Rex,

I would check the BGA ball composition as your problem may come from a 
head-in-pillow effect.
Low-Ag SAC BGAs are becoming more and more common and BGA ball 
composition is often not well specified (SAC is now a wide family...).
Low-Ag SAC ball composition may be the reason to call for higher reflow 
temperatures.
I have had the information (but not checked) that ATMEL AT91SAM9G20-CU 
(which looks close to your reference and has the same LFBGA217 package) 
has SAC105 balls.

Rex Waygood a écrit :
> We have built a batch of pcbs for a customer who supplied the pcbs. They
> are 2mm thick 6 layer with 0.2mm vias and ENIG finish. None work fully,
> they work intermittently at best. 
>
> The customer was suspicious that we had not reflowed the BGAs correctly.
>
> Our under BGA temp on a similar pcb is 245C (We now have some samples to
> profile correctly)
>
> The customer is suspicious due to the comment on page 753
> http://www.atmel.com/dyn/resources/prod_documents/doc6221.pdf
> <http://www.atmel.com/dyn/resources/prod_documents/doc6221.pdf>  where
> it states a min of 250C
>
> Close x-ray evaluation of the BGAs showed tear dropping along tracks,
> uniform balls, everything looked tickety boo.
>
> We offered to remove a BGA to see what was happening underneath.
>
> 2 corners showed a similar issue with the ball where it separated from
> the BGA rather than the pcb.
>
> The removal technique was to use a PDR IR rework station, under board
> heating to bring topside pcb temp to 170C and then a lead free reflow
> profile to bring the device up to reflow, manual removal.
>
> I didn't have dye for dye and pry so thought this might show if there
> was an ENIG problem.
>
> I was surprised by the almost complete removal of ball at the device and
> lack of evidence of solder.
>
> Was this just caused by my investigation processes?
>
> Was the ball device interface broken which prevented it from melting the
> ball?
>
> Any other clues?
>
> Thanks
>
> Rex
>
>  
>
> Hopefully Steve has received the images (sorry about the size!)
>
>  
> Rex Waygood
> Technical Manager
>
> Hansatech EMS provides value manufacturing through engineering and
> quality
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