Hi Steve,
To answer your question:
> is it REALLY necessary to underfill standard pitch BGA's?
The two main reasons for underfilling area array packages (mostly flip-chip
and some CSP):
1) to extend solder joint (fatigue) reliability when you do not expect the
solder joints will meet your reliability requirements for your end-use
environment (due to CTE mismatch and harshness of environment)
2) to help withstand mechanical shock (especially for some types of CSP)
I have worked mostly in Telecom environments where the thermal cycling is
quite benign. I have not found the need to underfill yet for 1.27mm pitch
devices - but beware of large CBGA (CTE~7ppm/'C).
So, the key questions that you have to ask yourself are:
- what is the expected solder joint (fatigue) reliability for your BGA/board
combination for your end-use environment ?
- do you expect mechanical shock to occur in service ?
Oh, and if you plan to put the product in a fighter jet or space shuttle,
vibration might be a concern ;->
Charles Elliott.
-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Wednesday, January 24, 2001 7:19 PM
To: [log in to unmask]; [log in to unmask]
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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