AN2035 Datasheet, PDF (1/11 Pages) STMicroelectronics – Control of whisker growth in Tin alloy coatings

AN2035
Application note
Control of whisker growth in Tin alloy coatings
1
Nature of whiskers and whisker mitigation
techniques
Some metals show an unusual metallurgical phenomenon: a single, microscopic crystal
filament of the metal grows “spontaneously” from its surface. The metals concerned include
Zinc, Cadmium, Silver, Tin and some of their alloys. Because of their likeness to microscopic
hair, these tiny filaments are commonly referred to as “whiskers”.
Scientists believe that whisker growth is mainly due to internal compressive stresses near
the metal surface. Under certain conditions the internal stress can reach a critical level,
leading to the formation of whiskers as a way of reducing the system’s internal energy.
Owing to their excellent electrical properties and solderability, and their low cost, pure Tin-
plated surfaces have been used for many decades by the electronics industry. Hundreds of
billions (trillions by some estimates taking passives and discretes into account) of
components have been supplied with pure Tin-plated surface finishes. On top of their low
cost, these components operate well and are highly reliable. Only does the occasional
occurrence of reliability problems caused by Tin whiskers tarnish their reputation. An easy
fix to whisker problems was found, that consisted in adding small amounts of Lead (Pb) – as
low as 3% – to the plating. In so doing, the growth of whiskers was effectively prevented.
With the recent European Directive to eliminate Lead from electronic products, there is a
renewed interest in Tin and its alloys as a replacement for Lead-bearing alloys. A better
understanding of the factors which influence whisker formation and the application of new
techniques to control these factors, along with the introduction of modern plating chemistries
and processes, allow the electronics industry to pursue this return to pure Tin-plating
surface finishes. Since whisker growth is mainly caused by internal compressive stresses, a
number of strategies have been developed to prevent stress development within the Tin-
plated film. Internal stress in Tin-plated films may originate from a number of causes, among
which are:
a) co-deposited impurities, e.g. organics
b) atomic defects, such as those caused by improper plating parameters
c) creation of new phases leading to local volume changes. These may be caused by
either metallurgical or chemical reactions.
d) thermal stress caused by mismatches in the Coefficients of Thermal Expansion
(CTE) between the Tin film and the base metal (and/or additional films beneath
the Tin film).
April 2006
Rev. 2
1/11
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