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I haven't noticed any tendency for kapton/polyamide stencils to lose
their curvature, even after a year or more. For fun I clamped one
between two sheets of metal with a vise and brought the whole thing
up to about 170F and held that for a few hours. and then sat it
outside to cool. When the stencil came back out it was exactly the
same shape: a little arc of roughly two inch radius waiting to be
annoying once again. So mylar is better than kapton on this score
too.<br>
-Pete<br>
<img src="cid:part1.58473BC1.05C2DBA2@soper.us" alt=""><br>
<br>
<div class="moz-cite-prefix">On 11/14/2016 12:02 PM, Brian via
TriEmbed wrote:<br>
</div>
<blockquote cite="mid:5829EE27.5070603@undecidedgames.net"
type="cite">On 11/14/2016 11:26 AM, Pete Soper via TriEmbed wrote:
<br>
<blockquote type="cite">I always enjoy your good cents.
<br>
</blockquote>
<br>
Bad cents also included, no extra charge.
<br>
<br>
<blockquote type="cite">If mylar from Pololu tends to sit flat
that would be worth some money.
<br>
</blockquote>
<br>
Mylar from Pololu is shipped in a tube, so it must be flattened
before use. Once that's done, it does sit reasonably flat.
Obviously care must still be taken.
<br>
<br>
My flattening methods have been the heavy-book-sandwich as well as
using magnets to affix it flat to one of my whiteboards (oriented
such that the curve tendency is convex). In a warmish room, a
couple days oughta do it.
<br>
<br>
Realistically though, a bit of smear and bleed is going to be
unavoidable unless you're using a machine. Hopefully your paste
has good flux and will (literally) pull itself together during
reflow. Mopping up afterward with more flux and solder wick is
very effective. Even QFN packages can be de-bridged that way,
especially on a board with a good soldermask.
<br>
<br>
SMT is fun!
<br>
<br>
-B
<br>
<br>
<br>
<blockquote type="cite">
<br>
-Pete
<br>
<br>
On 11/14/2016 10:00 AM, Brian via TriEmbed wrote:
<br>
<blockquote type="cite">Joining this conversation a bit late,
but having ordered stencils in a
<br>
variety of materials from a variety of vendors, here are my
three cents:
<br>
<br>
- OSH Stencils' kapton (polyimide) stencils are the least
pleasing to
<br>
use, IMNSHO. They are very flimsy and feel quite fragile.
<br>
<br>
- For flexible-substrate stencils, I've had better experiences
with
<br>
mylar stencils from Pololu. They're a bit more expensive, but
I'd say
<br>
it's worth the extra expense to have a sturdier material.
<br>
<br>
- OSH appears to offer stainless steel at a very competitive
price;
<br>
I've only ever ordered stainless steel from Stencils
Unlimited.
<br>
Stainless is definitely the best stencil to use, in no small
part
<br>
because it is flat and rigid. I think the premium for metal
stencils
<br>
is well worth it, especially for smaller designs of which you
plan to
<br>
build many.
<br>
<br>
IANAL, YMMV, OMG, WTF, BBQ!
<br>
<br>
-B
<br>
<br>
On 11/09/2016 03:55 PM, Pete Soper via TriEmbed wrote:
<br>
<blockquote type="cite">I made some notes while listening to
an interview of the guy behind OSH
<br>
Stencils:
<br>
<br>
<a class="moz-txt-link-freetext" href="http://triembed.org/blog/?p=1895">http://triembed.org/blog/?p=1895</a>
<a class="moz-txt-link-rfc2396E" href="http://triembed.org/blog/?p=1895"><http://triembed.org/blog/?p=1895></a>
<br>
<br>
By the way, I'm not in the position to publish this at the
moment, but
<br>
several weeks back I worked out the steps necessary to
"combine" Eagle
<br>
designs so one board file can be uploaded to OSH Stencils
for a
<br>
single metal stencil that can be used for the multiple
designs. By
<br>
having a set of fiduciary marks for each board (and adding a
few lines
<br>
to the stencil with a permanent marker) it is straight
forward to
<br>
minimize cost. (What the hell am I babbling about here? The
minimum
<br>
border size of the metal stencils make them *expensive* for
<br>
small designs.) This can no doubt be done with alternative
tools like
<br>
gerbmerge.
<br>
<br>
-Pete
<br>
<br>
<br>
<br>
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</blockquote>
<br>
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