🚀 go-pugleaf

On Fri, 25 Sep 2009 16:02:55 -0700 (PDT), mickgeyver
<eyezkubed@yahoo.com> wrote:

>I'm trying to make a 10x scope probe that is capable of dealing with
>slightly higher voltage levels of about +/- 20v yet at a high
>impedance ~20Meg. This link "http://picasaweb.google.com/
>UncleAltheKiddiesPal/Probe?feat=directlink" has some photo's showing
>the probe and board warpage. The board was made from double sided
>0.020-inch stock etched with a PCB milling machine (perhaps part of
>the problem). It is (obviously) hand assembled.  I would appreciate
>any tips or techniques that I could use to keep this board from
>turning into a potato chip during assembly.


<http://picasaweb.google.com/UncleAltheKiddiesPal/Probe?feat=directlink>

Is that a Polysulfone board?

With 0.020 inch anything, and all that copper, it's going to warp no
matter what you do.  Any differential heating, on one side of the
board, is going to cause the board to bend.  

I ran into the same problem years ago with wave and hand solder in
thin boards.  Going to thicker boards, with some kind of stiffener was
not an option because of weight limitations.  However, you apparently
don't have that problem.  Try 0.032 or 0.0625 boards, keep the heating
to a minimum, and it might work.

I think you might be entertained by what happens to just the board,
without any components.  Clamp one end of the board into a vice.  Heat
one side of the board with a hot air gun or soldering iron.  The
copper will expand, causing the board to warp.  In other words, the
large copper areas are what's causing the warping.

You can somewhat minimize the effect by pre-heating and post-heating
the board.  We found that freon vapor reflow soldering produced
absolutely flat boards.  That because the entire board was heated
evenly, and cooled down evenly and slowly.  Heat one side of the board
more than the other (i.e. differential heating) and it's going to warp
unless you're very careful on cooling it down evenly.  When we went to
surface mount devices, small traces, and 0.5 oz Cu traces, the warping
was gone.

0.020 is thin enough that the board can be straightened by the
mounting arrangement.  Instead of the round brass tube, it might be
possible to use an aluminum tube, with two grooves milled into the
inside diameter.  Your current board does not have enough unused area
along the lengthwise edges to do this, but it can be expanded
somewhat.  This also has the advantage of providing a nifty ground
connection to the case.  However, inside milling is expensive.  A
square aluminum extrusion is possible, but even more expensive.

Good luck.
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# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
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