ST AN2035 APPLICATION NOTE

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 “spon taneously” f rom its surface. The metals concerned include
Zinc, Cadmium, Silver , Tin and some of their allo ys. Because of the ir likeness to microscop ic
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 Tin­plated 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 w as f ound, that consist ed in adding small amounts of Lead (Pb) – as
low as 3% – to the plating. In so doing, the growth of whiskers was effectively prevented.

AN2035

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 f actors, alon g with the introduction of modern plating chemistries
and processes, allow the electronics industry to pursue this return to pure Tin-plating
surface finishes. Sin ce whisk er g ro wth is mainly caused by internal compressive stresses, a
number of strategies have been developed to prevent stress development within the Tin­plated film. Internal stress in Tin-plated films may o riginate from a number of cause s, among
which are:

a) co-deposited impurities, e. g. organics
b) atomic defects, such as t hose caused by improper plating parameters
c) creation of new phases leading to local v olume changes . These ma y 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

www.st.com

Contents AN2035 - Application note

Contents

1 Nature of whiskers and whisker mitigation techniques . . . . . . . . . . . . 1

2 Whisker assessment and process qualification . . . . . . . . . . . . . . . . . . 9

3 Whiskers and thermal cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2/11

AN2035 - Application note List of figures

List of figures

Figure 1. Natural growth of InterMetallic Compound (IMC) at room temperature . . . . . . . . . . . . . . . . 4

Figure 2. Microscope View of Protection by Post-bake Treatment (1 hour at 150°C) . . . . . . . . . . . . . 5

Figure 3. Protection by post-bake treatment (1 hour at 150°C) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 4. Protection by thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 5. Temperature Cycling SnPb on FeNi42 (250 Cycles of –35 to 125°C) . . . . . . . . . . . . . . . . . 9

Figure 6. Temperature Cycling Sn 100% (500 Cycles of –35 to 125°C) . . . . . . . . . . . . . . . . . . . . . . . 9

3/11

AN2035 - Application note

Experience has shown that proper plating practices and chemistries are important in
preventing whisker formation. Early Tin-plating chemistries were designed to produce a
cosmetically appealing shiny surface. This type of plating is known as “bright Tin plating”.
The shiny appearance of the tin-plate d surface is achieved by adding specialized chemicals
to the plating bath, that control the siz e of the gr ains (“gra in refiners”) and t he planarity of the
plated surface (“levelers”). Small grains and fl at surfaces help reflect the light, thus favoring
shiny surfaces. Due to the very nature of the chemistries and the high concentrations of
additives required to achieve bright finishes, these early bright Tin-plating chemistries were
prone to problems of organics co-deposition and atomic irregularities within the plated film,
leading to a higher susceptibility to whisker formation.

Modern chemistries and plating techniques have evolved with a view of preventing earlier
problems of contaminant co-deposition and atomic defect creation within the deposited film.
One major change in some Tin-plating chemistries is the use of much lower levels of grain
refining additives. The result is a duller (or matte) appearance of the Tin plating. For this
reason these chemistries are referred to as Matte Tin.

In a joint effort, Infineon, Philips, Freescale and ST Microelectronics (the so-called E4
Initiative) have tested a large number of modern plating chemistries for their resistance to
whisker growth. From this study a number of suitable commercial Matte Tin-plating
chemistries have been identified.

As mentioned previously, localized phase changes within the Tin film can also cause
localized compressive internal stresses. This happens when a volume increase is
associated with the phase change.

Since Tin and Copper normally form an intermetallic, Cu6Sn5, in a reaction which produces
a significant increase in volume, it is essential to take this into consideration for Tin-plated
copper leadframes.

When the Cu6Sn5 intermetallic forms at low temperatures (e.g. room temperature) the
reaction tends to take place more sp ecially along the grai n boundaries where the diffusion of
the combining elements is highest at lower temperatures due to solid-state diffusional
kinetics. The net result of the combined penetration and expansion of this growing
intermetallic may be envisioned as a “wedge” driven into the Tin layer at the grain boundary.
The penetration and growth of Cu6Sn5 intermetallics along grain boundaries is shown in

Figure 1 (schematic and photograph).

Figure 1. Natural growth of InterMetallic Compound (IMC) at room temperature

Tin Whisker

Cu6Sn

5

However, if the Cu

intermetallic is formed under higher temperature conditions (e.g.

6Sn5

around 150°C), a different and more desirable intermetallic structure forms. At higher
temperatures bulk diffusion is activated and the intermetallic reaction occurs more uniformly
across the entire Tin/Copper interface, not just at the grain boundaries. Since the reaction

4/11