Homebrew PCBs

One of the things that I've been having trouble with on toner 
transfer is smearing due to either incorrect pressure or too much 
heat.  My theory is that the smearing is happening because I have to 
move the iron around to compensate for the holes in the iron. (for 
various reasons, I'd prefer not to get a laminator) so I placed a 
thin piece of aluminum (about 24 gauge) on top of the board between 
the toner paper and the iron.  The idea is to more uniformly 
distribute the heat.  It worked pretty well but I found I needed to 
increase the iron temperature a bit over the direct contact 
approach.  This makes sense since the sheet metal is conducting heat 
away from the board.  I did move the iron a little during the heating 
phase but much less than with out the al sheet.

I got much less smearing than before and much more uniform line 
widths.  Significantly reducing movement of the iron seemed to really 
improve the quality of the product.  This leads me to think that it 
might work to just get two flat plates of steel, say 1/4 inch, heat 
them in the oven to the correct temperature and then quickly 
sandwitch the board/toner paper assembly between the plates.  The 
weight of the top plate may be sufficient to force toner-copper 
contact.  The thermal mass of the plates will need to be enough to 
maintain the temp above the fusing point for 30 seconds or so.

Phil