That was supposed to be "to think". Too bad Spell Check doesn't check that sort of thing.
Bev
----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Mon Dec 22 20:12:30 2008
Subject: Re: [TN] Underfill, inhibition due to leftover activators in the flux residue
Phil,
I have had more time outthink about this now. A good chili on a winter
night helps. :)
This could be really bad news. I have not heard of this before. The good
news is that the joints most at risk during thermal cycling and drop are the
ones the farthest from the neutral point, which is where (neutral point) you
are finding the phenomenon. Even so, I am concerned. Now I am wondering
how many other combinations are doing this?
Bev
RIM
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phillip Bavaro
Sent: Monday, December 22, 2008 6:02 PM
To: [log in to unmask]
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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