In a message dated 7/10/98 7:16:55 AM Pacific Daylight Time,
[log in to unmask] writes:
> Hi all,
>
> We are currently having problems with a laser-cut stencil that is
> electropolished and nickel plated...
Hello there Todd,
Laser, nickel plated, AND electropolished huh? Sounds to me like someone
is making some money. I've always understood that laser was supposed to
eliminate having to do things like electropolish or nickel plate, and that the
former two technologies were used to impove the paste release characteristics
of a standard chem etch stencil. Both are used to smooth aperture
sidewalls...electropolish to remove and smooth out the "hourglass" that forms
from a double sided etching, and the nickel plating to smooth the surface
graininess that results from the acid etch. I gotta say, this is the first
time I've ever heard of using all three on the same stencil...sounds like
overkill to me.
Have you actually measured the openings on the stencil and looked at them
under a microscope. That'll tell you a lot...take and compare what these
aperatures look like compared to your old foil (if you still have it around)
and I'll bet you'll see something quite different between the two provided the
aperture dimensions are still correct. I'm assuming that your aperture width
to stencil thickness ratio is good (approximately 1.5 : 1) as you said you
have the same dimensions as the previous stencil and that worked good.
Has anything changed with the board? Mask types, or thickness? Different
vendor? The reason I ask that is your comment about having to clean every
print, could be caused by bad stencil gasketing which can be caused by too
thick of a mask, or excess HASL giving you a rounded surface on your
pads...are you having to clean because your print is "fuzzy", or because the
paste won't release at all unless you do?
How about your solder paste, have you noticed anything different with its
consistency? To answer one of your questions, stencil quality DOES vary from
vendor to vendor, and along the same lines, OCCASIONALLY, I've gotten a bad
lot of paste from a vendor that has given me good paste for years...so that
might be something to double check.
One last thing, I'm gonna plug a stencil vendor I've worked with for years and
a method that they use to make stencils. To preface this, I just want to say
that I've NEVER bought a laser stencil...The finest pitch I've placed
regularly is 15.7-mil, and I've done a lot of micro BGA...these stencils were
used at the past NEPCON and SMI shows at our booths here in California. They
use what they call "Band-etch" or "Band-width etch" which is basically etching
the outline of the aperture, instead of the whole aperture itself. Much easier
to control from what I understand, which eliminates the "hourglass", and gives
nice vertical sidewalls.
Sammy Shina had taught a college class a couple of years ago during the summer
of 1996 at the University of Mass. Lowell, where the class did an objective
study comparing electroformed, laser cut, band etch, and standard chemical
etch stencil performances. They study was published and found that band etch
stencils performed just as good as a laser cut stencil. BeamOn technologies
(408) 982-0161 has copies of the report which is quite extensive...okay now,
I'm done with my plug...
-Steve Gregory-
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