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From: irwin@m.cs.uiuc.edu
Newsgroups: sci.electronics
Subject: Re: plated thru holes
Message-ID: <21000080@m.cs.uiuc.edu>
Date: 29 May 90 20:54:00 GMT
References: <1109@uvicctr.UVic.CA.UUCP>
Lines: 98
Nf-ID: #R:uvicctr.UVic.CA.UUCP:1109:m.cs.uiuc.edu:21000080:000:4726
Nf-From: m.cs.uiuc.edu!irwin    May 29 15:54:00 1990


/* Written  2:57 pm  May 28, 1990 by climber@uvicctr.UVic.CA.UUCP in m.cs.uiuc.edu:sci.electronics */
/* ---------- "plated thru holes" ---------- */
>I am designing double sided boards that I am planning to etch myself.
>Now, I want to start using pin arrays and realized that I will now need 
>plated thru holes (where the copper lines the hole itself making a nice
>connection between top and bottom traces).  Is there a way I,
>a hobbyist with measly resources, can build a two-sided board
>with plated thru holes?  I would like to avoid the following:
  
>  tremendous equipment expense
>  having to spend more than $10 per prototype board (outside the costs
>     of blank PC board)

>Has anyone tried this?  How does the big-wigs in the industry do it?

>Send any responses to: climber@sol.UVic.ca

>Any help would be greatly appreciated.  

>Craig
/* End of text from m.cs.uiuc.edu:sci.electronics */

Though I responded to Craig with e-mail, I thought I might put
some of what I sent to him here, as it may be of interest to
others.

I am with the Department of Computer Science, at the University
of Illinois. Years ago, about 15 or so, we used to have a complete
printed circuit board lab, complete with cameras, PCB layout group,
etch facilities, plating equipement and a wave soldering machine.

The process which we used for plated through holes may have changed
since then, and again, it may still be done in much the same manner.
I will describe the process for the hungry minds on the net.

We started by first drilling the boards (double sided), with what
I believe was called a Panograph. It was simular to the machines used
to make name plates and badges, or engraving. There was a master
template which one followed with the cursor, and the electric drill
followed your every move in the X/Y direction. We used a brass plate
to make the template, making a center punch in the template for each
hole to be drilled in the boards. The cursor had a sharp pin that 
dropped into the center punch hole, which positioned the drill over the
correct spot in the boards and they were stacked and drilled 4 or 5
at a time. The drill was air operated, so as one placed the cursor
into position, they pushed the air button and the holes were drilled.

After the holes were drilled, the board was placed into a copper bath,
and electrodes (2) were connected from a power supply to the opposite
sides of the board, such that when the board was placed into the bath,
a current flowed from one side of the board to the other, through the
bath. This plated the copper through the hole.

After the copper plate, the board was photo sensitized, dried and
exposed to the image via a negative, or I should say, a positive.
The image was developed, leaving the photo resist on the board where
you did NOT want to have any copper remain.

This stripped the resist from the area that would become the trace
or conductors on the board. It was then again placed in a bath and
plated, this time with tin. The tin was to act as the etch resist
in the final process, so, the trace and plated through holes were
tin plated. The image was then stripped, which removed the photo
resist, and the board was then etched, the tin being the resist.

There was also a seperate process for plating the contacts, if it was to
be a "plug-in type board". The contacts had to be I think first nickel
plated, then gold, over the original copper. This was done prior to
the tin plating.

After it was etched, it was trimmed to size, parts loaded and then
passed over the wave solder machine.

In summary, for one to do this on a small scale, one would need the
chemicals to make up the copper and tin bath, the power supply for
plating, the photo resist to apply to the board, an oven to dry it,
and so on. It takes quite a bit of equipement. We also emersed the
board in dye, which dyed the image of the resist on the board black
in color, and one could check the quality of the image. If flaws were
found, they could be patched up with nail polish (of all things) or,
the board could be stripped of the resist and done again.

The board had to be scrubbed with a fine powder like used at the kitchen
sink to clean copper bottom pans, before any of the above process was
started, to remove the oxide, or one would not obtain good results.

All in all, it was not an easy process. I do not know if the technology
has changed much, but I would suspect that the steps taken today are
much the same. I do think that there is a silkscreen process, that
provides the component identification on the board, plus acting as
a resist to the tin plating, though we did not do this with our boards.

Al Irwin
Univ of IL
Dept of CS
Urbana, IL
irwin@m.cs.uiuc.edu