HI, Joe,

No first hand experience, apart from re-using thermal profile boards a
number of times. These boards have BGA's on one side only, but do go
through a number of cycles.

The graininess probably means incomplete reflow the second (and/or first
time). What is the BGA's peak temperature and duration in the solder oven?
If you're using 63/37 solder, you should be aiming for a peak component
temp of 208 deg C for maybe 8 seconds. You might also try applying spray
flux to the already-soldered BGA joints prior to the second reflow.

Peter



"Macko, Joe @ IEC" <Joe.Macko 29/12/2002 12:43 AM
Sent by: TechNet <[log in to unmask]>

Please respond to "TechNet E-Mail Forum."; Please respond to "Macko, Joe @
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              cc:  (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
              Subject: [TN] 2nd side reflow problems








TechNets,

Many of our boards are densely populated, double sided, multilayered with
BGAs on both sides.   Side 1 is populated and reflowed, then side 2 is
populated and reflowed.  Side 1 therefore sees 2 reflows which appears to
make the surface of the solder balls on the BGAs reflowed twice appear
grainier after the 2nd reflow.   A water soluble flux based Sn 62Pb36Ag02
solder paste is used.  We have also tried using Sn63Pb37 alloy.

Has anyone else experienced similar problems with densely populated, double
sided boards where one side sees 2 reflow cycles?  Is there a practical
solution for maintaining BGA solder ball appearance thru the 2nd reflow?
Reflowing in a nitrogen environment is not an option.

Happy Holidays
joe

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