Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

----- Original Message -----
From: "Stuart Wallace" <stuartw@...>
To: <Homebrew_PCBs@yahoogroups.com>
Sent: Monday, June 19, 2006 12:09 PM
Subject: Re: [Homebrew_PCBs] My best fine-pitch PCB so far


> Leon Heller wrote:
>
>>What are you printing the transparencies on?
>>
>>
> I'm using a Canon i560 to print on IBM inkjet transparencies. I've tried
> a few different types of easily-available transparencies (the sort of
> stuff that places like PC World stock) and haven't noticed much of a
> difference between the types. I've made a number of test boards
> containing designs with 8/8 rules -- in each case the results haven't
> been nearly good enough to be usable. Something is definitely badly
> wrong with my process! I struggle to fully develop the boards without
> mechanical assistance (generally rubbing at the boards with cotton buds)
> -- this invariably leaves contaminants on the board, which in turn
> causes etching problems. Etching is in any case very uneven. This seems
> to point to either bad artwork or insufficient exposure.
>
> I expose the boards (6" x 4" DS FR4 stock) about two inches above two
> 15W UV tubes for three minutes. I'm extremely nervous about
> over-exposure: perhaps too nervous!
>
> I'd love to be able to make 8/8 double-sided boards at home; the reduced
> turnaround time would really help some projects. I suspect, however,
> that DIY PTH may be beyond my abilities. I'm getting ready to send
> artwork over to Olimex in order to get some proto boards made up, but I
> have my doubts: my requirements seem to be at the absolute limits of
> what they offer -- 8/8, lots of fine-pitch high pin count SMDs, and so
> forth -- so I'm not sure whether the results will be any good. Still, it
> doesn't cost much to find out!

I use Mega Electronics JetStar film. My UV exposure is 13 minutes (from
cold) at about 13 cm above two UV tubes. You might be too close, this can
cause uneven exposure. I'd rather have a longer exposure with some
collimation due to distance from the tubes. The PCB material should be quite
fresh; the resist goes off after about 6 months, this can cause lots of
problems with resist not being removed during development.

Try 10/10 or 12/12 first, and get that working before you try 8/8. You could
send me one of your transparencies to try, or I could send you one of mine.

I prefer PCB-Pool to Olimex, their quality is much better and they can do 6
mil tracks.

Leon