I had this idea for home-brew four layer pcbs, and finally got around
to trying it. It seems to work, too!
You need a few special items:
∗ Thinner than usual PCB stock. I got some 16 mil (1 oz) and 8 mil
(0.5 oz) SS off eBay.
∗ Sheets of double-sided tape. I got some 3M 7953 "membrane switch
spacer" which is about 4 mil thick.
I etched the four layers separately. For testing, I just did a grid
of vias of varying sizes and drills, with thermals to the ground and
power layers.
The stackup looks like this:
1/2oz copper
8 mil FR4
tape
1oz copper
16 mil FR4
tape
16 mil FR4
1oz copper
tape
8 mil FR4
1/2oz copper
I included small vias in the corners for registration, but next time I
need to be much more careful about etching and drilling those, as well
as the technique for lining them up.
Anyway...
The theory does like this: You don't use the inner layers for signals,
just power and ground. That way, you never have to connect both inner
layers together. Through vias are done as usual; etch, tape up the
board, drill, solder in a wire. I use wire wrap wire, which fits in a
13 mil hole. Oh - the drill "press" I built works great, I drilled a
couple HUNDRED 13 mil holes on this test board, and the only bit I
broke was the one I dropped on the floor.
http://www.delorie.com/pcb/dremel-stand/But with the above stack, you get blind vias almost for free! For
this, drill ONLY the outer layer, and make sure your inner etch
doesn't etch where you'd drill - you want a solid land under the via.
The hole needs to be big enough to get your iron in there; for mine
that means a 22 mil drill and 42 mil copper. You put the tape on the
outer layer, leaving the other backing on, and drill out all your
blind vias. Then peel off the backing and tape the layer on. Now,
poke the iron into the hold and solder, and the solder jumpers the two
layers.
To connect to the opposite power layer (i.e. connect layers 1+3 or
2+4), just drill a bigger hole on the other side (44 mil in my case)
before taping to expose the copper, then drill your via hole from the
other side after taping for the wire.
The 8 mil clad is thin enough to be see-through, so taping those to
the inner layers is pretty easy, just line them up and press. Taping
the two halves together is harder, I think next time I'll drill the
registration holes ∗after∗ taping up the halves, as it's more
important to line up the outer layers with each other than to line
them up with the inner layers (through vias have smaller annuluses).
The resulting stack measures 57 mil thick (the math says 64), but
still slightly flexible, unlike a the rigid board you get when you
epoxy the layers together.
You could use standard 32 mil DS for the inners if you already have
experience getting the two sides to line up. Just tape on the extra
layers afterwards. I wanted to try it this way to see if it was
easier to line up the sides, and to see if I got better results from
my laminator with thinner stock.