What can I do to keep this board from warping?

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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.

Thank you.

al

Re: What can I do to keep this board from warping?


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dang G00gle.....

"http://picasaweb.google.com/UncleAltheKiddiesPal/Probe #"

Re: What can I do to keep this board from warping?



mickgeyver wrote:
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   It looks like you used cheap phenolic PCB materials. Fiberglass is a
lot more stable.


--
You can't have a sense of humor, if you have no sense!

Re: What can I do to keep this board from warping?


On Fri, 25 Sep 2009 16:02:55 -0700 (PDT), mickgeyver

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  Use 0.032" media.  Bake under a flat press upon receipt of the bare
boards.

  Hand assemble...  no wave solder, no IR oven.

  Those are the only techniques that yield 100% positive results.

  Any automated process will certainly have a poorer prime pass yield.

  That is, unless the PCB maker was real good at stress relief during the
making of the boards.

Re: What can I do to keep this board from warping?


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I'll try to find thicker stock. Like Joerg there is no expense money
available for this project so I gotta use whatever is available.
There is a connector on the other end that I'm using to leech power
from the scope so I have limitations here.
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Done that. Perhaps if I make a frame to clamp it flat during assembly.
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I'm the PCB maker all contained in house with a mill machine.

Re: What can I do to keep this board from warping?


On Sat, 26 Sep 2009 06:34:12 -0700 (PDT), mickgeyver

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  Then your dwell time on each solder joint is way too high.

  Try 1/20th gram per joint, not the half gram per joint the picture
appears to show.  You know...  NASA soldering specs.  As little solder as
is possible and perform the operation quickly.  What I saw looked like
several time the amount needed in the dip chip.  Even the connector was
huge and blobby on the solder joints. The builder does not need to flood
those pins with solder.  The solder only needs to be between the pin and
the board ala SMD soldering.  FILLET not BLOB.

  The soldering iron should also be at 600 F, NOT 800 F.

  If you are having wetting problems due to age of the leads of your
parts or such, you may need to try different solders to see if a
different flux choice might yield a good solder joint faster than your
current choice.

  RMA is harder to clean (hot alcohol bath) but it does make a good, well
wetted solder joint faster in some instances.  I am not sure which flux
choices are the most active these days.  It has been a long time since I
did production level soldering, and we currently use odd technology since
we get exempted from RoHS compliance.

Re: What can I do to keep this board from warping?


On Fri, 25 Sep 2009 16:02:55 -0700 (PDT), mickgeyver

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<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.
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
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Re: What can I do to keep this board from warping?


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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.
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Can't go too thick here as I'm leeching power from a Tek P6204 probe
cable.
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I'll try that just for grins. Never thought of thin PCB material as a
bi-metalic thermostat element.
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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.
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Re: What can I do to keep this board from warping?


On Sat, 26 Sep 2009 06:52:30 -0700 (PDT), mickgeyver

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Ok.  I just noticed that the backlighting LCD inverter in a typical
laptop is constructed similarly to your prototype.  Very thin G10
board, 10mm wide, and all the parts on one side.  To make matters
worse, a big routed hole in the middle for the xformer.  Huge copper
ground plane on one side of the board.

Yet, all the boards in my collection are quite straight.  That
demonstrates that it can be done.  I'm not sure what you're doing
that's wrong, but I agree with others that suggest you're using far to
much solder and heat.  Smaller parts, less solder, and less heat will
help.

Incidentally, when I say less heat, I don't mean less temperature.  I
use the hottest iron tip I can get away with, and work fast.  That
minimizes the energy transfered to the solder joint and therefore
reduces the heat affected zone.

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PCB warping will certainly cause problems.  It really depends on the
physical size of the components.  Parts with flexible leads (xsistors,
IC's) are not much of a problem because the leads absorb the flex.
Parts that are soldered directly to the board, will either crack, or
rip the traces off the board.  If you can't fix the flexing problem,
switch to a flexible PCB design and apply an insulating stiffener
board on the circuit side.  It can't be prototyped with at PCB router,
but at some point, you're going to have to commit to a PCB layout.
Might as well do it early.

--
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: What can I do to keep this board from warping?



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I think one problem is that this is a home made board. The holes are
likely too large for the components (because most people don't have very
small drills on hand or even the drill chuck to hold them) and obviously
they aren't plated through. That can make it necessary to glob on the
solder.

As far as temperature, I agree with you. I've never found a need to
solder anything at less than 800F. Smaller parts just require shorter
dwell time.

I have no idea why the OP is using such thin stock, though. It looks
like he'd have room for something as robust as .050".

