On Sun, 5 Jul 2015 17:13:52 +1200, you wrote:
>Double sided boards as we all know can be a bit of a pain, the biggest
>problem is getting that precise alignment between sides correct so that
>when you drill in the center of your pad on one side, it comes out in the
>center of the pad on the other (and not, as is not uncommon, breaking out,
>and probably messing up the trace, or a trace nearby...). Especially if
>your only able to expose/imprint artwork one side at a time.
Even, then, you have to be certain that the top and bottom sources for
the artwork are properly aligned. Easier for photographic use, but
not perhaps for toner transfer
>
>Even more frustrating when one side comes out perfect in part of the
>process (eg, developing) and the other side is too flawed to be useful,
>meaning you get to start from scratch again.
Agreed.
>
>So, the thought randomly occurred to me today, if you could prepare,
>expose, etch, drill the two sides of a double sided board completely
>separately, and join them up at the end of the process, it would make
>getting precise alignment of those sides almost a non-issue, assuming you
>can drill even roughly vertically and hit the center of the holes it's just
>a matter of lining up the holes you drilled on each side. And also means
>that the success or failure of each side is independent of the other.
>
Exactly, but the drilling of the alignment holes is critical. More
later.
>Of course, it does double the drilling, but that's the easy bit really.
It doesn't if you align the boards first then drill.
>
>This is also of course more or less how multi layer PCBs are made by
>fabricators, a stackup of already prepared pcb layers and resin impregnated
>sheets.
>
however, they have cnc stuff that is far more accurate than the
average home fill-in-blank here.....
>Single sided laminate at 0.8mm thick (and you can get down to 0.5mm) is
>available out of China for cheap.
>
>Eg:
>http://item.taobao.com/item.htm?spm=a1z10.5-c.w4002-1192550948.12.TIEXsq&id=35870048695
>http://item.taobao.com/item.htm?spm=a230r.1.14.4.VdTsRT&id=21105435798&ns=1&abbucket=3#detail
>
>Has anybody done this... am I reinventing a triangular wheel here, is it
>doomed to failure? Suggestions on suitable adhesive... maybe just spray on
>contact adhesive might work, it's a pretty big flat contact area.
er... no, no contact adhesive ever, once it contacts, it grabs, and
you cannot adjust the alignment.
Short form of what I do:
1) create alignment holes (generally an unnamed via), Use 3 in an
asymmetric layout so that you can't misalign the board.
2) etch
3) drill holes for pins. I use map pins, which are short, moderately
fat, and I have a drill that fits them properly. You don't want
wobble.
4) cover the board surface except for the alignment holes with masking
tape, keeps epoxy off the board tracks.
5) mix epoxy, the 1 hour type is preferred. If you use 5 minute, then
you almost have to mix a new batch each board.
6) spread thinly on one board surface, poke pins through, align boards
7) press boards together and tape edges together. Masking tape is
fine.
8) place boards between two ceramic tiles (cheap is best) for 24 hours
9) remove tape, trim, and drill.
I use an upside down drill press with a TV camera for alignment. You
want the holes as accurately placed as possible. I can get within
about 0.002 or so (rough estimate) with this setup.
Harvey