Hi Steve,
I worked on a similar problem in the '94-95 timeframe while working for a
previous employer. We were discovering many solder joint failures in 1.5mm
pitch CBGA devices during shock and vibration environments, as well as a
few failures in larger PBGA devices. At the time, there were few people
working with BGAs, much less with underfilling them, so we wound up using
"traditional" flip-chip underfills (in particular, we used a Dexter-Hysol
formulation, which was excellent for its intended application). We
developed a process for using it under BGAs, and found that it solved the
environmental problems. However, it was unfriendly to our board assembly
and test process: it required high preheat and curing temperatures, and it
was unreworkable. This was a significant problem, since our assembled
boards were quite expensive.
We looked around for alternative materials and found a reworkable
formulation made by Mereco in Rhode Island which had been qualified for
mechanical constraint of QFPs in military applications by a large OEM. We
tried to use it for BGAs and found that it too solved the environmental
problems, and was much more compatible with our process (including being
reworkable). You could also tailor the viscosity of the formulation which
helped us work with BGAs with differing standoffs. I left the company and
learned that they discovered problems using this material with large PBGAs,
but this problem with apparently solved by changing the dispensing method.
I've since learned that other suppliers are now offering these reworkable
underfills (I think that Loctite and Emerson & Cuming are two names that
I've heard), but I have no knowledge of specifics.
Best of luck!
Greg Bartlett
Director of Engineering/R&D
Teledyne Electronic Technologies
Hudson, NH
603-889-6191 x239
"Stephen R. Gregory" <[log in to unmask]>@IPC.ORG> on 01/24/2001 07:18:37 PM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to [log in to unmask]
Sent by: TechNet <[log in to unmask]>
To: [log in to unmask]
cc:
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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