Having a brick form factor is great at resisting damage from drops and sudden impacts, but a compact footprint can only go so far. Just because something is shaped like a brick when clammed up doesn't mean that device is impervious to what most of would consider minor stressors. For example, a common avenue for cracking the top LCDs of New 2DS XL systems is to walk around with them in your pants pockets. It may look brick-like when it's closed, but the n2DSXL has the design flaw of poor back support for the top LCD.
In materials science, there are two failure modes that are often overlooked because damages caused by these are not instantly noticeable.
- (1) creep - slow deformation of an object from constantly applied pressure.
- (2) cyclic stress - repetitive (usually evenly timed) directional changes to applied pressure.
Placing a naked n3DS in a backpack while walking about provides the perfect environment for those two.
- Creep caused by wearing the backpack where the n3DS is pressed against the sidewalls.
- Cyclic stress from the up and down motion from the walking that jostles the backpack.
The exterior of your motherboard looks perfectly intact when seen by the human eye. However, the type of board damage that was explained earlier (ie, broken solder ball joints) are underneath those black square chips. One would need an x-ray inspect to spot those defects.
Here's an example comparison between visible light and x-ray imaging of a circuit board.
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Again, I mentioned there is a small chance your n3DS softbricked itself during sleep mode. Softbricking in open or stock firmware isn't common but not unheard of, especially in sleep mode.
If you want to know for sure the motherboard is still 100% functional, try ntrboot hacking the n3DS for signs of life or boot response that isn't black screens of death.
Here's an example of an New 3DS LL that turns on with black screens but doesn't react with ntrboot due to a dead SoC (ARM CPU).