My experience when going from a water soluble paste to various types of
no-clean paste was that an inert atmosphere had no significant effect on
the squeeze ball or as I call it the solder bead. I think the formation
of the undesirable squeeze ball is not only a function of volume of
paste but location of paste. If you have less volume by scooping it out
but there is still too much material under the body of the chip, I think
you will still get the squeeze ball most of the time. We used the
home-plate pattern to remove much of the paste from under the body of
the chip and that worked great.
Regards.
Wade Oberle
[log in to unmask]
-----Original Message-----
From: Andy Mackie [SMTP:[log in to unmask]]
Sent: Friday, January 29, 1999 8:11 AM
To: [log in to unmask]
Subject: Re: [TN] Design considerations for No-Clean
processes
Paul Klasek raises some interesting points. If you can't change
stencils,
then it may be possible to use a softer squeegee (less than
90durometer)
with a slightly higher print-pressure setting, to 'scoop out'
the amount of
paste you need. However, if you then try and slowly introduce
reduced-aperture stencils, you'll have to mix and match
print-processes
across the production floor.
I think there may be an opportunity for the solder paste
suppliers out
there to come up with exactly what you are talking about: a high
solids
content flux, with a reduced metal loading (i.e.
viscosity/rheology stays
pretty much the same). The problem with this is that hot-slump
becomes much
worse, so it may not actually buy you much reduction in the
solderballing -
I'm prepared to be convinced otherwise. May be a niche market
for some of
those 'polymer pastes' out there?
Alternatively, inert reflow, giving improved solderability to
all metal
surfaces, may be another way of reducing 'squeezeballs/capillary
balls/sideballs'. I'd be interested if anyone has any experience
with
this...
Andy
Paul Klasek <[log in to unmask]> on 01/28/99 08:41:21 PM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>;
Please respond
to Paul Klasek <[log in to unmask]>
To: [log in to unmask]
cc: (bcc: Andy Mackie)
Subject: Re: [TN] Design considerations for No-Clean processes
Steve ; Andy ; this is interesting ( am I glad we don't print
paste !) :
Do you (and Andy and Matt ) actually say you need different
apertures for
different fluxes (within realistically similar viscosities ;
releasability
;
etc.) ?
And the 50% volume flux ; would there be means of playing with
paste
composition contents to be able use the
400 at $ 400 ; uhhhh ; that IS A heavy pill to swallow with a
painful GRIN
upon hitting the budget bottom !
I believe Steve it made the beancounters do unmentionables .
Andy ; I know Steve will absolutely deny there is any other way
to no bath,
bull, and balls due to pending liability for $ 160 gran !!! ( no
wonder he
choked on calculations ! ) ;
but ; just between us & The Forum (ex the legend ) ; would there
be a no
paste composition allowing same paste stencil ?
This ball business is really so severe out there ?
If you'll keep solidarity cover just in case Steve's ex boss
with big
memory
watches the Net ; I understand .
The test strips are indeed good and seems versatile as well :
on SIR ; as you said on solderability ; we have a repeatability
shear pads
on them as well (interpolated to reliability test) .
On your note " bridged wide gaps are good guys" : one of the
cutest things
on the Ind. NC paste is it's wickability :
you can smear continuous line over the QFP and in reflow it all
wicks in to
individual pads evenly :
What I'm trying to say : I understand you test the adhesion ;
but on the
balance you may toss something very good out with something very
bad .
Hey Andy ; lettin' you know what I think just because you asked
for it .
Be gentle on me with the lead ball sizing ; I just find it hard
to believe
you have to chuck the car if you can change the seats to fit in
?
Thanks ahead for the insights
See you folks
Paul Klasek
http://www.resmed.com
> ----------
> From: Stephen R. Gregory[SMTP:[log in to unmask]]
> Sent: Friday, 29 January 1999 4:14
> To: [log in to unmask]
> Subject: Re: [TN] Design considerations for No-Clean
processes
>
> In a message dated 1/27/99 8:41:13 PM Pacific Standard Time,
> [log in to unmask]
> writes:
>
> > Here is one for the Design and Assembly folks.
> > We are a high volume OEM of consumer electronics.
> > My Assembly has converted to a No-Clean process.
> > What are the differences that I should be concerned with at
the design
> > level?
> > Should I have concerns about or modify any designs left
over from the
> > Aqueous days?
> > Thanks for the help
>
> > FNK
> >
> >
> > Frank N Kimmey, C.I.D.
> > EM Designer
> > C & K Systems, Inc.
> > 916-353-5366
> > [log in to unmask]
>
> Hi Frank!
>
> I was with a memory company when we converted from water
soluble to
> no-
> clean, and there were a few suprises for the finance
department that
> weren't
> realized when the decision was initially made to change
processes.
>
> One of he biggest expenses was stencils. We had over 400
different
> part
> numbers of SIMM's, DIMM's, Cache Modules, & PCMCIA
products...you
wouldn't
> believe the number of stencils we had. Well, they were all
made when we
> washed
> everything. Stencil apertures for all passive components were
1 to 1
> except
> for the industry standard etch factor compensation...and most
of you can
> guess
> what happened when we used those stencils with no clean. You
got it!
> Squeeze
> ball city! Not a big deal when the boards get a bath before
they leave
the
> factory, because they normally always wash off...(that's why
everybody
> lives
> with this process defect in a cleaning environment) but a very
big deal
> when
> you're no-clean. Every last one of the stencils needed to be
> replaced...and at
> a average of $400 a pop at 400 stencils...(that's
ummmm...lessee,...zero
> times
> zero equals zero, and then 4 times...oops wait, gotta take off
my shoes
> now..)....uhhhh...just TRUST ME, it's a whole lotta moola!
(GRIN)
>
> Another thing that drove the costs up was the fact that we
couldn't
> buy
> material the way we had in the past. Couldn't give those
100,000 board
> orders
> to fab vendors anymore to keep the prices down, and let the
PCB's
languish
> around on the stockroom shelves oxidizing and expect to solder
em' with
> no-
> clean. Same thing went for the DRAM. Used to buy from anybody
and
> everybody,
> but had to be a little more selective when the process
changed.
>
> One thing you might do in your designs, if you haven't
already, is to
> implement some sort of SIR comb pattern that you put on every
one of your
> designs. It's up to you where you want it to be. It could be
one like
> what's
> on IPC's B-24 board or as simple as two unmasked traces next
to each
> other.
> What we were doing since practically every one of the products
we build
> was
> panelized, we put them on the coupons, or skirts of the panel.
They went
> along
> in the process just like the SIMM's did so we would test them.
That way
> when
> we wanted to to a SIR test we wouldn't tie up any shipable
product.
>
> Another thing that I think that's pretty important, is to
standardize
> on
> a surface finish and mask type that you use on all your fabs.
We used o
> get
> boards from about 5 or 6 main vendors, and to try and get your
arms
around
> all
> of them so they're all giving you the same thing can be a
bear, but it
> really
> makes things easier and more predictable on the production
floor.
>
> Good Luck!
>
> -Steve Gregory-
>
>
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