Homebrew PCBs

On Sat, 2007-02-10 at 08:25 +0000, fourthdr wrote:
> Hi:
> 
> Does anyone have a recommended combination of
> toner/printer/paper/heat, to produce reliable reproducable circuit
> boards?
> 
> I've tried special transfer paper, transparencies, and plain paper
> using a regular iron with varied results. Most of the time the toner
> is over heated, or flakes off of the copper or melts and mirges with
> close traces.

That - on this list - is almost a 'religious' question along the lines
of 'what's the best programmer's editor?' or 'what's the best graphical
widget set?'. Different people get better results from different
choices, and in the end it all depends on who you are, where you're at,
and how you 'hold your mouth' during the process.

This said - I've tried many different processes, had countless successes
and failures, and finally settled on:

Toner/Printer:
 - hp2550l color laserjet - it gives good toner - but I'm sure that
   there are n brands/models of laser printer that perform just as well
   or better

Paper:
 - label-backing paper - the silicon-coated paper that's used as backing
   for printable paper labels. It prints well, it gives up its toner
   easily without a fight, peels off easily after the heat and doesn't
   need to be soaked/rubbed/scrubbed off. I first print out a 'guide
   sheet' on plain paper, then affix a rectangle of the backing paper
   over the artwork, with sticky tape holding it at the top edge and
   put it through the printer again. I've never failed to get a perfect
   print.

Heat:
 - I'm using an old Ibico laminator with cold spring-loaded
   rollers on entry and exit, and hot sprint-loaded plates in the
   middle. This laminator has digitally adjustable temperature and
   motor speed, but I always set it to its maximum 160degC and
   slowest motor.
 - Note that when using the silicon-coated backing paper, I only
   need to put the board through the
   laminator for one pass - if I use 2 or more passes, the
   traces/pads tend to 'squish' and short into each other

Post-heat:
 - I wait for the board to cool, hold it so the paper
   is on the bottom, then gently lift the board away from
   the paper

This is what's working best for me. YMMV. Others will find their best
results come from other methods.

One major point: the key to success in this - or *any* combination - is
how well the blank copper board is prepared. Poor prep will cause
errors, uncertainty, wastage of time and frustration in any process.
Good prep is orders of magnitude more critical than any choice of toner
transfer process.

Consequences of poor board prep:
 - toner failing to 'bond' with board surface, allowing the etchant
   to 'sneak in under' the toner in small areas. It's no fun having
   to manually solder-bridge a 1-mil break in a fine trace, too small
   for the naked eye to see
 - need for more passes through heat - causing distortion and
   'squishing' of toner deposit - which introduces the need for
   time-consuming examination of the board under magnification and
   manually separating joins with a fine knife
 - in worst case, toner continuing to cling more to the transfer
   medium than to the board, regardless of how much heat

One quick/easy prep process that's geting me zero defects is to scrub
the board gently yet vigorously, all over, with green kitchen scrubber
and hand-dishwashing detergent (my brand is 'Morning Fresh
Concentrate'). After scrubbing all over in rapid small circles and very
low pressure, paying special attention to edges and corners, I rinse the
board in water, then clean it in methylated spirits. 2-5 minutes should
be easily enough to complete the whole prep process.

Light scratching (and I mean *light*) can be quite desirable and can
promote better toner adhesion.

Also don't forget to file all edges/corners beforehand to remove any
burrs. Test for burrs by tracing a fingernail gently out toward each
edge and feeling for any bumps. This is just as important as the
scrubbing.

Other tools and transfer media processes I've tried:

 - press'n'peel film - nasty bitchy temperamental overpriced stuff.
   try it if you're curious and have money to waste. You never know,
   it just might like you

 - clothes iron - very hard to get even temperature coverage, as the
   iron surface is not guaranteed to be totally flat. It's not
   uncommon to have some toner overheat and squish out, and other
   toner areas get insufficient heat and stay clinging to the transfer
   medium

 - inkjet gloss paper - needs a few more passes through the laminator
   than the silicon coated label backing paper, but may give you 
   slightly better resolution, handy for really fine SMD layouts.
   However it introduces another step - the need to soak, peel, soak,
   gently rub, maybe apply packing tape and peel - to remove all the
   last remaining bits of paper, especially the little dots clinging
   stubbornly to the pad holes. Board prep is very critical here,
   since poor toner adhesion can cause you to remove not just paper,
   but toner as well. Before switching to the silicon paper, I was
   tempted to try an ultrasonic cleaner and see if that would
   'shatter' away the remaining paper.

 - wash-away paper - this paper is water-soluble (disintegrates and
   dissolves away completely in 10 seconds). For low-resolution
   boards, it's a more convenient alternative to inkjet gloss, but 
   does introduce more artwork distortion. But for me, it's more
   expensive, gives worse results and is no more convenient than
   the silicon-coated label backing paper

> Thanks in advance.

You're very welcome. Hope this info saves you some of the time,
frustration and expense I've had to go through.

Cheers
David