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December 2008

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Subject:
From:
"David D. Hillman" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Tue, 23 Dec 2008 06:54:57 -0600
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Hi Phil! Sorry to hear such bad news. I recommend you double check to make 
sure that you are indeed hitting the required minimum cure 
times/temperatures necessary to complete a full cure reaction. Some of the 
underfill formulation providers do have databases which include which 
solderpastes their underfill materials have compatibility issues. The 
cleaning chemistry suppliers and the fabricators of cleaning equipment do 
a great job of pulling together cleaning processes that do amazing jobs of 
removing residues given the size and complexity of our modern electronic 
products. But - if you have a BGA that is sufficiently large and has 
significant I/O, you are going to have some flux residue remaining under 
the component. Your underfill material must be able to accommodate that 
residue. Many folks don't realize that they should conduct solder paste - 
underfill compatibility studies. Good Luck.

Happy Holidays!
Dave Hillman
Rockwell Collins
[log in to unmask]




Phillip Bavaro <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
12/22/2008 05:02 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Phillip Bavaro <[log in to unmask]>


To
[log in to unmask]
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Subject
[TN] Underfill, inhibition due to leftover activators in the flux residue






I have made quite a few test samples and discovered that a certain 
underfill material will exhibit inhibited cure when it comes in contact 
with solderpaste flux residues at the center of a very dense BGA 
component.
The rest of the underfill beneath the BGA will harden like normal.
We don't clean nor plan on starting to clean our assemblies.
DVT testing does not show any defects due to underfill adhesion.


Background:
We used a four up ABAB panel which goes through SMT twice.
BGAs only passing through the oven once consistently show the problem.
Two passes through the reflow oven (ABAB flip) and the problem goes away.
This means that  half of our test samples still have soft material in the 
pocket at the center where the power ground pins are located.
We measure peak temperature in this area and have found that raising the 
peak temperature helps quite a bit but does not eliminate the inhibition 
entirely.
The manufacturers of the paste and the underfill acknowledge the issue but 

do not have a solution.

Has anyone else run into similar compatibility issues with a paste and 
underfill?

In the event that we cannot work out the root cause, and decide to make a 
change, I am trying to come up with a list of all the manufacturers of 
underfill.

So far I have found 8 major underfill manufacturers but would appreciate 
knowing about more if there are any.

3M
Ablestik
Epotek
Henkel
Hysol
SP&S
Sunstar
Zymet

Thanks in advance,

Phil

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