Understanding hidden reactions and the importance of profile in reflow soldering, part 2
joint, the common interactions of Sn are form intermetallic phases. Cu forms two isothermal aging at 150°C. It can be seen
observed with elements like Cu, Ag, Ni, different intermetallic phases (Cu
3
Sn and that the formation of Cu
3
Sn is significant
Au and palladium (Pd) depending on the Cu
6
Sn
5
), reacting with Sn in the solder after 500 hrs and thicker at 1000 hrs of
type of component and board finishes joint. The kinetics of Cu
3
Sn formation aging while Cu
6
Sn
5
IMC is visible even
used. Commonly known intermetallic at temperatures used in reflow soldering before the aging.
compounds formed by Sn during these is extremely slow and so its formation is
interactions are Cu
6
Sn
5
, Cu
3
Sn, Ag
3
Sn, not generally perceived. Hence, during Sn-Au system
Ni
3
Sn
4
, AuSn
4
and AuSn
2
. In the reflow only Cu
6
Sn
5
formation is observed. The Sn-Au system phase diagram shows
following section these interactions are However, during the growth of IMC layers, extensive intermetallic formation between
discussed with illustrations from the binary with time and temperature, the Sn in the the metals throughout the composition
phase diagrams. solder tends to diffuse into the Cu pad at various temperatures. Sn-Au, in the
region. At this stage, the availability of Sn eutectic composition (Au80%-Sn20%),
Sn-Ag system is limited, owing to the slow rate of the is used as a high melting solder alloy (MP
The phase diagram of this system (Figure 3) solid-state diffusion. Hence, the kinetics = 280°C) in die-level solder attachment
indicates the several phases of Sn-Ag at and reaction conditions favor the formation application. The comprehensive phase
different temperatures. The x-axis represents of Cu
3
Sn. Literature suggests that the diagram in Figure 6, shows the different
the composition in terms of weight formation and growth mechanism of Cu
3
Sn IMC phases formed. The composition
percentage of Ag, increasing from left to is driven by the thermodynamic instability of IMC extends from Au-rich side with
right. When solder is subjected to reflow between the Cu (pad) and Cu
6
Sn
5
layers Au10Sn to the Sn-rich side with AuSn
4
.
temperatures, molten Sn (from the solder) after the initial soldering process. The The latter is commonly observed in the
is free to interact and form intermetallic Cu
3
Sn is believed to provide an equilibrium reflow soldering process and its presence
compounds with Ag (present in PCB phase (from the phase diagram) between in the solder joint is known to reduce the
finish and solder alloy). When the molten the two layers. Hence, the formation and fatigue resistance drastically .
Sn encounters an Ag-rich phase, it forms growth of Cu
6
Sn
5
is observed even during Since most of the older systems contain
Ag
3
Sn precipitates. This is indicated by the reflow while the presence of Cu
3
Sn is not Sn in majority, the Sn-rich side (towards
vertical line from the x-axis at a 70-75 wt% perceived until the joint is subjected to the right-hand side) of the phase diagram
Ag. It could be observed from Figure 3 that aging. Figure 5 shows images of cross-section (Figure 6) is kept as the focus in this
the growth of Ag
3
Sn phase can result even of a Sn-Pb solder joint at different stages of section. Au, being an inert
in the solid state (below the liquidus line).
This type of intermetallic formation is a
consequence of solid-state diffusion of Sn
into the Ag-rich phase.
Sn-Cu system
The Sn-Cu system is relatively more
complicated than the Sn-Pb and Sn-Ag
systems. This is due to the fact that Cu and
Sn form more than one intermetallic phase
with distinct compositions. Figure 4
shows the binary phase diagram of Sn-Cu
Figure 5. Microstructure of Sn-Pb joint showing Cu6Sn & Cu3Sn interfacial intermetallic phases.
system. It can be observed from the
figure that the temperature to attain the
liquidus stage increases with increase in Cu
concentration. Cu
6
Sn
5
and Cu
3
Sn are the
IMC phases formed at or below the Sn-Pb
or Pb-free reflow temperatures. Above this
temperature two other IMC formations
observed are Cu
41
Sn
11
(~400°C) and
Cu
10
Sn
3
(above 500°C), which are not
encountered in the soldering process.
Kinetics of the formation of Cu
6
Sn
5
and Cu
3
Sn are strictly regulated by the
availability and concentration of metal
elements during the reaction. At higher Cu
concentrations (>60%) Cu
3
Sn is formed
from solid-state diffusion of Cu into the
Sn in the solder matrix, while at relatively
lower Cu content (<40% by wt) Cu
6
Sn
5
IMC is formed.
When subjected to reflow conditions,
the rate of intermetallic phase formation
is accelerated in molten state. Sn from the
molten solder reacts with Cu in the pad to
Figure 6. Sn-Au binary system phase diagram.
26 – Global SMT & Packaging - August 2008
www.globalsmt.net
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