Hi Amol - yes but the easiest way is to destructively mechanically remove 
the BGA and look at the "flux halo" to assess the level of spread.

Dave



From:   Amol Kane <[log in to unmask]>
To:     <[log in to unmask]>
Date:   11/13/2012 10:13 AM
Subject:        Re: [TN] Reflux and reflow BGA rework
Sent by:        TechNet <[log in to unmask]>



To add to this, most rework persons add more flux as a means to ensure the 
flux spreads out and wets the entire BGA during reflow. I have seen that 
this is a strategy not without pitfalls as you end up with pools of flux 
that still do not spread, especially for larger BGAs and you end up with 
conditions not unlike what was described in the original thread. That is 
why I do not like to perform a "reflow only" operation. 

As an aside, Dave, is there a method to determine how much flux has spread 
underneath the component during a reflow only operation?

Thanks,
Amol
    

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Tuesday, November 13, 2012 6:22 AM
To: [log in to unmask]
Subject: Re: [TN] Reflux and reflow BGA rework

Hi Julie - I don't know of any standards that cover this topic unless 
there is something in the IPC-7711 specification. When we use our BGA 
repair procedure, we use specific fluxes that are easily removed and we 
are very conscious about the volume of flux used. "Bigger the glob, better 
the job" is not a applicable process philosophy when utilizing fluxes for 
BGA repair. Our focus for the process is to insure that we are using flux 
in such a way that we don't cause a reliability issue that was not there 
prior to our BGA repair actions. The example we discussed of the flux 
packed under the BGA in the previous Technet thread is a classic case of 
not understanding how much flux was necessary for successful BGA repair.

Dave Hillman
Rockwell Collins
[log in to unmask]



From:   Julie Silk <[log in to unmask]>
To:     <[log in to unmask]>
Date:   11/13/2012 07:51 AM
Subject:        [TN] Reflux and reflow BGA rework
Sent by:        TechNet <[log in to unmask]>



A discussion of the practice of re-flux and reflow of BGAs to rework them 
has emerged within the "limits of flux residue" discussion.  I'm taking it 
out into a separate thread.  This rework process injects flux under the 
BGA, then reflows the BGA.  The part is not removed.  It will frequently 
make a part that wasn't working work again.  The heat damage to the board 
is less (fewer cycles) than a replacement process.  The question Joyce 
asked about whether there are standards and reliability studies has not 
been answered.  What is the proper procedure for this rework process?  Are 
there official standards?  Studies of effectiveness / reliability?




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