Phil,
I used some of Indium's no-flow underfill recently, and while I can't
tell you how it works, I can tell you that it does, and I applied the
material right over the pads. (I used it on a flip chip, not a PoP.)
For PoP I wonder what ball coverage you can achieve. The goal of
underfill is to completely encapsulate the ball from board to package
body. But with a PoP, if you make a ring of material covering the pads
that fills the gap between the board and the component, then you are
creating an air pocket that will want to go somewhere when it expands
through reflow.
Perhaps you could apply material to the corners only, or leave a gap on
one side?
Ben
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phillip Bavaro
Sent: Friday, December 12, 2008 4:44 PM
To: [log in to unmask]
Subject: Re: [TN] POP underfill?
Hi Mike,
I spoke with Indium today and their no-flow is typically dispensed.
This is theoretically feasible so I could envision dispensing the
no-flow
between the pads and on the inside of the pads.
This memory chip POP configuration only has two rows of 0.5 mm
solderballs
around the perimeter.
This would mean that the upper BGA underfill would not be continuous
support like the bottom BGA underfill would be, but I suppose it would
add
a lot of strength for high impact.
The thing that I am curious about is how do they make an epoxy-flux
mixture
that acts like a flux until the cross link is activated at 245 and then
sets up as the material cools.
I would think that one would need to keep the no-flow off of the bga
pads
so that proper joints could form.
I guess there are lots of options out there, just not a lot of people
talking about how they have tacked this problem.
Phil
At 10:41 AM 12/11/2008 +0000, Mike Fenner wrote:
>Could you use a no flow under-fill?
>This replaces flux in assembly cycle and cures during reflow so no
>additional 'fill needs to be wicked in.
>
>Regards
>
>Mike Fenner
>Indium Corporation
>
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Phillip Bavaro
>Sent: Wednesday, December 10, 2008 7:04 PM
>To: [log in to unmask]
>Subject: [TN] POP underfill?
>
>We normally underfill our BGAs by dispensing next to the device and
>allowing capillary forces pull the material under the BGA.
>
>Now we have a Package on Package (POP) BGA assembly to build........is
the
>upper memory BGA expected to be underfilled as well?
>
>And if so, how is that accomplished?
>
>This chip on top has the same dimensions as the BGA below so there is
no
>shoulder to dispense upon.
>
>Thanks in advance,
>
>Phil
>
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