Carl,
Inspection of Underfill can be carried out by X-Ray if you have a system
with bags of resilution and excellent computer enhancement capabilities,
but the recommended method is by using a Scanning Accoustic Microscope.
I haven't found any specs for accept/reject criteria either, so had to draw
up my own. Of prime consideration was the amount of rework involved should
you decide to reject an underfill because of voiding. Using "Flow" type
underfill (fill the air gap underneath a part after the soldering
operation) is bad enough to rework - you have to remove the BGA, clean up
the site to remove epoxy as well as solder, re-ball the BGA, re-attach,
re-underfill and re-inspect. "No FLow" type underfills are worse because
they're not reworkable. Once you fill a BGA with this stuff, it's meant to
be there for keeps.
I decided that "failure" occurs where a void leaves one or more balls more
than 50% by surface area unsupported. Multiple small voids are not
significant, but should be minimised. Void-free underfilling is not
actually critical, but the permissible degree of voiding is up to the
product engineers to determine from their Qualification tests. As a rule of
thumb, I used the same acceptance/rejection criteria as for voids in the
BGA solder joints, but gave a bit more lattitude/benefit of the doubt
before rejecting. After all, underfill voiding will not directly cause a
functional failure, though too many voids of the wrong size or in the wrong
places may reduce the enhanced reliability of the BGA's solder joints.
Solder joint life will still be longer than with no underfill under
conditions of temperature cycling.
Does this help any?
Peter
Carlr Ray <[log in to unmask]> 24/05/2003 02:01 AM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to Carlr Ray
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] Checking and measuring voids in underfil
Hey Techer's,
Currently I an assisting with the development of a underfilling a FPBGA.
As part of the qualification I was wondering what amount of voiding should
I expect and accept. I have been unable to find any standards for underfil
voids and methods of detection.
Any information would be greatly appreciated.
Carl Ray
Sr. Manufacturing Engineer
Huntsville, AL 35807
Phone: 256-882-4800 ext. 8845
Cell: 256-990-1990
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