Sorry, Jack - more questions.
How is your stencil made? Laser cut, etched, spark-eroded?
Is it parallel sided or angled?
Stepped stencils work well enough, though can be a bit hard on the
squeegee's life. I would be hesitant, too, for these small areas unless you
have a pretty good area beyond the stepped screening area to give a good
lead-in for the squeegee. Otherwise, and you're going to have less pressure
at the sides from the squeegee than you are further towards the centre, and
this would probably lead to slightly thicker deposits at these edges.
Given that eutectic solder-balled BGA's don't actually require additional
solder to solder to the board, can you devise a scheme for blanking these
devices from the solder paste screening process? You could then introduce a
means of dot-applying flux paste, in a fashion similar to glue dispensing
to bond components to boards for wave soldering. That would save a lot of
angst about stencil thickness and aperture sizes - albeit you may have to
invest in a dispenser.
I wouldn't like to comment of the 12 mils aperture you propose, as my
knowledge in that direction borders on the dangerous (i.e. it's a little).
Good luck, though!
Peter
"J&J Technologies" <[log in to unmask]> 06/02/2003 08:13 PM
To: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group@ST Domain, "TechNet E-Mail
Forum." <[log in to unmask]>
cc: <[log in to unmask]>
Subject: RE: [TN] BGA land pattern design for .65mm pitch device
Hi Peter,
The balls are eutectic and are .0157 diameter.
My problem is with printing on such a small pad (.0098 dia). I am using
a 5 mil stencil. I calculated that to attain an area ratio of greater
than .66, for a pad this size I would need a 3.7 mil stencil! With that
thin of a stencil, I think everything else on the board would be
insufficient. I have not heard many good things about stepped stencils,
so what I'm considering is making the apertures 12 mils. I am worried
about going that much larger than the pad. What do you think?
-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Wednesday, February 05, 2003 7:15 PM
To: TechNet E-Mail Forum.; J&J Technologies
Cc: [log in to unmask]
Subject: Re: [TN] BGA land pattern design for .65mm pitch device
Hi, Jack,
I think you've probably travelled into that country where you have to
calculate your land pattern and try it for size. Use the formula near
the
front of IPC-SM-782 or the on-line IPC calculator, if Hitachi themelves
cannot offer adequate advice.
What stencil thickness are you using for these, and what are the solder
balls made of - usual eutectic or something with a higher melting point?
If
they're high melting point, you only need enough pad to allow a solder
fillet to form, since the ball will just be in point contact with the
board. On the other hand, if the solder is eutectic, the small pad may
be
deliberate in order to try and maintain a reasonable stand-off height
for
the component. The larger the pad, the lower the component is likely to
settle onto it, and also the greater the increase in variance of solder
joint volume unless your paste printing volume is very tightly
controlled.
One other thought in conjuction with maintaining a maximised stand-off
height / minimised additional solder, is minimisation of the risk of
short
circuiting between the contacts at that very small pitch. What ball
diameter do you have at that?.
I'm just hypothesising here, as I guess you realised, but I hope I've
triggered some more-useful thoughts.
Peter
Jack McNally <[log in to unmask]> 05/02/2003 08:52 PM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to Jack
McNally
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] BGA land pattern design for .65mm pitch
device
Can anyone tell me where I can find land pattern design guidelines for
BGA's with .65 mm pitch? IPC-SM-782 only has pad dimensions for BGA's of
1.0 mm and greater.
I am placing a BGA which Hitachi calls its BP-240A package. The pad
design
on the boards seems too small, only .0098" while the ball size according
to the spec is .0157".
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