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P r o c e s s e s a n d e q u i p m e n t f o r t h e P W B i n d u s t r y
OnBoard Technology September 2004 - page 30 www.Onboard-Technology.com
PCBFABRICATION
In order to maintain the solder-
ability of printed circuit boards
during storage, it is necessary to
protect the Copper surface mount
pads with a solderable surface fin-
ish. Immersion Tin is one of the
materials used as a PCB surface
finish. It is expected that the use
Immersion Tin as a surface finish
will grow substantially and become
a high volume technology, espe-
cially in the course of the global
switch to Lead-free processes fol-
lowing the Reduction of Hazardous
Substances (RoHS) and the Waste
Electrical & Electronic Equipment
(WEEE) directives. Moreover, the
further miniaturisation of struc-
tures and high volume production
of PCBs require advanced alterna-
tive surface finishes.
With the growing global demand
for Immersion Tin, the require-
ments placed on product and pro-
cess technology are increasing
significantly. Market leading OEMs
require surface finishes with in-
creased Tin thickness, improved
solderability at multiple cycles and
higher temperatures, enhanced
surface flatness, fine pitch compat-
ibility and – ever more important
– limited whisker growth. Immer-
sion Tin can meet these require-
ments and has proven its advan-
tages with market leading PCB
manufacturers.
When talking about immersion Tin
as a final surface finish for PCBs,
whisker growth is a topic of con-
cern. Ormecon International has
spent a significant amount of time
and resources investigating the
whisker formation phenomenon
and its prevention. Based on this
experience, the company has de-
veloped a whisker-reducing im-
mersion Tin product: Ormecon
CSN FF-W. All chemicals used are
Lead-free and halogen-free.
This product, as well as the already
established Ormecon CSN and the
recently introduced CSN FF, are
based on the Organic Metal Poly-
aniline, which is responsible for
a completely different deposition
mechanism and Tin layer struc-
ture. This is due to the fact that the
Organic Metal not only passivates
the Copper (and later the Tin), but
also exclusively forms Cu(I) cat-
ions and transfers electrons to the
Sn(II) ions for the reduction and
deposition, thus acting as a cata-
lyst.
Whisker formation phenomenon
and preventive actions
Whisker formation is a typical fea-
ture of metallic Tin. There can be
two kinds of whisker phenomena:
at the component itself and also at
the Tin surface finish. Various fac-
tors can cause whisker formation.
One key factor is internal stress.
There are two stress categories:
• compressive stress, which drives
the whisker growth, and
• tensile stress, which reduces the
propensity of whisker growth.
Internal stress mainly develops
during storage as a result of diffu-
sion processes at the Cu-Sn bound-
ary. With Tin plated over Copper,
compressive stress is built up, be-
cause Copper diffuses much faster
into Tin than Tin diffuses into Cop-
per. With continuous diffusion over
time, compressive stress increases
in the Tin layer. Most often the
presence of tin oxides prevents the
stress from being released. How-
ever the tin oxide layer is never
A Superior Whisker-Reducing
Immersion Tin Technology
by N. Arendt, F. Baron, V. Benz, M.Letterer, H. Merkle, S.Schroeder, B. Wessling,
Ormecon International
Figure 1 – Diagram illustrating
whisker formation process
Figures 2 and 3 – Ormecon Immersion Tin deposit reaction vs. standard immersion Tin reactions
P r o c e s s e s a n d e q u i p m e n t f o r t h e P W B i n d u s t r y
OnBoard Technology September 2004 - page 31
www.Onboard-Technology.com
PCBFABRICATION
perfect (defects are the norm) and
when the internal stress becomes
high enough, it will break through
the defect in the oxide layer and
a whisker can form to release the
stress (Figure 1).
The products currently available
on the market for whisker reduc-
tion are mainly based on reduced
Copper-into-Tin diffusion because
of the more compact nature of the
Tin layer. Also whisker reducing ef-
fects are produced by a Silver layer
on top of the Tin layer. Such prod-
ucts, however, do not satisfy some
of the more stringent “whisker
reduction” specifications. Some
manufacturers require no whiskers
with a length of more than 20µm
and even less.
The Silver-topped Tin layer is rela-
tively expensive, as it requires an
additional process step and a sig-
nificant amount of Silver. There-
fore, a new Immersion Tin product
and process technology for whisker
reduction is necessary to meet all
the requirements of the market
leading OEMs and PCB manufac-
turers.
The new reduced whisker Tin layer
As in Ormecon CSN and Ormecon
CSN FF, the pre-dip of the Orme-
con CSN FF-W product contains
the Organic Metal in aqueous
dispersion, which catalyses the
Tin deposition resulting in a very
dense Tin layer with a big grain
size morphology. Figures 2 and 3
show the difference between the
Ormecon Immersion Tin deposit
reaction compared to standard im-
mersion Tin reactions.
The key benefits of the Ormecon
deposit are:
• A large grain size Tin layer is
thermodynamically more stable
than Tin with a small grain size
and it is less susceptible to re-crys-
tallization. It is therefore more dif-
ficult to “squeeze” out a whisker
from a large grain size Tin.
• A large grain size Tin layer fur-
thermore reduces the speed of
Copper diffusion, and thus less
stress is produced compared to
a rapid diffusion of Copper into a
small grain size Tin.
