I have had to cross section my fair share of underfilled BGAs recently and
have learned a few things the hard way.
1) Insufficient:
Visually being able to see a continuous fillet around the four sides of the
BGA in question is one criteria I can agree with. This is nondestructive
and a good criteria.
2) Excessive:
Inspecting how far the underfill has spread beyond the fillet is another
important criteria. Far too often, there is excessive application of
underfill.
3) Voids:
Verifying the coverage (which to me is checking for voids) is what I think
this thread actually was about. We cross section actual samples and grind
off the body of the IC which exposes the bird's eye view of the
underfill. The criteria is that a void may not connect two balls, however
it is difficult to see through the material if it is not 100 clear of
bubbles, and this seems like it is never the case except in lab conditions.
I have heard that SAM works for detecting the voids, but that it takes an
expert to truly be consistent in detection of these types of voids.
Phil
ps: I have a related question about going to OSP/Cu finish from ENIG and
being able to eliminate the underfill requirement altogether.
At 12:21 PM 11/2/2007 -0500, David D. Hillman wrote:
>Hi Tom - Just an correction - the IPC-7095B guideline document and not a
>requirement specification document. The solder voiding requirement of BGAs
>is per JSTD-001. I don't believe that there is an underfill voiding
>requirment in the JSTD-001 specification yet.
>
>Dave Hillman
>Rockwell Collins
>[log in to unmask]
>
>
>
>
>"Gervascio, Thomas" <[log in to unmask]>
>Sent by: TechNet <[log in to unmask]>
>11/02/2007 08:49 AM
>Please respond to
>TechNet E-Mail Forum <[log in to unmask]>; Please respond to
>"Gervascio, Thomas" <[log in to unmask]>
>
>
>To
>[log in to unmask]
>cc
>
>Subject
>Re: [TN] verifying BGA underfill coverage
>
>
>
>
>
>
>Curious what criteria are you using for BGA underfill (i.e. voiding,
>coverage, etc). IPC-7095B section 7.2.2?
>
>
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Joe Macko
>Sent: Wednesday, October 31, 2007 7:34 PM
>To: [log in to unmask]
>Subject: [TN] verifying BGA underfill coverage
>
>Fellow Techs
>
>
>
>We purchased a GPD automated epoxy dispenser and I want to proof out the
>new automated underfill application process on BGAs. We use a wickable
>Loctite underfill that requires an elevated temperature to flow properly
>under BGAs, etc.. Currently we successfully underfill by hand and want
>to switch to an automated process using the GPD.
>
>
>
>I want to verify the underfill coverage under BGAs using a
>non-destructive test method. The only non-destructive method I am aware
>of is acoustic imaging using an instrument such as Sonoscan, which I
>believe will show voids in the underfill material. I do not know how
>industry non-destructively verifies underfill coverage. I plan on
>physically removing some BGAs on scrap boards to verify coverage but do
>not want to do this on good boards for obvious reasons.
>
>
>
>Question - What is the commonly accepted non-destructive method for
>verifying BGA underfill coverage? Look forward to your comment. thanks
>
>
>
>Best Regards,
>
>- Joe
>
>
>
>
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