Re: What can I do to keep this board from warping?



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  Good observation. The lead size, and hole size and even the annular
ring width all inter-relate if one is to produce minimalist 'NASA' style
soldering results.

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  This one is off the mark.  Perhaps you meant "at MORE than 800F"
That would certainly be an upper max for me, and it is quite easy to
solder at much lower temperatures.  Well over a 100 degree differential,
and that is significant.

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  He could make little SMD circuits that he places on Silica wafers, and
then place those wafers on the substrate with epoxy, and wire the
sections together.

  I used to use 0.020 wafers up to 2 x 2 inches to make little smd proto
circuits on.  I have glued smd parts on, and hand wired point-to-point
the nodes between the parts.  I have also 'dead bug' glued thru-hole 8
pin dips onto it and wired those nodes with hook-up wire. The leads were
shortened, of course.

  That sounds like a good proto build solution, but to make a production
run would require a different decision.

Re: What can I do to keep this board from warping?



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Pardon my ambiguity. What I meant to say was "anything other than 800F."
That's the upper limit on most fixed-temperature tips, or adjustable
irons.

Re: What can I do to keep this board from warping?



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 Yes, and despite most folks cranking the damned things up that high,
basic soldering operations (pre-RoHS) were NEVER meant to be performed at
that high a temperature.  Even on big 0.092" stock with lots of Copper.
In that case, one should pre-heat the whole assembly a couple hundred F,
which will insure that all the solder joints wet well and flow through
properly.

Re: What can I do to keep this board from warping?



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Uh-huh. I've personally hand soldered hundreds of thousands of joints at
800F, and overseen the soldering of millions. Turning down the flame has
only one result: It takes too damn long to build the board. It's sort of
like the clock speed on a computer. It doesn't matter much if something
takes 1 second instead of 1/10 second, but it makes a big difference if
it takes an hour instead of 6 minutes. When I was a kid I built some
Heathkit stuff and spent 30 seconds making every solder connection,
because the iron was underpowered and I didn't know enough to know it.
Machine soldering is different, of course, but for hand work, 800 is
perfect for everything and anything.

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I concur - 700F tips are great - no problems in the last 15 years. The only
time I go for a 700F is  terminating some cables where the insulation is
super sensitive to the heat and shrinks back or melts through.



Re: What can I do to keep this board from warping?



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Typo - should have said 800F tips......

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Re: What can I do to keep this board from warping?



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  You are both idiots that would never sit very long at one of my
benches.

  Not to mention that neither of you would make it more than a day in a
NASA cert course either.  Not with a stupid mind set like that.

Re: What can I do to keep this board from warping?



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  The sinking capacity of a soldering iron has little to do with the
temperature it sits at and claims to be maintaining.  The ability of the
iron to keep a tip at a given temperature is what makes a good iron, and
no, those shit dial types with 800 at the upper end of the dial are NOT
good at maintaining a set point, and there is NO spec ANYWHERE that uses
800 as an iron tip temp far assembly, even by hand.

   And even at 650 or 700, a good soldering tech can perform a good
solder joint in the same amount of time with a proper iron that holds its
set point properly.

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  Proof that you do not really know anything about it.  So despite all
the talk about dwell times, and claim of "oversight" of others, your
decisions about ignoring the facts and standards the industry has known
about for decades means that your credibility for actually knowing what
is going on fell right through the floor.

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Obviously you have the engineering mindset, while I have the practical,
get-it-done mindset. Even NASA gave up their low-temperature bullshit
when they figured out that components are subjected to *less* thermal
stress with high temps and short dwell times. Sure I know the difference
between power and temp, but as far as production soldering, a lack of
either has the identical result, and that's a lack of production, which
translates to a lack of income, which translates to going out of
business for a job shop. Many industries are, or were, profitable enough
to have people working along at a snail's pace, but contract
manufacturing doesn't have that false luxury.

How long does it take you to solder a standard thru-hole 14 pin DIP IC
at 650? Anything more than about 1 second per leg, and you'd be fired
from *my* production line. The same IC in a surface mount package, and
you'd be expected to skate down each side and make seven perfect solder
joints in a total of about 2 seconds, once the IC was tacked at opposite
corners and fluxed.

I'll make the implicit explicit here, and let you have the last word.
But it won't make you right.

Re: What can I do to keep this board from warping?



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  You're a retard, and we ain't making nor talking about some crap that
barely makes class one standards.

  Your mind set is so archaic it is funny.

  Just so you know, I could properly produce 10k solder connections a day
in my heyday.  I am quite sure that I still could.

  I used to teach soldering processes as well.

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