Hi Ron
>I've been following the Markus' posts for a while. It appears he is really
>getting a process down for PTH on 2 layer.
I definately do have trouble with my english - not sure what you try
to tell, but I DO create 4 layer boards at home on a regular basis not
only 2 layer boards. Even 6 layer is not at all out of scope. A 6
layer board would just take 2 more hours to process.
>What frustrates me is the time it takes to get a 4 layer proto. If I can run
>them at home, even if it takes 30 hours, I'm still way ahead of the
>commercial houses who take a minimum of 5 days plus another day for
>shipping.
Welcome to the club. You definately have discovered my primary
motivation to having built my own through plating station. Another
point where I live (Switzerland) are the extreme high prices
boardhouses want if you say want a board within 3 working days. As an
example, my last quote for a 4 layer 200 hole 10x10cm board was
CHF3500 which is about $2K!
In short the process to create a multilayer at home is as follows:
1) Use photoresist laminate - that's a requirement in order to through
hole plate or else the just copper plated holes would loose the
copper.
2) First create the films for the inner and outer layers. Note, they
should be made so as the holes of the pads later on remain covered by
copper. Due to the nature of photoresist laminate they have to be
negataive - but that's not difficult either.
3) If you can't use a CNC drill machine, create one aditional film of
one of the outer most layers (does not matter which one) where the pad
holes are not covered. This will later serve as a helper mask to
manually drill the holes.
4) Put all layers on a stack. Create your desing so as you define two
aditional alignement holes of 3 or 4 millimeters diameter at diagonal
positions outside of the effective PCB area. Drill those two holes so
as you later can insert a shaft? (hope this is the right word for it)
into them. Drill one hole first, insert the shaft, then drill the
second one to make 100% sure all layers will fit together. Cut the
pieces of the layers with some oversize. This does not matter as you
can cut off thereby creating nice sides once you are done.
5) disasemble the stack, laminate the inner layer on both sides,
expose develop and etch it. But DON'T drill any holes yet. Use the two
diagonal alighement holes with the shaft to align the films (note the
films must get those holes also). I create the holes in the film with
a little stamp kind of tool I made with some stainless steel whire
where I drilled a small hole on one end with a center drill. They have
the right angle to create a very sharp ending that can later be used
as the stamp. It's otherwise quite difficult to "drill" holes into
film material precisely enough.
6) put a very thin layer of 2 minute epoxy (devcon or araldit is fine)
on both sides of the inner layer. Apply it to the INNER layer and do
it all in one step. The reason to apply it to the inner layer is to
make sure there is glue in the etched away parts. Then put the stack
together again using the two center shafts. Put as much pressure on
the stack from both endes as you can. I use two wooden blocks on both
endes and built a wooden construction that holds two vises which
actually press the stack togeter. I used two vises to have more
pressing area and power. Of course if you have a press available
that's better. Let it dry for 5 minutes.
7) Laminate one outer layer, expose and develop it with the special
film mentioned in step 3) Of course don't etch it! Use this as the
drilling mask and now drill all holes.
8) Remove the laminate clean the board and run it through the through
hole plating process. The just drilled holes will cross the copper
parts of the inner layer pads at those spots where they have to. As a
result the trhough hole plating process will create connections where
needed in the inner layer. To answer your question about blured /
burried vias - of course by selectively drill the holes before you
glue and eventually some cleaning after glueing you shold be able to
create such beast.
9) from this moment on, the remaining process is identical to
processing a two layer board. If you are about to create a 6 layer
board, you simply would continue with applying glue to the "semi"
inner layer, but would leave out the drilling and through hole plating
until you are finshed with glueing all layers etc. I figure you get
the picture.
10) laminate, expose develop and etch the outer layer - you are done
now.
I created picutres of the wooden construction to press the stack as
well as of the stamping tool mentioned. Again, I currently don't have
the time to put this onto a page. It's really not that complicated
once you have the aligment hole thingy working (which is also not so
complicated). In fact, this technology works so well that I sometimes
even use it to asure alignement of two layer boards.
I hope the above brabble makes some sense to you. Feel free to ask
whatever questions you might have.
Markus