Re: Homemade PCBs!

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Jan Wagner <jwagner-at-cc.hut.fi> 


On Tue, 27 Apr 2004, Tesla list wrote:
 > Original poster: Fucian-at-aol-dot-com
 >
 > What are all these layers for? What exactly IS a layer?

Layers are exactly what the name says, 'layers'. Sort of a 'paper sheet'
which contains something. The PCB is then stacked/sandwiched together from
these layers.

A typical single sided pre-UV-resist-coated DIY PCB:
1 photoresist layer
2 copper layer
3 glue layer
4 epox&fiberglass lamination layer (the "circuit board")

If you order commercially produced single sided PCBs, typical order of
layers might be:
1 silk screen (component names and other text printed on the PCB)
2 solder mask layer (the often deepgreen protective coating you see on
   PCBs, it helps against corrosion and prevents solder flowing onto
   copper tracks where you don't want it)
3 copper layer (this is where the signal traces are)
4 glue
5 epoxy&fiberglass board

Then there are a bunch of additional "non-physical" or "info" layers that
are defined in the software PCB file. For example, where the automatic
drill should be drilling, to what dimensions the board should be cut, etc.

You could have a try on Eagle (www.cadsoft.de) and wonder at how many
layers the PCB board editor contains  ;-)

If you meant what the use of multiple copper layers is -
oversimplified, they reduce the number of jumper wires you need. If you
look inside cheap TV, video or stereo sets, there's LOTS of jumper wires
there, a real mess. Nearly all those jumpers could be replaced with copper
traces on a new, added second copper layer. Much cleaner result and less
work, but it costs a bit more.

OTOH if you look at your PC motherboard, there are thousands of component
pins that should be connected together. A single copper layer
obviously isn't enough, and jumper wires are a really bad idea.
Additionally taking into account RF layout requirements (>100MHz to
 >10GHz signals there, needs ground and DC supply planes), you end up with
at least a 4-layer board. I don't know exactly where PC computer boards
are going nowadays, but I'd think they typically use from 8 up to 16
layers.

To keep this even remotely TC related: if you build SSTC or flyback
boards, it is a good design practice (IMHO) to use two layers. For
example, one larger combined ground and power plane on the bottom layer,
and logic and RF signals on the top layer. That way the circuit is, for
one, much less susceptible to HV RF interference (e.g. more bullet-proof).

cheers,
  - Jan

--
****************************************************
  Helsinki University of Technology
  Dept. of Electrical and Communications Engineering
  http://www.hut.fi/~jwagner/ -