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)
b)
c)
d)
co-deposited impurities, e.g. organics
atomic defects, such as those caused by improper plating parameters
creation of new phases leading to local volume changes. These may be caused by
either metallurgical or chemical reactions.
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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Contents
AN2035 - Application note
Contents
1
2
3
4
Nature of whiskers and whisker mitigation techniques . . . . . . . . . . . . 1
Whisker assessment and process qualification . . . . . . . . . . . . . . . . . . 9
Whiskers and thermal cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
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AN2035 - Application note
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Natural growth of InterMetallic Compound (IMC) at room temperature . . . . . . . . . . . . . . . . 4
Microscope View of Protection by Post-bake Treatment (1 hour at 150°C) . . . . . . . . . . . . . 5
Protection by post-bake treatment (1 hour at 150°C) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Protection by thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Temperature Cycling SnPb on FeNi42 (250 Cycles of –35 to 125°C) . . . . . . . . . . . . . . . . . 9
Temperature Cycling Sn 100% (500 Cycles of –35 to 125°C) . . . . . . . . . . . . . . . . . . . . . . . 9
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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-plated surface is achieved by adding specialized chemicals
to the plating bath, that control the size of the grains (“grain refiners”) and the planarity of the
plated surface (“levelers”). Small grains and flat 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 specially along the grain 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
Cu
6
Sn
5
However, if the Cu
6
Sn
5
intermetallic is formed under higher temperature conditions (e.g.
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
AN2035 - Application note
rate is virtually uniform across the Tin/Copper interface the resulting Cu6Sn5 structure has
practically no “wedges”.
The difference in the Cu6Sn5 structure is clearly demonstrated in the micrographs in
Figure 2.
In these micrographs the Tin was selectively removed by chemical etching, thus
exposing the Cu
6
Sn
5
intermetallic as well as any copper not covered by the intermetallic.
In the micrographs on the left hand side of
Figure 2,
the intermetallic was allowed to form at
room temperature over a period of one month. The result is a large, blocky and irregular
Cu
6
Sn
5
structure located almost exclusively at the grain boundaries.
Note:
In this image the large flat spaces between the blocky intermetallic are exposed copper.
By contrast the image on the right hand side of
Figure 2
shows a Cu
6
Sn
5
intermetallic
formed by baking the part within 24 hours of plating at a temperature of 150°C. In this case
the image shows a very uniform layer of Cu
6
Sn
5
with virtually no “wedges”.
Note:
All of the materials in this image are the Cu
6
Sn
5
intermetallic. There is no exposed Copper
after etching.
Figure 2.
Microscope View of Protection by Post-bake Treatment (1 hour at 150°C)
Large and Irregular IMC
(1 month at Room Temperature)
Thin and Uniform IMC
(1 hour at 150°C)
The ability of a 150°C bake to virtually eliminate the intermetallic “wedges” that lead to large
compressive stress at the grain boundaries serves as the basis for the second component of
the whisker mitigation strategy adopted by STMicroelectronics.
The second component of the strategy to mitigate whisker growth is to apply a 1-hour 150°C
bake within 24hrs to freshly plated Matte-Tin finishes. This has several beneficial effects: the
film is annealed and stress due to atomic-level plating defects is reduced. In addition, and
more importantly, a stable and uniform Cu
6
Sn
5
layer is created that protects against further
localized penetration of the intermetallic at grain boundaries, thus avoiding the “wedge”
effect that creates compressive stress. Furthermore, the bake creates an additional
beneficial layer of Cu
3
Sn, underneath the Cu
6
Sn
5
layer.
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