Power (VDD/VSS) is actually important
I was working at an ASIC vendor in the 80's, and there was an effort to
make the design flow simpler and more automated so that less experienced
engineers could do more of the work. One chip that was
done by new engineers came out of the fab, and the tester found that
(1) it failed most DC parametric tests, and (2) it ran really slow -
10's of KHz instead of ~5 MHz.
It turned out that although pins/pads were defined as VDD and VSS, they
were not connected to the internal chip power rails. So the chip worked
only because current could leak through the ESD protection diodes on
inputs and source/drain of output drivers to the VSS and VDD rails.
Adding power rail connections had always been done as a manual step by
layout designers on CALMA systems after the design had completed
automatic routing on our VAX systems. The instructions for these designs
neglected to mention the VDD/VSS pads. The new engineers checked the
layout plots, but were not experienced enough to remember power/ground.
No one else checked it.
- Inexperienced engineers don't know what it is that they don't
- When developing procedures for inexperienced people to follow,
check them over VERY carefully - someone with experience should try to
follow them, and pay attention to the details that might have been left
out. Everything should be in instructions.