Understanding hidden reactions and the importance of profile in reflow soldering, part 2
solder balls) on the reaction mechanisms. other metals such as Cu on the pad is interconnection process. The interaction
critical to the reflow soldering process of Sn with each of the elements is
Sn-Pb system since it determines the formation of uniquely different, and most of them
The phase composition diagram of Sn-Pb a metallurgical bond. The time and form intermetallic phases with different
system, as shown in Figure 2, is amongst temperature determines the kinetics of the stoichiometric compositions. In a solder
the simplest, due to a single eutectic
composition. The x-axis indicates weight
percentage of Pb, increasing from left
to right, and weight percentage of Sn,
increasing from right to left. Temperature
is indicated on the y-axis. At any given
temperature and composition it is possible
to completely define the Sn-Pb system.
Areas represent bivariant regions, lines
represent univariant regions and points
represent invariant systems in a two-
component phase diagram.
The solder alloy under examination
has a near-eutectic composition containing
63% Sn and 37% Pb by weight. Eutectic
composition of the alloy is one that
directly transforms from solid state to
liquid state (or vice-versa) without an
intermediate phase. The temperature at
which the eutectic composition undergoes
this transformation is referred to as the
eutectic temperature. The intermediate
phase formed in case of non-eutectic alloy
compositions is termed as the pasty region.
When subjected to reflow temperatures,
the eutectic alloy traces Path 1 to
transform from solid state to liquid state.
In the liquid state, Sn and Pb in the
Figure 3. Sn-Ag binary system phase diagram.
alloy are free to diffuse and react with
other elements present in the system.
Such reactions result in the change in
composition, leading to a shift in the
weight percentage of the individual
elements in the alloy. Since some amount
of Sn gets consumed in the formation of
intermetallic, the amount of Sn in the
alloy is reduced, thereby shifting the cool
down path to Path 2 as indicated in
Figure 2. The location of Path 2 will
depend on the temperature to which the
alloy is melted (peak temperature) and the
time duration in which it is maintained in
the molten state (TAL). Longer the time
the alloy is maintained above the melting
temperature, more the amount of Sn is
consumed, resulting in further shift of
Path 2 to the right.
The reaction of Sn with other metals
is generally accompanied by the formation
of stoichiometric line compound phase
(IMC). These compounds, in a binary
system such as Sn-Ag, Sn-Cu, etc., are
indicated by a vertical line rising from the
composition of individual elements. From
Figure 2 it is evident that no such line
compounds are formed between Sn and
Pb at any temperature or composition.
However, the interaction of Sn with
Figure 4. Sn-Cu binary system phase diagram.
24 – Global SMT & Packaging - August 2008 www.globalsmt.net
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