Homebrew_PCBs

Exerpt from an article in Electronic Design - maybe the epoxy is
available without the other paraphanelia. I haven't looked into it -
just got the e-mail article this morning. (the foil mentioned appears
to be just a mask to keep the epoxy restricted to the desired holes).
For the complete article:

www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=9984


Norm

PLATED THROUGH HOLES
The ProConduct system is designed to create plated through holes without
resorting to electroplating. The approach starts by applying an adhesive
foil to both sides of a board and then drilling the holes that will be
plated /(Fig. 4
<http://www.elecdesign.com/Articles/9984/Figure_04.gif>)/. Application
of a conductive epoxy paste follows in a method of placing the board on
a vacuum table, applying the paste, and pressing it through the holes
/(Fig. 5 <http://www.elecdesign.com/Articles/9984/Figure_05.jpg>)/. This
occurs for both sides of the board. After that, the foil on both sides
is removed /(Fig. 6
<http://www.elecdesign.com/Articles/9984/Figure_06.jpg>). /

Next, the board is placed in an oven for about half an hour /(Fig. 7
<http://www.elecdesign.com/Articles/9984/Figure_07.jpg>)/. At this
point, the board can be populated. The conductivity of hole plating is
nearly as good as the electroplated approach, and ProConduct is
significantly cleaner and easier to use. Typical resistance is 20 to 60
m(omega). The minimum size hole is 0.3 mm.

Plated through holes for multilayer boards are done in the same fashion
as double-layer boards. However, the layers are stacked before the
adhesive foil is applied.

LPKF Laser and Electronics
/*www.lpkfusa.com*/


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It is a very interesting article until I get to this statement:

"The complete system comes in under $25,000."

For corporate use though, it isn't a bad price.

mgray@... wrote:

>It is a very interesting article until I get to this statement:
>
>"The complete system comes in under $25,000."
>
>For corporate use though, it isn't a bad price.
>
>
>

That's why I said "maybe the epoxy is available separately". We could
play with application methods.

Norm


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It sounds similar to a technique I used years ago to make spacecraft structural parts. You need a vacuum table which consists of a piece of metal - maybe 1/8" thick - with holes drilled all over it. Below this perforated metal plate is a vacuum plenum. This plenum is simply a layer of coarse fabric covered with a piece of heavy gauge Mylar and sealed at the edges. This eliminates the need for a heavy steel plate that would resist the force of the vacuum. A tube from a vacuum pump is connected to this plenum.

On the top you build another structure consisting (from the plate up) of:

1 - A breather ply - a coarse non-woven fabric that will allow air to flow laterally
2 - A release ply - this is just a high count polyester fabric. After the epoxy cures, this is simply peeled off along with any excess epoxy
3 - The drilled board with the adhesive film attached to both sides. There is no need for this to be a foil. A polyester based adhesive tape would work just fine as long as the adhesive would take the cure temperature.
4 - A polyester film cover sealed at the edges of the vacuum plate and at the edges of the board. The area of the board with the drilled holes of course must be exposed so you can get the epoxy into them.

The sealant customarilly use is called 'zinc' tape. It is not zinc, but rather a tackey rubbery tape about 1/8" thick and 1/2" wide commonly used for sealing vacuum bags. You might cobble up a mechanical seal with a rubber back up, but that's a lot of work.

Start the vacuum pump and then just squeege the epoxy over the surface of the board. Apply it in a thick layer. Once all the holes are covered the vacuum will go to work and pull the epoxy down into the holes until it hits the peel ply. The fabric is so thick the epoxy can't readily flow through it so all the air is pulled out of the hole and is replaced with epoxy. Just squeege off the excess epoxy and release the vacuum. Put the whole thing in the oven to cure. After cure peel off the adhesive tape and peel ply. You should have epoxy bumps the thickness of the tape at each hole.

The most expensive component is the vacuum pump. A good one will run $1500, but you might get away with a diaphragm pump that costs about $200. The rest can be made from stuff from a scrap yard and the local hardware store. The release and breather ply sell for about $2 a yard.

Your ultimate success is highly dependant on the viscosity of the uncured epoxy. There are lots of conductive epoxy adhesives around. A little experimentation might find one that is reasonably priced and works.

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