Homebrew PCBs

Nice drill!

Not sure i can correctly envision your 4-layer method.

ST

On Nov 19, 2007 10:21 PM, DJ Delorie <dj@...> wrote:
>
> 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.
>
>
> Be sure to visit the group home and check for new Links, Files, and Photos:
> http://groups.yahoo.com/group/Homebrew_PCBs
> Yahoo! Groups Links
>
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