Those copper MCPCB's can be hard to solder to unless you have a fairly powerful soldering iron. Especially when in the host as the host will also help suck away the heat. I generally prefer to solder the driver last. If you solder leads to the MCPCB, make sure you clean any splattered flux off the emitters before powering them. You probably know this, but saying just in case.

Best wishes on your build.