Conduction reflow would help.  In theory, you have few degree difference
between the fully formed solder joint (old joint) and fine solder sphere
paste for reflow.  The paste would reflow a bit lower temperature.  As
long as all the surface tension still hold for the old joint, you should
be OK, unless the components is in the "cavity down" and weight a bit
too much = can't hold by surface tension..... ;-(.

jk

>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Rick Thompson
>Sent: Thursday, April 24, 2003 12:46 PM
>To: [log in to unmask]
>Subject: [TN] 'Unique' BGA Reflow Problem
>
>
>We have a customer who had a custom BGA built with some
>additional components besides the die under the encapsulant.
>Unfortunately, these parts were soldered with Sn63Pb37 and the
>solder reflows and bridges under the encapsulant during
>reflow.  Due to time constraints they would like to find a way
>to use these while they wait for new parts to be fabbed.
>We've tried various types of shielding, but the convection
>systems we have heat the entire part causing the internal
>reflow.  Does anyone know of a focused reflow system that
>would allow reflowing the eutectic balls of a BGA but not
>apply enough heat to the top of the component to reflow the
>other parts?  Would one of the focused IR systems (Ersa, PDR)
>do this?  Other possibilities?
>
>Thanks in advance.
>
>Rick Thompson
>
>Sr. SMT Process Engineer
>SMTEK International, Inc.
>+1 (805) 532-2800
>[log in to unmask]
>
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