Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

Hi,

It is not worth the effort.

The best way of doing it is just drilling 3 referent holes (that should be placed in advanced in layout) and using them for mask alignment for both side of double sided boards.

I tried to make multilayer boards and I almost done it (I have diy CNC and heating press with 20 tons for gluing cores and prepregs and chemical process for copper plating holes....).

It is interesting that my CNC has 0.01 mm precision but holes positions weren't precise (let say that the worst precision was 0.1 mm). I later discovered that I can use some aluminum sheet to enhance hole positions ...

Prepreg is a uncured laminate that acts as a glue (but in my case I manage to get 20 years old prepreg.... and actually it was working where FR4 prepreg faced the FR4 core, despite 20 years old the it glued well, but where prepreg glued to copper that was weak.... and I had problems - micro air bubbles where prepreg meets the copper and when I tried to solder the board in the oven it slightly delaminates ....).

I had 3 major problems: weak connection between prepreg and copper (I tried to make the copper surface rough by micro etching but that didn't work), weak connection between copper that plates the hole and the hole wall (I didn't find out the stuff that dissolve FR4 and make surface rough for better plating, well... I didn't find commonly available stuff for dissolving FR4).

Other than that my multilayer boards prototypes worked even at 6 Gb/s speed (I made some FPGA transceivers small boards....).

Thanks,
Petar 
 
 

 

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 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.

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.

Of course, it does double the drilling, but that's the easy bit really.

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.

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.