http://www.tycoelectronics.com/environment/leadfree/pdf/tyco_lf_customer
_packet_whiskers.pdf
Dr Higgins:
The article in the above web address shows
a reflowed tin surface, please comment.
Regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leo Higgins
Sent: Thursday, April 20, 2006 12:03 PM
To: [log in to unmask]
Subject: Re: [TN] Testing to determine matte tin finish vice bright tin
fi nish and for annealing after plating
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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