April 2006

Rev. 2

1/11

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).

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