Hi Ed,
Underfill makes solder attachments sufficiently reliable in cases where without it solder joint failures would occur prematurely for the application in question. As work by Motorola [see my reliability column Engelmaier, W., “Reliability Improvement with Underfill,” Global SMT & Packaging, Vol. 6, No. 3, March 2006, pp. 66-67] has shown, not all underfills are of equal benefit and that increases of up to 6x are achievable in praxis.
If the reliability without the underfill is sufficient—because of small component size, high solder joint stand-off, built-in component compliancy [Tessera patents], small CTE-mismatch, small delta-T, then there is no need for underfills.
Werner
-----Original Message-----
From: Edward Rios <[log in to unmask]>
To: [log in to unmask]
Sent: Wed, 3 Jun 2009 5:45 pm
Subject: [TN] When not to underfill
Hi TechNet,
OK I'll admit it, I'm fairly new to the field of solder joint
reliability. ;) Therefore, the recent thread about underfill inspection
criteria prompted me to do additional reading on the subject. It seems
like there is consensus on the positive effect of underfilling on BGA
solder joint reliability. On the other hand, are there cases where
underfill doesn't have a significant impact? Maybe in smaller BGA
components?
Thanks,
Ed Rios
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