Understanding the hidden reactions and the importance of profile—part 3
Ni diffuses and occupies the interstitial
2a. 2b.
lattices of Cu to form this compound
1
.
sn-ag-cu system
The phase diagram for the lead-free SAC
alloy is complicated by the presence of
three elements requiring a third dimension
to determine the state of the alloy at a giv-
en composition and temperature. Several
compositions of the alloy are commercially
available, but one of the most commonly
used compositions is SAC305 (Sn96.5%-
Ag3.0%-Cu0.5%). Two diagrams are
needed to elucidate the equilibria and the
metallurgical transformations occurring in
the alloy. In a ternary phase diagram, the
% weight of the two elements represented
Figure 2. Sn-Ag-Cu ternary system temperature composition phase diagram.
in the phase diagram (Ag & Cu) deter-
mines the % content of the third element
3a. 3b.
(Sn). The temperature lines (isotherms)
indicate the interactions at that specific
temperature. The points of intersection
of the isotherms on the composition axis
determine the composition of the alloy at
which the IMC formation takes place. The
isotherms are typically drawn increasing
from 218°C as that is the melting point of
the general solder paste alloys containing
Sn, Cu and Ag and IMC formation and
changes are active only in molten state.
Figure 2a shows the 3D phase diagram
of a SAC system. It can be noted from the
3D diagram that Ag and Cu phases are
in thermodynamic equilibrium even at
Figure 3. Sn-Ag-Bi ternary system temperature composition phase diagram.
temperatures above 700°C. This makes the
Ag-Cu system very stable and hence, any
interaction in the system is either in the
system requires a dual phase diagram in high-temperature applications. Hence,
Sn-Ag phase or in the Sn-Cu phase.
analysis to understand the behavior at the the application of Bi containing alloys
Figure 2b shows the Sn-rich region,
operating region (reflow process region). in assemblies possibly containing Pb are
which is commonly observed in ternary
In any Ag-rich region in the matrix, restricted to low temperature and medium
solder paste alloys. The eutectic composi-
the silver would readily combine with Sn to low reliability requirements.
tion of the system melts at 216.7°C and
to form the Ag Sn IMC (at temperatures
3
it contains 1% Cu (approx) and 3.7% Ag
above 200°C). The Ag region of the graph risk mitigation of thermally
by weight. The commonly used SAC305
is similar to that in the SAC system, while induced damage and degradation
(Sn-3.0Ag-0.5Cu at. wt.) is a near eutectic
the activity of Bi is dominant only in the of electronic assemblies
SAC alloy with a one degree centigrade
low temperature regimes (<200°C). Electronic assemblies are subjected to ther-
pasty region.
The diagram shows the ternary system mal phenomena such as reflow soldering,
When subjected to reflow, the consti-
with a eutectic phase that melts at lower repair, rework, encapsulation, underfilling,
tuting elements segregate into Cu-rich and
temperatures (<150°C). Prior experimental through out its life cycle. Further, product
Ag-rich phases within the interconnection,
results indicate the presence of a separate qualification and reliability evaluations
with Sn being the major element present
Bi phase and no combination with other involve numerous thermal exposures to
throughout the matrix. Ag and Cu in the
elements (except in the presence of Pb), varying severities and durations. In addi-
system when coming in contact with Sn,
suggesting that the metal does not show tion, the operational environments of the
especially in molten state, react to form
much interaction with other elements
2
. product also impose exposures to differing
Ag Sn, Cu Sn and Cu Sn IMC phases.
In the presence of Pb, Bi reacts and forms durations and temperature extremity severi-
3 6 5 3
The formation reactions of the IM phases
a low melting eutectic composition. In a ties over the designed product life.
are governed by the physical conditions
solder joint assembly containing Pb either The thermal exposure, depending
prevailing in the system. It is not con-
in the component termination finish or on the nature of design, materials, and
venient to discuss the formation of the
board surface finish, Bi is observed to processes, can at times degrade and dam-
individual IMCs using a 3D diagram. It
form a low melting eutectic phase
3
. It can age the product. Thus, there exists some
is better understood by using specific two-
be noted in the Sn-Pb-Bi system phase risk propensity associated with thermal
dimensional sections of a 3D drawing.
diagram (as shown in Figure 4). exposures. It is important to ensure that
Formation of a low-melting eutectic electronic product conforms to the quality
sn-ag-Bi system
phase (by Sn & Bi) is observed to have and reliability expectations. Risk can be
The SAB system (Figure 3), like the SAC
a deleterious effect on the performance defined as the probability of either damage
and long-term reliability of the assemblies to, and or degradation in the performance
www.globalsmt.net Global SMT & Packaging – September 2008 – 35
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