of most ICs is sufficient and will provide enough protection for normal usage by a user who is aware of ESD.

So if you have external connections on your board: use extra protection.

For the part that the user should not touch anyway: no extra protection needed.

Answer 2

Let me tell you a story.

In the early 80's all keyboards would have tiny red LEDs exposed and customers found their keyboards would die for no reason. Our company did research on our computer products and tested every new product with a Schaefer 15kV to 25kV ESD generator to simulate the human finger or cart model and found the zap could be silent and not felt by a user going past the exposed 5mm LED to the leads and either anode or cathode connect to CMOS logic could trigger an SCR latchup effect which could cause the so-called ESD protected CMOS to burnup ( internal short, not flames) from an SCR crowbar in the sublayer of all CMOS.

Since then all LED's were buried well below the surface and used light pipes to prevent this or very long epoxy bodies just below a plastic surface guard.

If you do not test your product for ESD susceptibility or design it to withstand 1 hundred random discharges @ 15kV from 300pF, to all user accessible interfaces

... you can expect failures unless the user is given clear instructions how to avoid this and accept the consequences of failure.

• I have dozens of similar stories of ESD failures at embarrassing times.

One day (1982) we were in a customer (MTS) owned "test home" with all our newly invented technology aka Project IDA, that offered digital telephony, pay-Tv , fire/burglar alarms , tele-shopping with remote keyboard, online TV opinion polling in real-time, digital weather maps, high speed serial ports in the phone jacks with dual RF TV converters outside on the poles and a whole 2 way 1.544 Mb sync modem to every home in the trial. Then the Mayor of St Louis was interested and brought his group for a demo and when the demonstrator touched the TV it conducted ESD to the motherboard in the basement and reset the unit into a fault mode, recovered by power cycling. ( so after quickly power cycling it .. dry winter in Winnipeg, the demo resumed) ( Mayor never bough it and was just trying to steal the technology to buy votes for next election) it ended up being sold off to a company that own Scientific Atlanta, Mattel, Intelivision etc. in Phili.PA and the company was bankrupt ahead of its time.

Next day we had 3 fixes for the ESD event to never happen again with improved ground shift immunity with CM choke , better grounding etc.