Static Electricity on Components

Discussion in 'General Electronics Chat' started by Dritech, Jul 7, 2016.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi all,

    When we order components, they are usually packed in anti-static bags etc.

    If the components are so prone to being damaged by static electricity, what measures are taken to prevent such damages on PCBs which have no protective case? For instance, most of the development boards are not protected by cases and they do not come in anti-static bags. Their packaging rarely contain labels warning that the device may be electrostatic sensitive.

    I am currently working on a PCB project which I intend to leave without any protective case. Is there any measures that I should take to prevent electrostatic sensitive devices from being damaged?
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,651
    632
    Without knowing more than what you posted above, the answer would be "yes".

    Post a schematic and a description of what it does and its environment and you might receive more specific advice.
     
  3. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,931
    382
    Most ICs have input protection diodes to the supply rails. While the IC is not in a circuit these protections are ineffective and so the IC needs the conductive etc. Once the IC is installed on a board there is a relatively low impedance (e.g a bypass capacitor) across the supply rails and that input protection is now effective.

    Having said that, I was working in a development lab where the floor was carpeted and this carpet generated large amounts of static electricity when you walked on it. I was using a piece of equipment (fully assembled into a case) with a built-in keyboard and the development manager approached and pointed to this equipment. I saw a blue spark leap from his finger to the keyboard :eek: and the equipment instantly died and had to be sent away for repair.

    Take care out there :)
     
  4. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the reply. I am currently still working on the design but I will post a schematic as soon as I finish.
    But the question is more general one as why all the anti-stric packageing when receiving individual components, but then some PCBs are left without any protection against electrostatic discharges? (such as most development boards)
     
    Last edited: Jul 7, 2016
  5. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,308
    884
  6. dl324

    Distinguished Member

    Mar 30, 2015
    3,242
    619
    They should. It's cheap insurance to avoid difficult to troubleshoot problems caused by ESD.
    Experienced people would know when to take precautions.
    I've seen resistors in series with board inputs/outputs. Resistor size is a compromise between limiting current from an ESD event and the RC delay it adds to the net.
     
  7. dl324

    Distinguished Member

    Mar 30, 2015
    3,242
    619
    But no competent engineer/technician would count on the protection diodes for worst case events. Protection diode testing is only done to a few thousand volts. It's possible to generate much higher voltages that will blow the diodes and the diodes are typically part of the MOS devices so the devices themselves are damaged.
     
  8. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,677
    2,729
    My 2¢:

    Modern silicon, in general, is far more impervious to static discharge damage than in the past. Bare chips should be handled as carefully as possible, but once assembled onto a PCB, impedances are, generally, low enough that static discharge is rarely a problem.

    Some days, I handle hundreds of PCBs with little or no attention to static discharge. It's been years since I've seen a board damaged by static.

    Yes, I live and work in Florida where the humidity is usually high -- but we also use A/C and I can generate sparks from the carpet on days when it is cranking.
     
  9. dl324

    Distinguished Member

    Mar 30, 2015
    3,242
    619
    That hasn't been my experience being involved with IC design. At the 130nm node, the gate oxides were so thin that the reverse biased diodes we traditionally used to protect gates during manufacturing stopped giving any protection. The gate oxides broke down before the diodes started conducting. We had switch to a different protection device.

    With lower operating voltages, oxides are scaled down. This makes ESD even more problematic. Even the voltages generated during manufacturing are sufficient to cause damage, so designs need to include even more design for manufacturing (DFM) considerations.
    This goes against conventional wisdom. If PCBs with static sensitive components don't need protection, why do companies manufacturing them continue to use static protection?
    You're lucky if an ESD event causes immediate failure. The more costly ones fail at some future time when field replacements cost more than just the cost to isolate and replace the defective board/component.
     
  10. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,677
    2,729
    I see product returns as well. Zero failures due to static discharge over many years.
     
  11. dl324

    Distinguished Member

    Mar 30, 2015
    3,242
    619
    Be that as it may, there's an entire industry that disagrees with you.

    When I was in R&D, I personally used many dozens of MOS devices with no explicit I/O protection and I only damaged one. This was while following established ESD procedures.
     
Loading...