FP Zero won't turn on

[Flavor]

Hey Nohika!

First, if you haven't already, check through all the pertinent stuff at
http://freeplaytech.com/troubleshooting

As you already have suspected, the solder on your Pi doesn't look great.  I would definitely suspect that it's likely at fault.  There could be other things going on, though.  If you can connect the Pi to HDMI without hooking the Pi to the Freeplay Zero main board, then we can make sure the Pi and the SD card are functioning properly.  If you can't, then I feel like we'd want to focus on gettin the soldering fixed up.

The good news is that it's very fixable.  There's a pretty good/extensive guide here that'd I'd recommend that you check out.
https://learn.adafruit.com/adafruit-guid...g?view=all

This PDF guide from Seeed Studio is also pretty good.  "13 Common PCB Soldering Problems to Avoid"

  13 Common PCB Soldering Problems to Avoid | Seeed Studio Blog - SMALL.pdf (Size: 606.87 KB / Downloads: 2)

Based on your photo:
- Make sure your iron is hot enough (400C give or take a bit)
- Get a wet sponge (or paper towel or whatever) and clean the tip of your iron (when fully hot) after each solder joint
- Make sure the solder wicks down into the joint (into the PCB hole).  If the solder is like a ball/mushroom on top of the pin/board, it's not wicking.

Let us know what you find out or if you have some luck re-doing the solder.