Kathy,
Based on our experience, the design of your stencil depends on how much
experience you have with your stencil printing process...at 30 mil
pitch, depending on the quality of your stencil printer and paste if you
use a stencil that is 6 mil thick with 14 mil square apertures with
rounded corners you should be fine...if you can print sucessfully for a
19.7 mil pitch QFP, you should not any problems with it....As for the
voiding issue, my suggestion is for you to read an artile written by
Banks, et. al on " The effects of Solder Joint Voiding on PBGA
Reliability" from Motorola Semi. Products Sector... I can't tell which
periodical I got it from since I interacted with the authors
themselves... but on a nutshell, their study appeared to indicate that
voids up to 24% of the pad area did not detrimentally affect their
reliability... of course another party results would help validate their
findings... but it is better than nothing...As for paste, Type 3 mesh
will do just fine... I have yet to see a microBGA bridging even with a 6
mil placement offset...
Regards,
Marcelo Chan
Advanced Manufacturing Technology
Harris Corp., FL
-----Original Message-----
From: Kathy Palumbo [SMTP:[log in to unmask]]
Sent: Thursday, February 12, 1998 6:32 PM
To: [log in to unmask]
Subject: [TN] Solder Requirements for Chip Scale BGA's
Could anyone tell me what the Class II requirement is for Micro
(Chip
Scale) BGA's solder voids?
The size of the balls are .325mm +/-.025mm with a pitch of
.75mm.
Also, could anyone recommend a stencil thickness for printing
this, and
what mesh solder paste you would use?
Would you use round, oval , or square apperatures on your
stencil?
The pads on the PCB are round, but some have say use round, some
say use
oval and some say use square apperatures on the stencil. Does
it really
matter? If so why?
Thanks in advance for your advice on these questions?
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