PHYSICAL METALLURGY OF LEAD-FREE SOLDER ALLOYS
Use of lead-bearing solders, such as the Sn-Pb eutectic solder, can be dated back to
early human history. In the last 50 years, the Sn-Pb solders have found extensive usage
in electronic and semiconductor industries for large volume production for printed circuit
board (PCB)/component assemblies with highly automated processes and process controls.
In principle, solders are used for joining purposes because they possess the following use-
ful characteristics: a liquidus temperature lower than melting points of the materials to
be joined; molten solders wet or spread on the substrate metallic or metalization surfaces
and form sound metallic bonds without significant erosion of the surfaces to be joined [1].
Strengths of the final solder interconnections are determined by the solder chemical compo-
sition, processing conditions, and particularly by the metallurgical reactions of the molten
solder with the metallic surfaces to be joined. The interfacial reactions, such as wetting or
spreading between molten solder and metallic surfaces, depend on many factors, such as
intrinsic chemical affinity, surface cleanness, thermodynamics and kinetics of intermetallic
formation and growth.
Tin-Lead Solders
The Sn-Pb binary system has a eutectic reaction around 183â—¦C with a composition
of 63Sn-37Pb (wt%). The eutectic reaction is taken as the following form [2,3]:
L → β Sn (solid solution with Pb, tetragonal lattice)
+ α Pb (solid solution with Sn, BCC lattice).
The solubility at the eutectic temperature is 19% of Sn in Pb, and 2.5% of Pb in Sn. The
Sn solubility in Pb decreases significantly to less than 2% at room temperature, while Pb
solubility in Sn is reduced to literally zero. Thus, during the solidification or aging at room
temperature, the secondary Pb and Sn will precipitate from the original Sn-Pb eutectic
structures. Also, Sn-Pb solders with compositions other than eutectic will also have primary
lead phase (for Pb-rich solders) or primary Tin-rich phase (for Tin-rich solders) forms as
dendrites, with subsequent precipitations of saturated Pb or Tin phases within these primary
phases.
The Sn-Pb solder alloys offer the following advantages as compared with other sol-
ders:
(1) Superior wetting and spreading characteristics on metallic substrates, such as, Au,
Pt, Pd, Cu, Ni, Ag, and other metals and alloys, with minimal substrate erosion.
(2) Satisfactory metallic bonding strength, ductility, stiffness, and fatigue resistance.
(3) Ready application as soldering preservation coatings on PCBs and on component
leads by electro-plating or dipping.
(4) Relatively inexpensive to produce and use.
However, lead (Pb), as a pure element or an additive and alloying element, is also toxic
to human beings. For this reason it has been or will be banned for use in many industries
and applications. Even with its superior manufacturing process attributes in the electronics
industry, the European Union has passed laws to ban the usage of Pb in electronic products
starting from July 1, 2006 [4,5].
Lead-Free Solder Alloys
For a smooth transition to lead-free soldering, ideally, the lead-free alloys would have
melting points around 180â—¦C, close to that of the Sn-Pb eutectic alloy, and are of their con-
stituent eutectic compositions. However, there are very few tin-based Pb-free solder alloys
that satisfy the above criteria. Sn-Bi (42Sn-58Bi) eutectic and Sn-In (48Sn-52In) eutectic
have relatively low melting points, 138â—¦C and 118â—¦C, respectively [6,7]. Due to relative
poor high temperature mechanical strength, lack of ductility (for Sn-Bi alloys), and limited
resource, these alloys are finding only very limited applications in the industry. Sn-Zn or
Sn-Zn-Bi alloys have melting points close to the Pb-Sn eutectic alloys, and have been used
with some success, particularly in East Asia for some consumer products [8]. However,
with Zinc being prone to oxidation in the high temperature soldering processes, and to
corrosion (possible conductive corrosion by-products), these alloys are unlikely to be used
for volume production as a general Pb-free solution. The currently most promising Pb-free
solder candidates are based on the Sn-Ag-Cu ternary system, which has a eutectic compo-
sition around Sn-3.8Ag-0.7Cu, melting temperature around 217â—¦C, about 34â—¦C above that
of the Sn-Pb eutectic alloy (183â—¦C) [9].
The potential Pb-free candidates are listed in Table 10.1 and shown in Figure 10.1.
Both eutectic composition and non-eutectic alloys are shown. For increased fluidity of
molten solders, non-eutectic alloy’s paste range (the temperature range from solidus to
liquidus points) should be kept as small as possible.
The binary system phase diagrams for Sn-Ag and Sn-Cu are shown in Figure 10.2
and Figure 10.3, respectively [10,11]. Unlike Sn-Pb eutectic alloy, the Sn-Cu and Sn-Ag
alloys form eutectic reactions with their intermetallic compounds (η Cu6Sn5 for the Sn-Cu
binary system and γ Ag3Sn for the Sn-Ag system) instead of their solid solutions like in the
case of the Sn-Pb binary system. Under a nearly thermodynamic equilibrium solidification
condition, tin will solidify as nearly pure β phase without any significant solid solution of
either Ag or Cu, co-existing with η Cu6Sn5 for the Sn-Cu system or γ Ag3Sn for the Sn-
Ag system. However, under most industrial solidification conditions, the eutectic reactions
will be off-equilibrium, thus beta tin could contain solid solute atoms of Ag and/or Cu.
Subsequent precipitations of stable or metastable intermetallic phases are possible at room
temperature and during solidification or aging at temperatures below melting points.
There is still no general agreement about the exact eutectic composition for the Sn-
Ag-Cu ternary system; the most reported eutectic compositions are Sn-3.8Ag-0.7Cu and
Sn-3.6Ag-0.9Cu (wt%). Currently, the most widely recommended and studied Pb-free al-
loys are SAC387 (Sn-3.8Ag-0.7Cu) and SAC305 (Sn-3.0Ag-0.5Cu), along with SAC369
(Sn-3.9Ag-0.6Cu) and SAC405 (Sn-4.0Ag-0.5Cu), and a handful of quaternary systems,
such as Sn-Ag-Cu-Bi and Sn-Ag-Cu-Fe [12].
The ternary eutectic reaction of the Sn-Ag-Cu system can be expressed as [13]:
L → Ag3Sn + Cu6Sn5 + β (Sn).
Depending on the exact chemical compositions and solidification conditions (cooling rates,
undercooling temperature ranges, etc.), Sn-Ag-Cu alloys may experience primary reactions
prior to the ternary eutectic reaction. For example, it was reported that for SAC387, the
solidification reaction sequence was found to be [13]:
L → L1 + Ag3Sn at 221.9◦C,
L1 → L2 + β (Sn) at 218.7◦C,
L2 → Ag3Sn + Cu6Sn5 + β (Sn) at 217◦C.
However, should the Ag concentration be less than the eutectic composition, the first reac-
tion would not take place.
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