Hi Rick,
This was a good move, because what the underfill did was not only let the solder balls not collapse, but stretched them to some degree. That increase in SJ height is what gave you the increased reliability. I, and others, have been pushing for this technique for years.
BTW, without the underfill, your SJ failures were not stress-initiated, but still creep-fatigue; because of the shorter SJs, the same thermal expansion mismatch caused higher strain levels at those SJs.
Werner
-----Original Message-----
From: Rick Bell <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>; [log in to unmask] <[log in to unmask]>
Sent: Wed, 18 Mar 2009 5:31 pm
Subject: RE: [TN] Underfill Application
Werner,
We placed the adhesive on four corners
and some cleared depopulated areas inside the ball array prior to smt device
placement, and cured it during the reflow process. Essentially, we didn’t
use it as an underfill, but more as an SMA. Of your two choices, I guess
that would be “before soldering”.
Our initial temp cycle failures with no
underfill were typical stress-initiated fractures on corner pins. After
adding the adhesive in this fashion, the parts passed 1k cycles. We didn’t
evaluate failure mechanisms past that.
rb
From:
[log in to unmask] [mailto:[log in to unmask]]
Sent: Wedn
esday, March 18, 2009
4:19 PM
To: [log in to unmask]; Rick Bell
Subject: Re: [TN] Underfill
Application
Hi Amol,
When you say: "we elected to just do a corner bond approach"--what exactly
do you mean? after or before soldering?
If after soldering, that may have helped the corner SJs, but how did it help all the others?
Werner
-----Original
Message-----
From: Rick Bell <[log in to unmask]>
To: [log in to unmask]
Sent: Wed, 18 Mar 2009 4:50 pm
Subject: Re: [TN] Underfill Application
Amol,
I have some admittedly limited experience with underfilling CSP's for increased
reliability. Some answers to your questions:
1.) Our material selection initially was governed by a host of parameters--
reworkability (see next point), minimum dispense gap, process temperatures, cte
matching, filler content, etc. In my experience, trying to get a reworkable
version will greatly limit your choices.
2.) We used a Loctite material, I believe it was 3549. Please be aware that the
term "reworkable" is relative. We tried using it on polyimide flex substrates,
and found its reworkability was a matter of interpretation. Your mileage may
vary...
3.) We used an Asymtek dispense jet p
latform with very good results as far as
material deposition. Our only inspection capability was outsourced CSAM..
4.) Pitfalls-- Our biggest issues were some minor problems with adhesion to the
substrate/device, and incomplete encapsulation of a given solder joint due to
poor capillary fill on our device (this has highly detrimental effects in temp
cycle). My only advice there is that if you're going to attempt a full
encapsulation, have CSAM inspection available to verify your coverage, or some
very tight process controls once you have a benchmarked process.
In the end, we elected to just do a corner bond approach, rather than trying to
fully encapsulate the BGA array, and we had reasonable success in reliability
improvement.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kane, Amol (349)
Sent: Wednesday, March 18, 2009 2:48 PM
To: [log in to unmask]
Subject: [TN] Underfill Application
Dear Technetters,
We are about the develop a process for underfilling BGAs for a high reliability
application. the underfill will be a rework able underfill. I wanted to poll the
technet on:
1. Guidance on mater
ial selection
2. most common materials being used out there and the process equipment
being used to do so.
3. Common pitfalls and things to lookout for
4. inspection techniques used to inspect the finished product
5. a good source for info on underfilling (books, papers etc.,)
Thanks in advance.
Regards,
Amol
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