The melting temp for pure Sn is 232C and Pb is 327C.  The
combination of the two @ 60/40  is about 183C. Something caused melt
separation into their pure states?. Mislabeled solder paste? It was
actually Sn100.
	
	2 cents 
	 



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: Friday, May 04, 2007 1:22 PM
To: [log in to unmask]
Subject: [TN] Metalurgists, need assistance

Have an issue with (randomly) lifted FPGA leads.
Pic 1: good joint after pull test
Pic 2: ditto but high magnification
Pic 3: "rotten" joint after pull test
Pic 4: ditto but high magnification
Pic 5: "rotten" cross section
Pic 6: ditto but high magnification

60Sn on ENIG, obviously a brittle region. Suspect Kirkendall voids, but
the pockets with zillions of Tin spheres (0.1 to 2 um in size) points at
insufficient peak temperature to make 100% melt and good solidifying. If
these little balls were Lead, I could understand, but they are Tin!
Never seen like. Any "quickhead" out there, need fast advice. 

Note: Pic 2, you can see the underlying board pad nickel as dark areas. 
An experienced analyst will hopefully recognize. No Phosphorous peaks!

My Friday headache.

Inge

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