Hi Steve,
One thing i've learnt is that underfilling serves a few basic purposes,
first to compensate for the CTE mismatch - increases reliability, second is
to inhibit contamination from entering the matrix of electrically biased
solder bumps, thirdly, retard the oxidation of any cracking solder and
lastly to improve the solder joint life during mechanical shock testing
(drop tests).
As you have mentioned, reworkability is also an issue, but it all depends
on the application. eg, BGA/CSP packages in cellphones and other handhelds
are mostly underfilled for mechanical shock purposes and i would think
reworkability would not be an issue in this case.
Best regards,
Hann
"Stephen R. Gregory" <[log in to unmask]> on 01/24/2001 06:18:37 PM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to [log in to unmask]
To: [log in to unmask]
cc: (bcc: Hann Pang/US-Corporate/3M/US)
Subject: Re: [TN] BGA Underfill...
In a message dated 1/24/01 1:20:24 PM Central Standard Time,
[log in to unmask] writes:
<< As you know, underfill is rarely used for higher pitch BGA (1mm or
1.27mm). Unfortunately, I've lent my proceedings to someone, so I can't
give
you much detail.
Hope this helps.
Charles Elliott >>
Hi Charles!
That's what I'm learning, from everything that I've been reading from the
stuff I'm finding on the web (when talking about underfill), are mostly
dealing with flip-chip, or other CSP packages...
Yes, when dealing with CSP's and flip-chips, it has been shown that
underfilling increases the reliability (depending on what you read), two to
four fold...
So the next big question is; is it REALLY necessary to underfill standard
pitch BGA's? Maybe our customer has seen the data for flip-chips and CSP's
and feel that if it helps with these packages, it must also help with
standard pitch BGA's. Our customer is a defense supplier by the way...
What I have learned is that if you want to increase BGA reliability, use
PCB
materials that have low CTE's...that's where the real problem is, since BGA
solder joints aren't as compliant as gull-wing joints...
I have learned that our customer hasn't really spec'd out a material yet,
researching the rework-ability of different materials...I've learned that
is
a pretty big consideration, whether the material is thermo-setting, or
thermo-plastic...
I've got so much to learn!
-Steve Gregory-
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