[tinwhiskers] Re: Tin whiskers with Immersion tin
- From: "Bob Landman" <rlandman@xxxxxxxxxxxxxxxxx>
- To: tinwhiskers@xxxxxxxxxxxxx
- Date: Fri, 14 Mar 2008 22:19:04 -0400
According to Vicor (see below), it's a bad idea.
GEIA-STD-005-2 suggests it may be ok
http://www.empf.org/empfasis/2006/mar06/geia-tin_whiskers.html
We use ENIG as Vioor recommends and we are happy with the results, esp. on very
small QFN packages which require a very flat surface as the solder paste pads
are so tiny, esp the center heatsink ground pad.
-Bob Landman/H&L instruments,LLC
See http://www.electronicproducts.com/rohs/?filename=vicor-rohs.sep2006.html
Tin whiskers and RoHS: a perspective
Here are practical strategies to mitigate tin whiskers in RoHS-compliant
assemblies
BY PATRICK LAVERY
Vicor
Andover, MA
http://www.vicorpower.com
The tin whisker phenomenon is a failure mode associated with a number of
plated low-melting-point elements (tin, cadmium, and indium) used to promote
good soldering. Recognized many years ago, the problem had been minimized by
using plating finishes with lead, now identified as a hazardous substance and
banned under the RoHS initiative rules.
Fig. 1. SEM images: Left—Bright tin virtually featureless submicron grains;
center—matte tin "A" small grained 1 to 2 µm (less desirable); right—matte tin
"B" large grained, >6 µm (preferred).
Investigations by Vicor over the last three years have resulted in some
practical strategies to mitigate tin whiskering. Exactly how and why a tin
whisker grows is not completely understood, but because they are conductive
and can carry high current they can certainly cause serious problems.
Whisker growth from copper-containing substrates finished with standard pure
tin is of the greatest concern. Research has shown that whiskers will grow
from tin-lead surfaces, although the length of those whiskers is much shorter
than with pure tin.
Choice of plating materials
Precious-metal plating, such as gold and palladium, are not susceptible to
whiskering. Trials with pure tin finishes, which are much lower cost, have
revealed that large grained 100% matte tin plating on copper-lead frames
resists whiskering even when exposed to compressive forces and held at
elevated temperatures in humid environments.
Bright tin, however, had whisker growth reaching 700 µm even at ambient
temperatures. Extensive cyclic compression tests with select matte tin
chemistries, considered a worst-case evaluation, yielded whisker growth of
less than 20 µm.
Simple SEM examination confirmed that all matte tin plating may not be whisker
resistant because of the presence of small grains structures. Figure 1 shows
some SEM images of tin finishes observed at 4,000X.
Use of a nickel barrier
In power device products, tin plating is used on IC packages, passives,
internal connection pins, and external I/O pins. In the normal fabrication
processes, tin-finished leads and pins are regularly subjected to Pb-free
reflow during surface-mount operations, reducing residual plating stress and
leading to reduced risk of whiskering. Where possible, a minimum 2-µm nickel
barrier layer under tin plating is used, based on iNEMI (International
Electronics Manufacturing Initiative) and plating chemical supplier
recommendations.
A nickel barrier under tin helps prevent the formation of compressive stresses
caused by the growth of CuSn intermetallics and hence reduces the one driving
force for tin whisker growth. If a nickel barrier layer is not possible, tin
plating is annealed after plating for 1 hour at 150°C to promote controlled
intermetallic compound formation and reduced tin whisker risks.
Vicor has chosen to ban the use of immersion tin plating on pc boards,
primarily because the resulting thin layer, only 20 µin. thick, is susceptible
to whiskering at higher voltage levels. We have chosen, in most applications,
either silver or gold finishes, though they are considerably more expensive.
Of these finishes, ENIG has been shown to be the most robust for lead free
soldering. When hot air solder leveling (HASL) has been required,
tin-silver-copper alloy has replaced tin-Pb. Immersion tin plating may be
suitable for lower voltage applications, but we have found that the
chemistries are not as well documented.
Coatings can help
Conformal coatings or epoxies have been suggested as a way to reduce tin
whisker risks. A thick conformal coating (silicone-based polymer with 90% to
92% filled ceramic particles) ? used in power brick products as part of a
thermal management strategy—adds resistance to whisker growth.
This fill material encapsulates all components, wets leads, and fills the
space between package leads, components, and traces. Thinner coatings may also
reduce whiskering, but certainly not eliminate it.
_____
From: Tom Burek [mailto:Tom.Burek@xxxxxxxxxx]
To: tinwhiskers@xxxxxxxxxxxxx
Sent: Fri, 14 Mar 2008 17:56:30 -0400
Subject: [tinwhiskers] Tin whiskers with Immersion tin
Does anyone have any data concerning tin whiskers using Immersion Tin as a PWB
finish?
It is my understanding that whiskers are less likely with Immersion tin.
Tom Burek, CID+
tom.burek@xxxxxxxxxx
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