🚀 go-pugleaf

On Sep 25, 11:41 pm, Jeff Liebermann <je...@cruzio.com> wrote:
> On Fri, 25 Sep 2009 16:02:55 -0700 (PDT), mickgeyver
>
> <eyezku...@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?
No. Just a scrap of fiberglass board from long ago. I have some Rogers
flex board, but the surface appeared too rough to get good results on
my PCB mill.
>
> 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.

Can't go too thick here as I'm leeching power from a Tek P6204 probe
cable.
>
> 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.

I'll try that just for grins. Never thought of thin PCB material as a
bi-metalic thermostat element.
>
> 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.

I was concerned that too much bending would crack some of the
components giving me problems later on. I don't want this to get too
big as it  lets me attach this unit to a probe manipulator to use
under a microscope to hit those tiny nodes. Like I've stated
before.... I'm leeching power from the business end of a TEK P6204
probe. The 3/8 brass tube mates up nicely with the Tek probe cable
strain relief at the probe end.  I'll post more photo's of the other
end for the groups viewing pleasure.
>
> Good luck.
> --
> # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
> # 831-336-2558
> #http://802.11junk.com              je...@cruzio.com
> #http://www.LearnByDestroying.com              AE6KS