We have been soldering LED boards together for years with no issues. Recently engineering has gone to a new board design and we got the first samples in this week. Because of the improved design and better heat dissipation of the board my team is have great difficulty soldering lead wires and connecting boards in series with wire to the solder pads on the LED boards.
The solder will not stay molten long enough to insert a wire even with the soldering tip still in direct contact with the solder pad. I have tried different temperatures and methods but all have resulted in failed solder joints.
Looking for any advice on methods/ materials to try out.
We are currently using Kester lead free 275 .062 wire at 625 degrees
The easiest way to get the PCB soldered by hand is to heat it up to 50-80°C so that your soldering iron has less heat to provide. The best way, in my experience, to do this is to use a heat plate. There are special heat plates sold for soldering, but basically any that you can reasonably well control would work. Word of advice: because the PCBs you get are not humidity controlled anymore, do not heat the PCB up too much (i.e. going over 100°C) without first drying the PCB at around 50°C for a while. Otherwise your components will go *pop*.
75W seems fine (I use 90W but that's not a lot more).
Since no-one mentioned it: use a big thick tip on your soldering iron, as heat transfer is dependent on the cross-section of the tip. A chisel or "screwdriver-style" tip provides a large flat contact area that you can press down for maximum heat transfer into the work piece. A thin round tip won't work as well. Put a small drop of solder on the tip.
The best kind of soldering iron for aluminum core boards is probably Metcal's RF system, which uses a ~13MHz RF drive to send power to the tip. The temperature regulation takes place via the Curie point of a material in the tip similar to the old WTCP Magnastat from Weller, but better.
It appears that Metcal's patent has expired and you can buy similar and allegedly "compatible" systems from Hakko. Around USD $600 for a solder station. The tips have a fixed setpoint temperature associated with them.
You should ask for a demo before buying, or get a return authorization if it doesn't work for you. You could also consider the higher power conventional stations such as FX-951 and FM203-01. The latter is 140W which ought to be plenty.