Homebrew PCBs

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

----- Original Message ----- 

From: "Stuart Wallace" <stuartw@...>
To: <homebrew_pcbs@yahoogroups.com>
Sent: Thursday, July 21, 2005 4:59 PM
Subject: [Homebrew_PCBs] Uneven etching problem


> 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 



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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

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.

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

If the short etch does show signs of residue, can the board be rinsed 
and additional developing attempted? Or is it all over at that point?

Regards, Bob

--- In Homebrew_PCBs@yahoogroups.com, Adam Seychell <a_seychell@y...> 
wrote:
<snip>
> 
> Rinse then, etch for 20 seconds. If copper does not 
> turn matte pink in some areas then there this is signs of 
photoresist residue.
>

Bob_xyz wrote:

> If the short etch does show signs of residue, can the board be rinsed 
> and additional developing attempted? Or is it all over at that point?
> 
> Regards, Bob

Yes, I've done that before and it seems to work a 2nd time just ok.

Note: the residue is invisible. I've seen it as an extremely thin clear 
film that peals off the copper when scrapped with a pin while viewing 
under a 40X microscope.

> 
> --- In Homebrew_PCBs@yahoogroups.com, Adam Seychell <a_seychell@y...> 
> wrote:
> <snip>
> 
>>Rinse then, etch for 20 seconds. If copper does not 
>>turn matte pink in some areas then there this is signs of 
> 
> photoresist residue.
>