I encountered the exactly same symptom on another IdeaPad 3 with 10th gen i7 CPU.
Onboard RAM working fine as expected, passed any tests I put it through.

As soon as I add another SO-DIMM in the extra slot, the board doesn't POST, no matter what module I use, e.g. 4g, 8g,16g, different frequencies and various brands, same result.

Under the microscope I checked the SO-DIMM pads for any possible cold or cracked joints, but found no evidence of such problem. Nevertheless, I resoldered all the SO-DIMM pins, just in case, but no change afterwards either...
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Re: Lenovo Yoga 500-14IBD backlight flickering

Well... what if all this boost regulator circuit is totally missing from the board in some versions?

I'm trying to fix another Yoga 500-14IBD with the exact problem as discusssed here.
The screen flickers like hell when you disconnect the charger... not only in Windows, but in BIOS screen as well (so drivers or other software issues are out of the question).

This model is also powered by a 2-cell battery pack which appears to be in pretty decent shape, measured with a professional battery tester it's...

Re: XPS 13 9360 no CPU power

Lately I'm trying to fix another LA-D841P board who had the same fate (liquid damage) and I amazingly noticed in here the description of EXACTLY the same symptoms and behaviour, just as in @ytsoc 's case. Maybe it's the same board?

Plugging in the charger, PWR LED comes ON only for a couple of seconds, but no fan-spin and no POST.
If I press the power button, all major voltages seem to appear again for two seconds... well, all except +VCC_CORE and +VCC_GT.
When it's on, I also noticed some PWM waveform present...