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SOLDER FAMILIES
AND HOW THEY
WORK
Low melting-temperature alloys are vital to successful electronics assembly.
S
older is a critical material that physically
holds electronic assemblies together while
allowing the various components to expand and contract, to dissipate heat, and
to transmit electrical signals. Without solder, it
would be impossible to produce the countless electronic devices that define the 21st century.
Solder is available in numerous shapes and alloys. Each has its particular properties, providing
a solder for nearly every application. Many times,
solder is an afterthought in the design and engineering process. However, by considering the soldering step early in the design process, problems
can be minimized. In fact, with the proper information, the characteristics of a solder can be part
of an optimal design.
Solders for assembly of electronic devices melt
at temperatures below 350ºC (660°F), and typically
bond two or more metallic surfaces. The elements
commonly alloyed in solders include tin (Sn), lead
(Pb), antimony (Sb), bismuth (Bi), indium (In), gold
(Au), silver (Ag), zinc (Zn), and copper (Cu).
Another material commonly used in soldering
is flux. The primary function of flux is to remove
existing oxides on the solder itself and on the
metallic surfaces being bonded, and to protect these
metals from further oxidation while at the high temperatures of the soldering operation. Fluxes typically contain rosin and/or resin, and organic acids
and/or halides, which are combined to produce
the appropriate fluxing strength for a particular
metallization..
Electronic solders can be grouped into the following five families: tin/lead, lead-free, indium/
lead, low-temperature, and high-temperature. This
article discusses these five alloy families, and several
members of each family. It also describes the wide
variety of solder forms.
Tin/lead solder alloys
Tin/lead alloys are the fundamental solders, with
a history dating back to the early days of radio. This
alloy family consists of three basic compositions that
have melting points in the 180°C (355°F) region:
• 63Sn/37Pb: the eutectic composition with a
melting point of 183°C (361°F). The term “eutectic”
26
Eric Bastow
Indium Corp. of America, Utica, New York
indicates that the composition produces an alloy
with a distinct melting point, versus a melting
range.
• 60Sn/40Pb: a variation from the eutectic, with
a melting range of 183 to 188°C (361 to 370°F)
• 62Sn/36Pb/2Ag: a composition that is often
chosen for silver metallizations, with a melting point
of 179°C (354°F).
These alloys have reasonable melting points, adequate wettability and strength, and low cost.
Therefore, they account for perhaps 80 to 90% of
all solders in electronics assembly. The performance of these alloys is so well understood and documented that the electronics assembly industry has
designed and engineered products based on their
properties.
Increasing the lead content and reducing the tin
content results in solders with substantially higher
melting points. Common versions are:
• 90Pb/10Sn: has a melting range of 275 to 302°C
(527 to 575°F).
• 95Pb/5Sn: has a melting range of 308 to 312°C
(586 to 593°F).
These alloys solder the terminations within electronic components. High melting-point solders prevent the solder joint within the component from re-
Solder preforms are available in a wide range of shapes and
sizes, primarily for surface mount technology.
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melting when the component is subsequently soldered to the printed circuit board (PCB), a step that
typically involves the lower melting-point
63Sn/37Pb solder. High lead-containing solders,
in general, have better fatigue performance, higher
tensile strengths, and slightly reduced wettability
when compared to the lower melting-point tin-lead
compositions. Reducing-gas atmospheres, such as
forming gas or pure hydrogen, are effective fluxing
agents at these high soldering temperatures, and
often substitute for chemical fluxes that may char
at high soldering temperatures.
In spite of all the beneficial attributes and familiarity associated with these alloys, the presence of
lead, and its potential environmental impact when
products are discarded to landfills, has caused the
industry to seek lead-free alternatives.
Lead-free solder alloys
Legislation in Europe will ban lead-containing
solders, with a few exceptions, effective 01 July 2006.
As a result, manufacturers, regardless of location,
will have to comply if they plan to sell electronic
products into Europe after the deadline.
Lead-free alloy development (for replacing
Sn/Pb alloys) has largely focused on a group of alloys that have become known by the acronym
“SAC” for its Sn/Ag/Cu (tin-silver-copper) composition. SAC alloys have compositions that range
from 3.0% to 4.0% silver, and from 0.5% to 0.8%
copper, with the balance tin. They are generally regarded as eutectic, or nearly eutectic, at ~217ºC
(422°F).
It has been suggested that the properties of tinbismuth-silver alloys are better than those of the
SAC alloys, because they exhibit improved wettability and fatigue resistance. However, tin-bismuthsilver solders do have some drawbacks. When combined with a lead-containing solder metallization,
on the PCB or the component terminations, a small
amount of tin-lead-bismuth eutectic alloy will form.
