Things to check:
A shorted Capacitor (it happened to me once) or a transistor or the protection and/or MCU. Excesive current consumption. A component placed in the wrong position and/or orientation. And maybe a bad solder (specially if you hear faint sparks).
For reference on where to test with a DMM, I attach an schematic of a similar unit.
Pin PD6 drives an NPN which switches the LED and the PNP used as power switch. Since the LED is lit as soon as power is provided, despite the ATmega being removed, the NPN could have a short.
Sorry for the delay In replying didn't sleep too well last night and didn't feel too well waking up
Am gonna try to be as detailed as possible within my limited knowledge
So the first thing I did was to clean the top of the PCB with 70% alcohol and a toothbrush since I saw that some of the soldering paste I used had managed to seep into the other side and dried it with a hair dryer
Then I went and reflowed all the transistors again, and now the issue of the led lighting up as soon as battery was hooked up was gone
Then I checked the power draw with the dmm in series with the battery and he connector and saw 0 on power consumption
so I decided to press the encoder to see what it would do and surprisingly this time the led turned on and it stayed on (this was without atmega or screen in)
Put the atmega and screen in, and it it turned on sort of. The led lit up and stayed on and the screen was solid white but most importantly it stayed on, pressing the encoder the led flashed off, holding it down turned the led off but he unit wouldnt shut off the only was was to remove the battery
So I decided to reflow all the solder again with the iron, no improvements but either the weird fizzling/cracking noise came back or it was never gone.
Power draw with battery in bit turned off is now 0.10-0.12 mA (no atmega or screen in) turned on with atmega in but no screen is about 0.46 mA
And about the dp6 pin on the atmega (am gonna explain how I measured it don't know if it was the wrong way or not) I put the negative lead on the battery negative pad and the positive lead on the dp6 pin then I turned it on and I saw the voltage rise to around to a little over 3 volts then when it was on settle around 1.6-1.7 volts
(Keep in mind me multimeter update freq is around 3 times a second so maybe it isn't fast enough to see 6?
I've attached a few close ups of it, don't think any component is in the wrong orientation but you never know
I would attach more but the 8mb total limit isn't letting me