Homebrew PCBs

Stefan Trethan wrote:

>Anyway, i bought the more expensive silver paste because it must surely be  
>better ;-)
>
>BUT, it seems to reflow in the same strange manner as the leaded paste.
>
>When you heat it until it has a dry, grayish appearance, and then wipe it  
>off, it will come off as sort of dried paste/powder, BUT the board will be  
>tinned in a thin, shiny layer.
>
>When you heat it further, until it reflows, it will get a dull, rough  
>appearance from where the particles melted.
>
>
>I don't understand the first thing. How can it tin the board without  
>melting, at all?
>What happens?
>
>
>  
>
  Think of having your board right at the melt point, and taking a solid 
lead pencil and writing on the traces.  Pencil stays solid, but where 
you're touching the traces is melting.  Traces are above the melt temp, 
but you're only getting melt when you make it touch.  For sure it is 
melting some, just not a lot.  Remember it takes a lot more energy for 
the phase change, so right near the melt temp you'll melt the surface 
but not have near enough extra heat to melt the rest of the solder 
balls.  Might also be doing something with the flux at a higher temp, as 
someone else noted recently it stays on much longer just at the melt temp..

  Think of how you draw on your iron tip with solder, as the iron is 
just heating up, leaves a nice shiny path.  And won't melt the rest of 
the solder very fast yet, because it doesn't have enough extra heat. 

  Things well above the melt temp uaually oxidize much faster than 
things right at it, part of the reason a temp controlled soldering iron 
is a good thing too..

Alan