This resultant alloy has a melting temperature of
only 96ºC (204°F)! Because many temperaturecycling regimens do cycle up to 125ºC (257ºF), this
presents an obvious problem. As a result, tinbismuth-silver has been abandoned until the electronics industry is certain that all lead has been
“purged” from electronics manufacturing. This is
expected to take at least five or ten years.
Lead-free alloys, with all of their “environmentally friendly” hype, come with a few “issues” of
their own:
• Higher melting temperature: The ~35ºC (63°F)
higher melting temperature (vs. eutectic tin-lead)
has to be considered in component and assembly
design. Standard solder processing temperatures
of 240 to 260ºC (464 to 500°F), associated with SAC
alloys, can damage “standard” electronic components that are rated up to only 235ºC (455°F) because they were designed for eutectic tin-lead. This
higher processing temperature also results in higher
manufacturing cost due to the extra energy needed
to operate equipment at these higher temperatures.
• Greater fuel consumption: More energy
means higher fuel consumption, which in turn
These are solder balls on a ball grid array (BGA).
means more pollution. Thus, the environmental
benefit of lead-free alloys is somewhat mitigated.
• Multiple soldering steps: The other main issue
revolves around the high-lead alloys (>85% Pb) that
are often needed in assemblies requiring multiple
soldering steps. These high-lead compositions melt
in the 245 to 327ºC (473 to 620°F) range. To date,
the only lead-free alloy that can exist at these higher
temperatures is 80Au/20Sn (eutectic at 280ºC,
536°F). The gold cost associated with this alloy, and
the fact that no lower-cost alternative lead-free com-
S
older forms
Solder is typically provided in these common forms:
• Bar/Ingot: Typically cast and used in solder pot or wave soldering applications.
• Shot: Small tear-drop shaped pieces of alloy. The relatively small
size offers flexibility in applications in which the alloy has to be weighed
to a particular amount, such as filling crucibles for vapor deposition.
• Spheres: Also called precision solder balls, spheres are supplied with
diameters from 0.012 to 0.032 in. They are deposited as bumps on electronic packages such as BGAs (ball grid arrays).
• Ribbon and foil: Typically thin (0.002 to 0.010 in.+ thick) pieces of
solder, foil often has a square or rectangular geometry. Ribbon, on the
other hand, is more of a long, narrow strip wrapped on a spool. Both
can be hand cut to form simple preforms or to make shims and thermal
interfaces.
• Wire: Often applied in rework or cut to lengths and formed into
rings or other simple shapes, wire diameters typically range from 0.010
to 0.030 in. However, smaller and larger diameters are available, depending on the alloy. Solder wire can be produced with a flux core.
• Preforms: Typically punched, these thin pieces of solder are manufactured as squares, rectangles, frames, disks, washers, and custom geometries. Solder preforms can be applied in surface mount technology (SMT),
which is common to the manufacture of most consumer electronics such as
cellular phones and computers. Preforms separately attach a component to
a pad, or they augment the solder volume of the solder paste. Washers
serve as pin connectors or other through-hole components.
• Paste: A mixture of prealloyed spherical solder powder with a
flux/vehicle to form a pasty material. Paste is dispensed or stencil-printed
onto the metallization pads of a printed circuit board, and components are
automatically placed onto the solder paste. The tacky nature of the solder
paste temporarily holds the components in place. The printed circuit
board is then reflow soldered, attaching the components to the pads.
Solder pastes are available with RMA, no-clean, and water-soluble flux
vehicle formulations.
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Selected lead-free solder alloys
Indalloy No.
Composition
Solidus, °C
Liquidus, °C
Patents
1E
52In/48Sn
118
(Eutectic)
—
Comments
Lowest melting-point practical solder.
281
58Bi/42Sn
138
(Eutectic)
—
Good thermal fatigue performance;
established history.
227
77.2Sn/20.0In/2.8Ag
175
187
5,256,3702 and
5,580,5202
Not for use over 100°C due to Sn/
In eutectic at 118°C.
254
86.9Sn/10.0In/3.1Ag
204
205
5,256,3702 and
5,580,5202
No Sn/In eutectic problem; potential
use for flip chip assembly.
241
95.5Sn/3.8Ag/0.7Cu
217-218
(Eutectic)
—
Common lead-free alloys.
246
95.5Sn/4.0 Ag/0.5Cu
217-218
(Eutectic)
—
Petzow (German) prior art reference
makes this alloy patent-free.
2521
95.5Sn/3.9Ag/0.6Cu
217-218
(Eutectic)
—
NEMI promoted alloy (average
composition of Indalloy #241
and #246).
249
91.8Sn/3.4Ag/4.8Bi
211
213
5,439,6393
121
96.5Sn/3.5Ag
221
(Eutectic)
—
Binary solder has history of use,
marginal wetting.
244
99.3Sn/0.7Cu
227
(Eutectic)
—
Inexpensive, possible use in wave
soldering.
