Your mail implies otherwise good wetting/satisfactory appearance.
A grainy or rough appearance can be a feature of prolonged time at temp or
slow cool down, especially with Sn62, and or with certain finishes which can
also help promote a coarser grain structure in the joint. [The coarse
structure is visible at the surface as roughness.]

Regards

Mike Fenner

Applications Engineer, European Operations
Indium Corporation
 T: + 44 1908 580 400
M: + 44 7810 526 317
 F: + 44 1908 580 411
 E: [log in to unmask]
W: www.indium.com
Leadfree: www.Pb-Free.com



-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Macko, Joe @ IEC
Sent: Saturday, December 28, 2002 4:44 PM
To: [log in to unmask]
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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