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March 2010

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Subject:
From:
Bob Landman <[log in to unmask]>
Reply To:
Bob Landman <[log in to unmask]>
Date:
Thu, 25 Mar 2010 13:13:39 -0400
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Also problems with IMC according to this paper

http://www.springerlink.com/content/671k1j765838u166/

Anupam Choubey1, Hao Yu1, Michael Osterman1, Michael Pecht1 Contact Information, Fu Yun2, Li Yonghong2 and Xu Ming2
(1)  	Center for Advanced life Cycle Engineering (CALCE), University of Maryland, College Park, MD, USA
(2)  	Aero Combined Environment Laboratory (ACEL), P.O. Box 1673, Beijing, China

Received: 17 July 2007  Accepted: 2 April 2008  Published online: 28 May 2008

Abstract  Solder interconnect reliability is influenced by environmentally imposed loads, solder material properties, and the intermetallics formed within the solder and the metal surfaces to which the solder is bonded. Several lead-free metallurgies are being used for component terminal plating, board pad plating, and solder materials. These metallurgies react together and form intermetallic compounds (IMCs) that affect the metallurgical bond strength and the reliability of solder joint connections. This study evaluates the composition and extent of intermetallic growth in solder joints of ball grid array components for several printed circuit board pad finishes and solder materials. Intermetallic growth during solid state aging at 100°C and 125°C up to 1000 h for two solder alloys, Sn-3.5Ag and Sn-3.0Ag-0.5Cu, was investigated. For Sn-3.5Ag solder, the electroless nickel immersion gold (ENIG) pad finish was found to result in the lowest IMC thickness compared to immersion tin (ImSn), immersion silver (ImAg), and organic solderability preservative (OSP). Due to the brittle nature of the IMC, a lower IMC thickness is generally preferred for optimal solder joint reliability. A lower IMC thickness may make ENIG a desirable finish for long-life applications. Activation energies of IMC growth in solid-state aging were found to be 0.54 ± 0.1 eV for ENIG, 0.91 ± 0.12 eV for ImSn, and 1.03 ± 0.1 eV for ImAg. Cu3Sn and Cu6Sn5 IMCs were found between the solder and the copper pad on boards with the ImSn and ImAg pad finishes. Ternary (Cu,Ni)6Sn5 intermetallics were found for the ENIG pad finish on the board side. On the component side, a ternary IMC layer composed of Ni-Cu-Sn was found. Along with intermetallics, microvoids were observed at the interface between the copper pad and solder, which presents some concern if devices are subject to shock and vibration loading.  

- I would never trust a tin plating.  To quote Lyudmyla "sometimes whiskers"  

- Why look for trouble? Why not use ENIG?

Bob Landman
H&L Instruments, LLC

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Kelly
Sent: Wednesday, March 24, 2010 1:31 PM
To: [log in to unmask]
Subject: [TN] Tin Whiskers

Hi All,

Two questions today:

1)      Any known instances of tin whiskering when the finish is Immersion Tin

2)      Any known instances of tin whiskering after the immersion tin is soldered with a RoHS compliant solder such as SAC 305.

Thanks. Steve Kelly













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Steve Kelly

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