Leon Heller wrote:
> From: "Stuart Wallace" <stuartw@...
>>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.
>
> I get boards looking like that when I use old PCB stock. The resist is only
> good for six months or so after it has been applied.
>
> Leon
I agree. The solution is to lay the board flat in a shallow container of
ferric chloride and do the etching by brushing the etcher over the pcb
with a paintbrush for a few minutes until done. I'm using 5-year old pcb
like that and it comes out as sharp as fresh stuff.
The other problem you have is fine tracks being destroyed. This can be caused
by two things. If you remove the plastic film from the pcb then lay the pcb
printout against it from an inkjet, the outgassing of the ink can affect
the photoresist coating, as i found with epson ink and spray-on prp resist.
You must dry the printout in front of a fan heater for upto 10 or 15mins
until you can't smell the ink much.
If you leave the protective plastic film on the pcb, the ink of the printout
is then further away from the copper, and because the uv light is so close,
lots of undercutting and fuzzy edges can result. I fixed that by making a
uv light box that has a 125Watt mercury HID UV lamp backed with a cylindrical
parabolic reflector, and the globe is 60cm from the pcb.