Homebrew PCBs

How are you getting circulation of your etchant?  I don't know if
that's your problem but when I've had uneven etching, it's been due to
poor circulation.  A good bubbler solved the problem for me.


--- In Homebrew_PCBs@yahoogroups.com, Stuart Wallace <stuartw@a...> wrote:
> Hi,
> 
> 
> Bit of a ramble -- sorry about this...
> 
> I've recently developed a bit of a problem with my etching process and 
> I'm hoping that someone can give me some suggestions for improvement.
> 
> Up to now I've been etching FR2 boards at roughly 10/10 (10 mil tracks, 
> 10 mil spaces) with good yield. I've recently moved to FR4 substrate
and 
> 8/8 rules in order to make a board for a slightly higher-speed design, 
> and yield has now dropped to zero. I've uploaded a couple of
pictures of 
> my latest effort -- they were taken with a camera phone so they may not 
> be very useful. I have been experimenting with UV exposure times: the 
> top side of the board was exposed for 2 mins 45 secs; the bottom side 
> for 2 mins 30 secs. I normally expose for 2 mins 15 seconds but this 
> doesn't seem like enough for the FR4 board.
> 
> 
> Top layer: http://www.atom.net/stuartw/board-top.jpg
> 
> Bottom layer: http://www.atom.net/stuartw/board-bottom.jpg
> 
> 
> The details:
> 
> Artwork: transparencies printed at 1200dpi on a Canon i560
> Board: Farnell FR4 double-sided photosensitised
> UV exposure unit: home-built, 2x15W tubes approx. 5cm from the subtrate
> Developer: Farnell KOH/NaOH pre-mix
> Etchant: Farnell FeCl3 crystals made up to near-saturation
> 
> All the chemicals are less than one week old. I've used them to develop 
> and etch three Eurocard (160mm x 100mm) boards so far. My process 
> involves the following steps: drill some alignment holes in the board, 
> remove the protective sticker from one side and expose it, then remove 
> the sticker from the other side and expose that. I then develop the 
> board in a tray of developer solution at room temp while manually 
> agitating the board and turning it over frequently in order to try and 
> develop both sides evenly. After about 2 mins 30 secs I wash and etch 
> the board.
> 
> The problem is that I'm getting regions of board that are clearly 
> over-etched (e.g. top right corner of the top side in the picture
above) 
> and other regions that have barely etched at all (left hand side of the 
> same picture). The under-etched regions emerge from the etchant stained 
> orange, which seems odd. I suspect that the over-etched regions occur 
> because I'm etching for too long as a result of the other areas which 
> refuse to etch.
> 
> I'm going to put my (home-built) bubble etcher back in to service this 
> evening to see if it improves matters -- so far I've been manually 
> agitating the boards in a bath of etchant.
> 
> Can anyone suggest any likely causes of this problem? I'm willing to 
> accept that my process may never be good enough to produce boards at 
> 8/8, but the fact that I can no longer reliably etch even heavy tracks 
> is depressing!
> 
> Incidentally, I'm not at all happy with the quality of the Farnell 
> (Newark in the US, I believe) FR4 board -- I have found scratches and 
> areas of missing photoresist in the latest batch. Has anyone else had 
> this problem?
> 
> Thanks for your help.
> 
> 
> Stuart