When I worked at an electronics plant the surface mount boards were soldered by passing them through a conveyer oven. It would melt the paste and bond the parts. I do not know what temperature was used or for how long it was in there.
From: Homebrew_PCBs@yahoogroups.com [mailto:Homebrew_PCBs@yahoogroups.com]
Sent: Wednesday, August 31, 2016 8:39 AM
To: Homebrew_PCBs@yahoogroups.com
Subject: [Homebrew_PCBs] Re: Exposed pads
Yes, it is possible. You can use a heat gun. There is another method, that requires only soldering iron,
but I never tried it. They say it works with SO8 packages.
Use a thin material for PCB. 1 mm is the maximum. Drill a hole in the center of the thermal pad. Put some solder on the PCB pad AND on the IC pad. Solder pins as usual. Use a thin tip with flat end for your soldering iron. Inject some solder into the hole and heat it there. The solder will be sucked into the gap between the IC and the PCB due to surface tension. This method is not as good as using an oven, but it is simple.