• The big, flat Tin crystals of an Or-
mecon CSN FF / FF-W finish offer
significantly less room (pores) for
impurity inclusions compared to
a small grain size Tin deposit. The
small Tin grain size compared with
the Ormecon large Tin grain size
contains more pores (Figure 4).
However, as documented in Table
1, this is sometimes not sufficient.
Therefore, Ormecon has complete-
ly changed the layer structure by
adding an additional whisker in-
hibiting ingredient in the pre-dip
process step. This can be selected
from various metal ions. In the new
product Ormecon CSN FF-W, Sil-
ver is used. Thereby a metal alloy
nanolayer (10-20nm) containing
Ag and Cu is formed on the Copper
surface, again induced by the action
of the Organic Metal catalyst. The
subsequent Tin layer deposition is
then carried out in a way that a new
sandwich layer structure is set-up.
There is a precisely defined Tin
layer on top, followed by a broader
intermetallic layer. All layers show
a much smoother concentration
gradient than in other types of Tin
surfaces. Such a new sandwich lay-
er construction has proven to have
a superior characteristics in all
respects. Based on Secondary Ion
Mass Spectrometry (SIMS) (Figure
5), it was possible to prove that:
• Sn can be found at a much great-
er depth in the Cu layer (up to 3
µm or more) compared to standard
immersion Tin products, where
Sn can only be found up to about
1.5µm deep;
• Cu can be found directly at the
surface (in very low concentra-
tions) in contrast to standard
products, where Cu is only present
starting at a depth of about 0.8 to
1 µm;
• Ag is not present in the form of a
separate layer, not even below the
Tin layer (which was deposited af-
ter Ag deposition), but everywhere
in the Tin-Copper and more deeper
in the Copper-Tin alloy, with a
maximum concentration slightly
below the surface;
• the concentration of all three
metals gradually changes with in-
creasing depth.
The fact that Ag is not present in
form of a separate layer eliminates
the risk of Silver migration over
time.
Figure 4 – Diagram of pores of small grain and large grain Tin finishes
Figure 5 – SIMS results showing diffusion of Tin, Silver and Copper in
various sandwich structures
P r o c e s s e s a n d e q u i p m e n t f o r t h e P W B i n d u s t r y
OnBoard Technology September 2004 - page 32 www.Onboard-Technology.com
PCBFABRICATION
Figures 6 shows the difference in
the layer structure resulting from
the use of standard immersion Tin
products and the new whisker pre-
venting sandwich layer structure.
The concentration gradient of both
Cu and Sn is much less steep com-
pared to the previous layer struc-
tures. Diffusion is dramatically
reduced because of the fact that
the concentration of Cu in Sn in
the surface regions and of Sn in Cu
in the deeper regions is relatively
high to begin with. This fact, to-
gether with the ennobling effect of
Ag and the Organic Metal, is also
the reason for the extremely good
solderability and ageing behaviour
of the Immersion Tin layer.
This new sandwich structure ef-
fectively reduces and even pre-
vents whisker formation, as shown
in Table 1, because less stress is
induced due to the much lower
diffusion of both Cu and Sn. This
innovative process fulfils the re-
quirements of several currently
implemented OEM whisker reduc-
tion specifications. The product
was introduced in 2004 with some
high volume PCB manufacturers.
The PCB manufacturers choosing
to apply Ormecon Immersion Tin
technologies can easily change be-
tween the standard immersion Tin
product Ormecon CSN FF and the
high-performance, whisker reduc-
ing immersion Tin product Orme-
con CSN FF - W. The difference
between such two products is only
the pre-dip step.
Solderability
Due to the deposition process (us-
ing noble Organic Metal and noble
Silver), the resulting Silver-con-
taining sandwich composition and
the compact and big crystal grain
size structure, the ageing behaviour
of this surface finish has proven to
be superior. The diffusion rate of
Cu into Sn at
155 °C is even
lower (less
than 3.3 nm/
s) than in Or-
mecon CSN
and Orme-
con CSN FF
(around 4.0
nm/s, which
is signifi-
cantly lower
than with
any other
conventional
immersion
Tin technique). Oxidation is also
strongly reduced. Consequently,
after thermal ageing and / or mul-
tiple solder steps, the solderability
is preserved to a superior degree.
In particular:
• electrochemical analysis (GCM,
SERA) shows remaining pure Tin is
about 0.2 – 0.5 µm thick (depend-
ing on starting Tin layer thickness
and ageing conditions);
• solder tests with a meniscograph
show a wetting angle of less than
55° after ageing or multiple re-
flow;
• wave solder tests show complete
solderability after thermal ageing
and / or multiple solder steps (re-
flow).
For the electrochemical Tin layer
thickness measurements (GCM or
SERA), samples with comparable
virgin Tin layer thickness were
made: 0.58 µm for both Ormecon
CSN FF and FF-W. After thermal
ageing at 155°C for 4 hrs, a Tin
thickness of 0.17 µm remains on
the standard surface, whereas for
the new FF-W, a thickness of 0.24
µm remains, corresponding to the
lower diffusion rate.
Process description
Figure 7 shows the process tech-
nology of the two products. For an
accurate process control, Ormecon
International recommends the
measurement of the layer thick-
ness with the coulombmetric mea-
surement system (GCM, SERA),
before and after aging.
Figure 7 – Process technology for CSN FF and CSN FF-W
Figure 6 – Layer structure with and without the “whisker protection”
pre-dip step
Table 1 - Whisker formation with various Immersion Tin surface finishes