Board and component metallizations
must be lead-free.
133
95Sn/5Sb
235
240
—
—
209
65Sn/25Ag/10Sb
233
(Melting point)
—
Die attach solder, very brittle.
1. Alloy of choice for general SMT assembly; 2. ICA patent; 3. ICA licensed Sandia patent.
positions exist, has forced the industry to reconsider a total ban on lead. As a result, the European
lead-free legislation exempts lead-bearing alloys
that contain 85% or more lead. Certain defense,
telecommunications, and space applications are
also exempt from lead restrictions.
Other lower melting-point lead-free alloys that
are of some interest include 58Bi/42Sn (138ºC,
281ºF); Bi/Sn/Ag (~140ºC,~284ºF); and In/Sn
(118ºC, 244ºF). They offer alternatives for applications with temperature-sensitive components
and materials. They also serve well in step-soldering applications in which the first level of assembly may have been constructed with a SAC
alloy.
Low-temperature alloys
When added to various solder alloys, both indium and bismuth reduce the melting point. Additionally, high indium-containing, low meltingpoint solders have good ductility that often can
compensate for mismatches in the coefficient of
thermal expansion (CTE) between component and
board materials.
Low temperature solders are useful in the soldering of temperature-sensitive components or substrates, as well as in step soldering. Step soldering is
the process in which an initial soldering step is
made with a relatively high-melting point alloy,
followed by a soldering step with a lower-melting
point alloy that can be applied without re-melting
the previously soldered joints.
Examples of low-melting point solders are:
• 52In/48Sn: a eutectic alloy with a melting point
of 118°C (244°F).
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• 58Bi/42Sn: a eutectic alloy with a melting point
of 138°C (281°F).
• 80In/15Pb/5Ag: melting range of 142 to 149°C
(287 to 300°F).
High-temperature solder alloys
In addition to the 90Pb/10Sn and 95Pb/5Sn solders discussed earlier, other high-temperature solders have melting points in the 300°C range. For
example, 80Au/20Sn is a eutectic composition
having a melting point of 280°C (536°F). This high
tensile-strength, precious metal solder is often selected for the “gold to gold” sealing of large packages. When processed in an inert gas environment
such as nitrogen, this solder has the advantage of
requiring no flux when soldering to two gold metallizations.
The alloy 92.5Pb/5.0In/2.5Ag has a melting
range of 300 to 310°C (572 to 590°F). This solder has
excellent thermal fatigue properties and is frequently chosen for applications in which the electronic assembly is subjected to large thermal excursions.
Indium-lead for thick gold metallizations
Anyone who spends time perusing the various
solder compositions will quickly realize that tin is
one of the main constituents in most solders. However, tin has an affinity for alloying with precious
metals such as gold. Studies indicate that
63Sn/37Pb at 200ºC (392°F) will dissolve one micron (~40 micro-inches) of gold/second/unit area.
As tin reacts with gold, a brittle Au/Sn intermetallic
forms. When the concentration is high enough,
these intermetallics have a deleterious effect on the
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thermal fatigue characteristics of the joint, and make
it susceptible to fracture during thermal cycling.
For tin-bearing solders in applications with goldplated materials, it is advisable to keep the gold
layer thin, < 0.38µ (15 micro-inches), thereby reducing the concentration of Au/Sn intermetallic
that can form. However, many applications such
as optoelectronics packages and defense/space electronics call for thicker gold metallizations. In such
scenarios, in which the need for reliability is high,
tin-bearing solders are not appropriate.
Unlike tin, indium has a much lower affinity for
precious metals and dissolves gold at a rate 13 to
14 times slower than tin. Also, in devices with operational temperatures below 125ºC (257°F), the intermetallic that forms between indium and gold is
of a much more compliant and ductile nature, and
is not susceptible to embrittlement.
Therefore, the family of In/Pb solders is beneficial when soldering against thick gold film metallizations. The In/Pb alloys are a solid solution
system in which the liquidus and solidus temperatures are close for all compositions (near-eutectic at
all compositions). The indium-lead system offers
alloys of varying melting points, with indium-rich
compositions having a lower melting range, and
the lead-rich compositions having a higher melting
range. For examples: 70In/30Pb has a melting range
of 165 to 175°C (329 to 347°F), and 81Pb/19In has
a melting range of 260 to 275°C (500 to 527°F). ■
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Paste is dispensed or stencil-printed onto the metallization pads of a printed circuit
board, and components are automatically placed onto the solder paste. The tacky nature of the solder paste temporarily holds the components in place.
For more information: Eric Bastow is a Technical Support Engineer at Indium Corp. of America, 1676 Lincoln
Ave., Utica, NY 13503; tel: 315/853-4900; fax: 315/8531000; e-mail: [email protected]; Web site: www.
indium.com.
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