820-00165 Bouncing Bitch (no sign of H2O)

The Bounce. Pulls .3A and shuts down. From no power it will bounce twice in about 4 seconds. The SMC appears perfect. The SMC gives it a second try. after that it settles at 0.030A and just sits there.
the 5V rail and 3V rails come up but the 3V3 rail stays up.

All rails enter S0. All sys PWR Good is present about .2 seconds.
All rails have normal resistance to ground.

No signs of water. Power washed under BGA packages as a last effort.
Bios is loading. CPU is not booting and accessing RAM. Only U3900 lights up for a brief second on thermal. Which is totally normal for this.
Thermal shows no shorts. "none I can see".

What should I check next? How do you trouble shoot bounces like this?
 
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