Hi,
It has been almost a week since I have been trying various techniques to
fix the ESD problem in my PCB. I am particularly thankful of the
suggestion by John Barnes to place a copper tape on the insulator over HCP
and then move the EUT over the tape. John, I improvised it further by
connecting a small piece of copper tape to the probe tip. I also connected
the copper tape directly to the ground through a long wire. I could then
move the probe tip over the circuit board with the ESD gun turned on. This
way, I was able to accurately pinpoint the sensitive locations on the
board.
Currently, I have loaded special microcode which only reads the on-board
flash and blinks the LED on the board, which simplifies my debugging
process. I have tried placing low value decoupling capacitors (47 pF) on
the following signals:
JTAG Reset
POR
Analog VDD
PLL VDD
Couple of other lines used for ASIC configuration
Every step has helped raise the threshold of failure. Now I can contact
discharge directly on the board ground screws up to -7 kV without failure.
I also have no problem with -7 kV contact discharge on the HCP. On
Positive polarity, I can pass 8 kV. However, I am still failing at 8 kV
Negative contact discharge on the board screws, and HCP. I would like to
know if I am expecting too much from a complex logic (meaning should I
declare victory with this level of improvement), or keep trying to make it
more robust. I would welcome all suggestions. The ASIC has an embedded
processor, and there are no external interrupt lines to filter. According
to the ASIC designer, no other pin should impact the working of embedded
processor.
I still have to test the board with the actual microcode running real
application.
Once again Thanks to All for your valuable suggestions.
Regards, Ravinder
Server PCB and Flex Development
Hitachi Global Storage Technologies
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