Tom:
What you are saying about the diffusion rate of nickel being slower is correct read:
DISCUSSION
A review of the SEM EDX analysis reveals an interaction of the non-uniform microstructure and the solder joint crack. Figures 29-30 illustrate further evidence of degradation of the BGA solder joint integrity. Figure 30 shows a Sn/Ag intermetallic platelet intercepting a void region along the crack/pwb pad interface. The presence of the Sn/Ag intermetallic platelet intruding into the void space demonstrates the presence of the void during platelet growth or formation of the void upon solder joint solidification - physical evidence of nonwetting/dewetting behavior of the nickel surface during solder joint formation. Soldering nickel surfaces is more complex than soldering copper surfaces due to nickel oxide/flux reactions and the slow diffusion rates of nickel into solder [8]. Secondly, the SEM EDX measurements show that the solder joint microstructure was not uniform. Several industry studies [2, 9, 10, 11] have documented that the formation of a 177ºC ternary eutectic alloy (62.5Sn/36.1Pb/1.4Ag) is possible in a mixed surface finish manufacturing scenario. This investigation results show the 177ºC ternary eutectic alloy formed in Zones C/D in the interdendritic regions. The combination of a nickel surface and the non-uniform solder joint microstructure resulted in solder joint degradation leading to solder joint failure. Other industry studies [2, 5, 6, 7, 12] have reported that allowing the formation of a uniform solder joint microstructure with mixed surface finish manufacturing assemblies results in a Pbfree solder joint integrity equal to or better than Sn/Pb solder joint integrity. The non-homogeneous microstructure seen in this study is the result of a Pbfree solderball subjected to a eutectic solder reflow temperatures and lead to solder joint failure/
Full Article:
http://www.aciusa.org/leadfree/leadfree_impact_of_reflowing_a_pbfree_solder_alloy_using_a_tin-lead_solder_alloy_reflow_profile_on_solder_joint%20_integrity.html
Slowing the chain down will give the same results.
Regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Friday, January 19, 2007 10:20 AM
To: [log in to unmask]
Subject: Re: [TN] Minimum solder reflow temperatures for ENIG board finish
Werner Engelmaier published an excellent article "Soldering:Quality and Reliability Issues" I believe in Global SMT magazine. I have a paper copy but it does not show the date it was written. Werner, perhaps you could help with this?
The dissolution rates of various alloys into tin are detailed in Klein-Wassink's Soldering in Electronics.
You may wish to perform a search in the forum using the word "dissolution", and you may find a number of postings on the subject.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gervascio, Thomas
Sent: Friday, January 19, 2007 8:05 AM
To: [log in to unmask]
Subject: [TN] Minimum solder reflow temperatures for ENIG board finish
One rule of thumb that I used over the years was when soldering eutectic solder (Sn 63 Pb 37) on ENIG board finishes to run the profile a bit hotter than HASL. Instead of say 205 C I remember literature citing 217 C as the minimium temperature with the rationale being that nickel diffusion rates into solder are much slower than copper.
I did several internet searches for some supporting literature and couldn't readily find any. Any ideas where I could find some supporting evidence and/or literature?
Thanks
Tom Gervascio
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