So I'm back from the conference, and I finally got to try the solderpaste that arrived before I left. I just quickly went to a local pharmacy to buy the thickest needle they had, I cut that needle with a dremel, and I was ready to go:
First, I applied a small drop of the solderpaste on each of the pads:
Then I carefully placed all the diodes, making sure they are all aligned properly:
Next, I went to the kitchen and switched the stove on:
I waited a bit until the plate got hot, and pushed the board onto it. Then waited until the solderpaste was properly melting, and pushed the board off the plate.
The final effect looks good:
However, it doesn't work. Only one LED ever blinks, and that is only for a short moment.
I think I must have applied too much soldering paste, and there are some internal shorts of the signal pins. Now I'm thinking about some way to recover the LEDs -- I think I will put the board on the hot plate again, and pick the LEDs from it?