The IC packaging community is very aware of the greater whisker risk with
Bright tin, so I would hope the use of a bright tin lead finish would not
occur.
Bright tin lead finish characteristics: Relative to standard matte tin or
the newer satin finish matte tins, bright tin is much shinier, or
reflective. Matte tin is dull gray and even the satin finish tins are
really not shiny. The problem here is that this is a relative, visual
assessment, not quantitative.
Bright tins are characterized by having the finest microstructure with the
grains typically sub-micron (iNEMI characterizes the typical bright tin
grain size to be 0.5 - 0.8 micrometers). Even this can be problematic. The
older matte tin chemistries were characterized by a coarse grain structure,
with Lucent/Agere stressing the benefit of 7 - 10 micrometer grain sizes
(from memory...may be slightly off). More recent matte tins / satin tins
have finer grain sizes that may be in the range of about 1 - 5 micrometers.
Since the microstructure can be affected by the plating conditions and bath
chemistry maintenance, a microstructural evaluation under a microscope may
also not be convincing.
The high purity matte and satin tin finishes are very low in grain refiners,
with total carbon content typically in the 0.005 - 0.05% range (iNEMI
range). The high purity tin finish is supposed to be 99.9% Sn (min.), but
chemical analysis of tin finishes with common XRF equipment typically used
for finish thickness measurements really cannot guarantee this purity level
due to the measurement accuracy and repeatability (Check the Seiko
instruments web site where they provide some good app notes on this and show
GR&R results that demonstrate this problem.
Verification that the device was annealed at 150C for 1 hr within 24 hours
of plating is also not verifiable via visual inspection techniques. If
cross-sectioning of the leads shows a nice uniform IMC layer between the Sn
and the leadframe metal, it was likely to have been annealed, but the temp
and time of annealing, and the staging time from plating to annealing
cannot be determined.
What you need is good certification from the supplier.
Best regards,
Leo
Leo M. Higgins III, Ph.D.
Vice President, Technical Support Operations
ASAT, Inc.
3755 Capital of Texas Highway, Suite 100
Austin, Texas 78704
office phone 512-383-4593
mobile 512-423-2002
[log in to unmask]
www.asat.com
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of - Bogert
Sent: Wednesday, April 19, 2006 6:03 PM
To: [log in to unmask]
Subject: [TN] Testing to determine matte tin finish vice bright tin
finish and for annealing after plating
April 19, 2006
Many electronic part manufacturers because of RoHS/WEEE are changing their
part termination finish from a SnPb finish to a 100% matte tin finish.
However, it is not clear if all folks are using matte tin vice bright tin.
Bright tin is more prone to growing tin whiskers than matte tin is.
Does anyone know if there is a test method that one can use to test parts at
receiving inspection to determine if the tin finish is matte tin vice bright
tin? Would XRF testing determine this with proper calibration of the XRF
test instrument?
Also, is there a test method that could determine if a matte tin finished
part was annealed after the tin plating process. I am not aware of any such
method (other than the long term cycling test of JESD STD 101 for
determining the propensity for growing tin whiskers). Annealing part
terminations at 150C for 1 hour after matte tin plating
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is an industry recognized method for mitigating tin whisker growth.
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