Hi Dave,

I have gone over this many times already and raised the temperature as much 
as I possibly can (right at the max peak for the chip in question).

If there is a test plan laid out already for testing compatibility of paste 
and underfill, it would be very helpful at this stage.

The underfill supplier uses only DSC to determine the incompatibility but 
it seems like there should be more ways to examine this property.

I shudder to think of all the real life flux residue underfill combinations 
there can be when a Contract Manufacturer is used to assemble product.

Solderpaste,  paste only rework flux, solder wire with flux core, liquid 
rework flux.

A nightmare would be to implement a scenario where the incompatibility 
under the BGA takes a year in the field to cause a failure.

Phil


At 06:54 AM 12/23/2008 -0600, [log in to unmask] wrote:

>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]
>cc
>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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