I'm regularly working with <100um track and gap and up to 6 layers. The
problem lies with whether your PCB mfgr can reliably produce at these
geometries and keep registration - especially of the soldermask. A solder
splash at this level could need a microscope to be seen - unless you watch
for the glow at power-on :-).
Of course I'm speaking of 3-5V power and signal. If your voltages are higher
eg mains, then you'll have to keep you gaps larger to prevent air breakdown
and arcing from occurring.
-----------------------------------------------------

John Walshe         walshej@nmrc.ucc.ie

National Microelectronics Research Centre,
Prospect Row,  Cork,  Ireland.

Direct Line: + 353 21 904082
Switch: + 353 21  903000
Fax: + 353 21 270271

Home page:    http://nmrc.ucc.ie/
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