Thanks for the many suggestions. In particular, the following ideas:
Larwe: spark gaps Jamie: TVS diodes Johns (Larkin/Devereux): RC clamps
Additionally, I found that HCT claims better ESD immunity than ALS. Both are much better than CMOS micros.
The QFN package has 15 mils gap between signals and ground pad. I will reduce it to 7 mils in PCB.
The unnamed manufacturer (many of you would have guessed) could have done a better job in ESD protection, but the high failure rate indicates otherwise.
I am going to install two diodes back to back between two pins (around
8 mils apart). 0603 is slightly too big. 0402 would be perfect. It would help if I can pre-joint two diodes before mounting on the PCB. What can I use to pre-join them? High temperature soldering? Micro- welding?
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