Two sided PCB production question.

[Pack34]

I've heard that when doing SMD assembly on a two sided board, you'll use solder paste with different melting points so that the first side doesn't fall off when you're reflowing the second. My question is, where do you find paste with different melting points? I'm not seeing this criteria mentioned on the usual suspects (Digikey, Mouser, etc)

I've seen a single melting point solder paste used but with glue to hold the bottom components on. How does this work? Won't you need to stencil both the glue and the paste? Or are you just supposed to do a stencil and then manually go in and drop glue in the middle of all of the larger packages?

[free_electron]

no, that is not correct.

the surface tension of liquid solder is high enough that parts , below a certain weight, won't fall of the board.

in an industrial process the bottom of the board only sees a temperature below the actual liquidus point of solder. circuit board material is a relatively bad thermal conductor. and you can create a topside that is hotter than the bottom side.

[rx8pilot]

I do my 2 sided PCB's with all the heavy components on one side. Then I do the side with only small parts, followed by the side with bigger things like inductors. I have a convection batch oven and it seems to work ok.