MENDA - Questions And Answers

Are there any reports of ESD damage to BIOS and system chips when installing or removing expansion boards in an ATX powered computer case (there is a standby voltage on most of the ISA and PCI board pins)Also, I was prodding around my system board with the main power off, but the unit was still plugged into the wall outlet, and the standby power was still active. I was probing to determine the standby voltage (which was around 50-100mV) and I may have shorted some leads of some chips in the process, (due to the large size of the voltage probe in comparison to the chip leads)Would a voltage level of this magnitude have caused any damage to system BIOS chips or expansion card components? - David Chan, Waterloo, Ontario, Canada

Bios and system chips when installing or removing expansion boards are highly sensitive to ESD damage and should be handled properly, according to standard ESD control methods. Your specific question regarding standby voltages on the system board falls into EOS (Electrical OverStress). If electronic chips are powered up and any of the I/O (input/outputs) are shorted, you can EOS the device causing permanent damage. The voltage of 50 to 100 millivolts is more in line with the emf (electro-motive force) between two dissimilar metals and may not necessarily be due to the local power supply or capacitor bank. Either way, it is highly advisable to not have your power supply plugged in when operating on your PC, but rather remove the power cord and attach just a ground cord to a localized ESD dissipative mat/work surface. Also, you should wear an ESD wrist strap and ground yourself when handling these sensitive devices. Additional Info: How to prevent unwanted ESD from cruising through your PC? Remove all unnecessary objects, especially non conductors from around your PC. Make sure you have a proper power ground for your PC and peripherals. Ground all conductive objects that come in contact with your PC. Ground all conductive objects that are within the near vicinity of your PC. Keep your monitor screen clean with an antistatic spray or wiping cloth. Ground yourself and anyone else working near or on your PC when repairing or adding components. NOTE: Make sure there is no exposed high voltage (<250VAC) within your reach when grounded. For repairs or add-ons, keep your PC components in their shielded bags during transportation. Before removing PC components from shielded bags, place them on a grounded work surface. Remove the PC components only when you are grounded via your wrist strap. EOS? ESD? When you walk across a carpet and then reach for a door knob and get ZAPPED, you have experienced an ESD event. As you walked across the carpet, you charged your body with 6,000 - 20,000 Volts by means of triboelectric generation. When reaching for the door knob, the difference in voltage between the door knob and your body became so great that the air physically broke down and created an electric current discharge. The discharge occurred to bring the voltage of your body to the same voltage of the door knob. If the door knob was your computer keyboard the discharge (current) could travel through the I/O cables into your computer and cause an electrical overstress to any IC in its path. An overstress voltage as low as 10 Volts could permanently damage a CMOS Chip (IC). The computer can receive charges/discharges from any contact via the keyboard, chassis, drive doors, connecting cables, monitor, etceteras. The most common occurrence of an ESD event effecting the PC is through the handling of components when upgrading, adding, changing, troubleshooting or repairing your PC or other electronic equipment. These failures can be latent or catastrophic. Latent failures will not be evident at first but may cause problems at a later time leading to catastrophic failures. Catastrophic failures are noticed immediately because your system will “crash” or have an equipment malfunction.

Related Categories: