Joe wrote:
. . .
I was wondering about a post I read somewhere else a long time ago. Is there
such a thing as a paste (that comes in a tube) that can be applied to the
terminals of a surface mount device and then when it is stuck to the board,
the paste hardens into something like hardened solder? So it is mounted to
the board and soldered with this paste?

One-part conductive epoxy fits that description. It's commonly used to
mount SMT parts on hybrid circuits. However, the ones I'm familiar with
require curing at an elevated temperature (e.g., 150C for an hour). I
believe there are also some UV curing conductive epoxies. Conductive
epoxy can be used for mounting parts on a PCB, too, but I don't think
it's commonly done because it's considerably more expensive than solder.
There might be two-part conductive epoxies that cure at room
temperature, but I've never used one and am not sure they exist. The
one-part epoxies I've used aren't conductive until they're cured -- the
tiny conductive (gold or silver) particles in the paste don't contact
each other until the curing process causes the epoxy to shrink and pull
them together.

Then there's solder paste, nearly universally used for mounting parts on
PCBs. This also fits your description and can by applied by hand with a
syringe, then melted by a number of means -- hot air, IR, soldering
iron. It's actually a slurry of flux and tiny spheres of solder. It's
not really sticky, but sort of gummy, so some other means (like
superglue) has to be used if the parts need to be kept in place when the
board is inverted or severely disturbed before the paste is melted.

Roy Lewallen, W7EL