> I'd like to know how ESD actually damages a PCB and whether it can be
>reversed. If microchips are solid material with contacts how can
>static electricity damage a chip that would lose electrical data
>anyway on power-down!?
ESD doesn't damage the PCB, it damages the chips. It's nto repairable without replacing the defective chip.
Basically, it fries transistors. Transistors are made from a few layers of different things sandwiched together. On the bottom is :diffusion" area, which isn't great at conducting. The two ends of this area are connected to wiring, one is the input, one is the output. If left like this, the two ends would electrically not be connected to each other.
On top of this diffusion layer is a thin layer of silicon oxide. On top of that is the transistor gate made out of polysilicon.
When the poly gate is charged, an electical field gets pushed down into the diffusion layer below, and this field makes the diffusion able to conduct electricity, turning the transistor "on". If you discharge the poly gate, the electrical field goes away and the diffusion below again is unable to conduct electricity, turnign the transistor "off".
Now, I haven't said much about that middle silicon oxide layer. Really all it does is insulate the poly gate from the diffusion below it. If these twothings touch, they cause a short and things won't work anymore. ESD problems apply so much voltage between the gate and the diffusion that the thin silicon oxide can no longer withstand the pressure, and it breaks down, allowing the poly and diffusion to short together. Bam, your chip is fried and needs replaced.
I hope we've all enjoyed this afternoon's CMOS lesson. Be prepared for a quiz next Thursday.
