If you follow the underfill manufacturer's process instructions, which are identical on the three flux/underfills I use from Kester, Alpha, and Zymet (none are used here at GD), and bring the soak temp to 150 deg. C for a full two minutes before going up to reflow, the flux underfill (and the regular underfill epoxies also) will break loose and allow the CSP or BGA to be removed without any damage to the soldermask or substrate.  I do this: http://zymet.com/p-demoreworkable.php, except I use the Detail Master I discussed in my posting earlier to remove the fillets on the periphery, then we use the same tool to remove the balance after the part is removed. I have been able to do this many times, even for small flipchips, without any damage whatsoever.
The key is to make sure it is at 150 C for at least two minutes. Three might be better for some epoxies.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Wettermann
Sent: Wednesday, June 19, 2013 6:32 AM
To: [log in to unmask]
Subject: [TN] Calling all chemists-the dreaded underfill

Dear TechNetters:

Most of the epoxy underfills on all of our handheld devices are not practically reworkable, even if you believe the marketing hype on reworkable underfills. The adhesive force of the underfill either damages the pads upon device removal, the mask is damaged during the "scrape off"
process. the chemical antidotes sold by the same underfill vendors either damages nearby parts or is highly caustic to employees.

Has anyone approached this rework challenge differently or could conjure up a unique way to suggest trying?

Thanks!

Bob W


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