Bob,
Those voids are too big. Since many of the voids are near the edges
of the spheres, you may get some of them out will a little more time in
reflow. (However, other voids will become larger when two or more small
voids combine). Most voids will never come out of the BGA sphere because it
has nowhere to go but up to the pads on the BGA side.
As a general rule, voids are bad. The small voids in your X-ray
photo have been shown by a Motorola study (SMI 96 Proceedings) to be better
for joint reliability. But as Gabriela said, there is a limit to the size
the void can be.
The best explanation I have found for the source of voids is Dr
Lee's flux exclusion rate theory. You can view his paper that was presented
in SMT magazine (Jan 96) at Indium's web site. If you don't want to read
the paper, the two sentence summary is: voids are caused by entrapped flux,
and flux residue. (Note that entrapped flux will still volatize). Since
paste does not transform from solidus to liquidus instantaneously, (much
like melting ice cubes), the liquid solder will drive the flux and flux
residue out of the solid solder powder until all of the solder powder is
liquid. The less viscous the flux and residue is, the less likely it will
get entrapped by the liquefying solder.
If you have the luxury of using Nitrogen, that will help control
voids immensely. This is because it reduces the amount of oxidation that
occurs, thus reducing the amount of gummy residue that is generated. (Gummy
residue is more viscous, so it does not move out of the paste as easily.)
Certain paste will control voids as well. There is an aqueous paste
that is advertised as reducing voids in BGA balls. (They advertise in SMT
and Circuits Assembly). I have used this paste and it does work to control
voids.
As far as the reflow profile, I think it is still somewhat of a
black art. (even with the flux exclusion rate theory.) You will just have
to play around with it a bit. I have not found anyone who has explained the
dynamics required to reduce BGA voids with the profile, and I haven't had
time to try out my own theories. i.e.. reducing the time to reflow and
slowing down the reflow spike.
Good luck. (I would be interested in hearing what you end up doing).
Ryan Grant
> -----Original Message-----
> From: Bob Perkins [SMTP:[log in to unmask]]
> Sent: Monday, March 20, 2000 2:01 PM
> To: [log in to unmask]
> Subject: [TN] Voids in BGA Balls after reflow
>
> Hello Technetters
>
> Recently with the help of a high end piece of x-ray equipment we
> have noticed voids in the spheres of the bga after reflow, "see
> attachment-file". Has anyone seen this before, or has any idea how the
> voids are created? Also does anyone know if this is a bad or good thing?
> Does anyone have any recommendations to the profile?
>
>
> Thanks
> Bob Perkins
> Automation Technician/Manufacturing Engineer
> Aimtronics
> [log in to unmask]
>
>
> <<wboard6.jpg>> << File: wboard6.jpg >>
##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe: SUBSCRIBE TECHNET <your full name>
To unsubscribe: SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
##############################################################
|
