I tried this recently, and noticed something similar.
In my experiment I used a lead free acid based plumber's paste, and
an old clothing iron at the highest heat setting. This did not burn
the board, and did seem to leave a very thin plating of tin. I was
very liberal in applying the paste.
I tried another experiment later with a hotplate. At some point, the
paste did melt (what you're calling the second stage, I think),
leaving me with a big mess. Also, the board was scorched.
I'm guessing that the first stage was not actually melting the
solder, but rather some kind of chemical plating, but accelerated by
the heat.
Possibly this might actually be a better way to process circuit
boards than the higher temp reflow. My only complaint was that the
plating was extremely thin. Also, I'm a little concerned about the
environmental effects of washing off what must be the bulk of the
solder.
Kevin
--- In
Homebrew_PCBs@yahoogroups.com, "Stefan Trethan"
<stefan_trethan@g...> wrote:
> Hi,
>
> tried it today, with mixed results (but definitely worthwhile).
>
> A few things i noticed:
>
> There seem to be two "stages" of tinning.
> Where the heat of the heatgun was greatest, just below the center
of the
> nozzle, the reflow was more complete. What i see there is s rough
surface.
> Like the paste was much coarser as it actually is. On the other
hand in
> areas with less heat it appears that the board is still tinned
nicely, but
> with a shiny, uniform, and thinner. I wonder how the paste can tin
the
> board without actually reflowing...
>
> Also, i could not get the paste to reflow to a silver appearance
before
> burning the board. I had expected it to go shiny silver at some
stage,
> like SMD paste, but it didn't. It just stays the same dull grey,
but after
> wiping the board off underneath is a shiny silver tinning?
> The packaging says one should see when it reflows.
>
> What about silkscreen? will it work over a board tinned that way?
> Do you drill before or after tinning?
>
>
> Note that i used a old solder paste with lead content, which gives
a less
> silvery appearance anyway. I am planning to get tin only paste,
also
> because it is a slightly messy procedure and i am not fond
of "loose" lead
> particles around and in the water.
>
> I will make several sample pieces and heat them with different
temperature
> settings, to find out about the strange reflow...
>
> ST