Homebrew PCBs

Stuart Wallace 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
> 
> 
> Stuart


Is does appear like your getting uneven etching.
There are two main causes.

1) non-uniform solution agitation.
2) photoresist residue.

To solve problem 1, you have to attack your etch tank design. This 
problem creates areas of the PCB that are over etched (traces get overly 
thin or completely removed), while other areas remain slow to remove. 
You picture shows the fiber glass weave pattern on the partly etched 
copper foil. this leads me to think you have a resist residue problem.

I only have experience with negative dry film photoresists, and come
across the residue problem many times. The longer the film has been on
the copper the more difficult it is to remove the residue in the 
developing step. PCB fabricators won't process a PCB with resist applied 
longer than a day. I've had PCB stock with resist thats over a year old, 
which can only be developed with difficulty.
The subtle problems of resist residue generally cause trace shorts and 
various spots on the PCB, especially in "tight" corners where the 
developing action was least.

The only fix I've found to using aged photoresist coated boards is to 
be more aggressive with the developer. There is a fine balance between 
over developing (causing traces to swell and lift) and under developing 
(causing incomplete removal of invisible residue).
Use hotter developer solution ( 35\ufffd ~ 40\ufffdC), and longer times with the 
paint brush strokes. Rinse then, etch for 20 seconds. If copper does not 
turn matte pink in some areas then there this is signs of photoresist 
residue.

Good luck

Adam