My very first attempt at building a light was a P60 in which I failed miserably. Part of the problem was my cheap soldering iron, the other was bridging the gap between the driver and the pill. No matter how much I tred, the bridge kept failing. My solder was always breaking away from the pill when the large spring rubbed against it. The solder always failed where it had to jump accross that dead void where the driver press fits into the pill. Being nothing but air, and the fiber edge of the board presents a very iffy soldering territory.
I'm much better now, but I also developed a little technique I use to guarantee a solid negative connection to the pill with no weak solder gap.
A 1.4A Nanjg driver and P60 pill
I then attach copper braid to the neg. edge on the top of the driver in a few places.
I then snip the copper braid and bend it to the bottom side of the board, and press the driver into the pill. I've never used a 1.4A driver in a P60 before, but it sure makes things easy. Without any chips on the bottom side, I could just whack it in with a mallet.
Crop the copper braid short and bend it over the board, using a tweezers to squish it down.
The braid comes pre-impregnated with flux, but I add more anyway. Heat the braid with the tip of your soldering iron, touch the solder to it, and it will absorb the solder from the bottom of the driver to the top, filling the gap and ensuring a complete circuit and a strong bond.
That's just one way to do it. I hope this method can help someone.