Reliable PCBs at home

From: Marvin <marvin_at_rain.org>
Date: Sun Oct 24 23:48:29 1999

Richard Erlacher wrote:
>
> First of all, I haven't read ALL of this thread, but I recall Tony or
> someone else replying to him saying something about the method for making
> plated through two-sided boards in your home. I've never met anyone aside
> from professionals with scads of equipment who could do that, but it seems
> to me that the method which was described to me was to start with bare
> fiberglass/epoxy panels, drill them, then apply a slightly conductive
> coating in liquid form which had to be forcibly dried (perhaps baked) before
> the resist was applied. The boards were then exposed, the films applied to
> registration targets on each side, to a powerful UV light, for which some
> prefer to use direct sunlight, and the boards subsequently developed, then
> etched.
>
> Has any of you ever encountered an approach to this that could be managed in
> the home environment with equipment costing, nominally, less that a k-buck
> or two and achieving nominally 10-mil traces with 8-10 mil separation or
> anything close to that? How about a dry-film solder mask?

First of all, there are service shops that will take a drilled board, do the
PTH process, and electroplate the desired amount of copper onto the board.
The normal process for making PTH boards is as follows.

The copper clad laminate is cut to panel size and drilled. The drilling is
usually done on an NC machine. The NC program can either be supplied as a
drill file, or it can be hand programmed. Hand programming involves taping
the artwork (or more likely a copy) to a programing table, marking the rout
to follow for each drill size, and then just the grunt work of centering
each hole in a scope, and pushing a foot pedal that records that location.

There are a number of different processes for doing PTH, but the most common
is to take the drilled panel(s), run it through an electroless copper line
(cleaning, catalist, accellerator, electroless copper) that will put about
30 millionths of copper on the board, and electroplate about .3 mill or so
of copper on the bare panel (enough so the rest of the process doesn't
create problems with the plated through holes.)

The next step is imaging and how that is done depends on the required
quantity and line density. What I used (prototype/short run shop) was use
dry film. The board is cleaned and laminated with a photosensitive film. The
artwork was transferred to diazo film, and the diazo films were used to
actually image the board. At this point, the board is developed and the
copper you see is what you want.

The rest of the process is fairly short. Electroplate copper up to the
desired thickness, electroplate tin-lead, strip the dry film, etch, gold
plate the fingers if necessary, fuse the tin-lead into solder, route, clean,
and ship. The etching is usually done by machine using an alkaline etching
solution.

Doing the process at home can be done with a minimum of equipment if service
shops are used for parts of the process. A small copper plating tank,
tin-lead tank, and peroxide-sulfuric etchant along with fusing oil and flux
can be set up at home for probably a couple hundred dollars. To set up a
fairly complete shop including drilling and imaging would probably cost
between 2K and 3K. This would provide the capabilities of producing
reasonably high quality boards. Oh, did I forget to mention getting the
experience to know how to do it :)?

BTW, I think you just meant dry film above. Dry Film Solder mask does
require UV curing and is probably impractical for home use. However silk
screening the soldermask and legend is easy and inexpensive to do at home.
Received on Sun Oct 24 1999 - 23:48:29 BST