57Bi-42Sn-1Ag: A Lead Free, Low Temperature Solder for the Electronic
Industry
Ernesto Ferrer and Helen Holder
Hewlett-Packard Company
Aguadilla, PR and Palo Alto, CA
Abstract
The ternary alloy 57Bi-42Sn-1Ag is a general-purpose solder with mechanical properties that are comparable to
63Sn-37Pb. This alloy is appropriate for many electronics applications and is particularly useful when components
cannot tolerate Sn-Pb or high temperature, lead free reflow processes. An SMT process was developed for volume
manufacturing using this material, and this article describes design guidelines, materials, and processes for no-clean,
air-reflow assembly. This process was developed for BGAs, CSPs, TSOPs, QFPs, passives, connectors, and two
thicknesses of FR-4 (0.062” and 0.016”), but it can easily be extended to many other technologies because the
majority of industry guidelines apply when using this material. Exceptions, additions, and modifications to existing
documents and processes are described here. 57Bi-42Sn-1Ag is a lead free solder and should not be used when lead
(Pb) is present in board or component finishes.
Background
HP began developing Bi-Sn alloys in the early 1990s
in response to legislation proposed in the U.S.
Congress to ban lead (Pb) from a variety of uses,
including electronic soldering. From a technical
perspective, lower melting point solders in reflow
were considered as an alternative to wave soldering
temperature sensitive components.
57Bi-42Sn-1Ag and eutectic 58Bi-42Sn have been
studied extensively as an alternative to 63Sn-37Pb or
Sn-Ag-Cu solders in SMT and wave solder within
HP and at other companies and outside groups,
including Fujitsu, Panasonic, NEC, Matsushita, and
Motorola, AT&T, Honeywell, Motorola, IBM, and
UC Berkeley.1-50
57Bi-42Sn-1Ag is a very promising system for low
temperature or hierarchical soldering because it has:
• A lower melting temperature than 63Sn-37Pb,
• High enough melting temperature for most
applications,
• Similar mechanical properties to 63Sn-37Pb in
many cases, and
• Contains no lead.
Despite the fact that 57Bi-42Sn-1Ag was researched
as a mainstream replacement for 63Sn-37Pb, high
temperature Sn-Ag-Cu alloys dominate lead free
development in the electronics industry. 57Bi-42Sn1Ag is not prohibitively expensive nor does it
provide significant cost savings on its own, however,
when used to enable a technology or component
choice, it can be the most economical solution.
HP has encountered many temperature sensitive
components requiring lower temperature processing,
including the application for which this
manufacturing process was developed. In this case, a
product required components with a maximum
temperature of 180°C. A project was started with
these goals:
• Develop a lead free process that the part could
survive, with
• Minimal capital investment for process
implementation, where
• Current manufacturing equipment could be used
57Bi-42Sn-1Ag had previously been studied and
tested in small scale manufacturing, but had not been
developed for volume SMT manufacturing. This
paper describes the results of the process
development project.
Review of 57Bi-42Sn-1Ag Properties
Although this paper focuses on the manufacturing
process, a brief overview of 57Bi-42Sn-1Ag may be
helpful for those considering implementation. The
material properties and reliability of Bi-Sn solders
have been investigated for many years and have been
written about extensively elsewhere, as noted above.
It is beyond the scope of this paper to thoroughly
review all work on 57Bi-42Sn-1Ag. For more
information, consult the reference section at the end.
Results of shear strength, creep resistance, fatigue
resistance, and other mechanical testing show 57Bi42Sn-1Ag has properties approaching or surpassing
63Sn-37Pb under most conditions, including
reasonable strength up to 90C, despite its low melting
point, and is considered acceptable for most
applications where 63Sn-37Pb is adequate.
Shear strength: Shear tests on bulk tensile specimens
show 57Bi-42Sn-1Ag has higher shear strength than
63Sn-37Pb at 20C, is comparable at 65C, and shows
more degradation than 63Sn-37Pb at 110C, but is still
comparable.51 58Bi-42Sn with small additions of Au
or Ag has a higher strength than 63Sn-37Pb and
about the same shear ductility.22 The bulk strength of
57Bi-42Sn-1Ag is sufficient for most product
environments where 63Sn-37Pb performs acceptably.
Creep: Creep is a critical factor in joint failure
because it is the main deformation mechanism in
solders. 58Bi-42Sn creep resistance exceeds 63Sn37Pb in the range 20-65C.5-7,30,40,44 We do not have
separate creep data for 57Bi-42Sn-1Ag.
Thermal fatigue: 57Bi-42Sn-1Ag thermal fatigue is
comparable or superior to 63Sn-37Pb, even in 0100C cycles.2,34 A standard 0-100C cycle will only
produce valid results for 57Bi-42Sn-1Ag with no lead
present.
Lead contamination: 57Bi-42Sn-1Ag joints on leadcontaining surfaces will form the ternary eutectic
52Bi-30Pb-18Sn. Even small amounts of this phase
will become molten at or above 96°C and promote
highly accelerated grain growth, destroying the
mechanical integrity of the solder joint.20,21 The low
melting point failure mode is limited to cases where
the temperature exceeds the melting point of the
ternary eutectic (96°C). Thermal cycling to 75°C had
little effect on microstructural coarsening both with
and without lead.2
Isothermal fatigue: Glazer,46 Mei,51 and Hua2 report
a shorter isothermal fatigue life for 58Bi-42Sn at
large strains (10-20%), but comparable life at low
strains. Guo52 observed a sharp break in CoffinManson for 58Bi-42Sn, where it became much more
sensitive to strain rates over 4 x 10-2. We do not have
separate isothermal fatigue data for 57Bi-42Sn-1Ag.
Isothermal fatigue data is often used as the input to
life predictions, however, since there is a large
difference between isothermal results at high strains
and measured thermal cycling results for 57Bi-42Sn1Ag, the implication is that highly accelerated tests
may have limited predictive value. If 57Bi-42Sn1Ag assemblies are tested in highly accelerated
conditions, the acceleration model for this material
should be developed in order to make life predictions
based on the failure data.
Shock and vibration:
Board level shock and
vibration tests have been conducted on large and
small assemblies, and product level testing has been
done on small assemblies. 57Bi-42Sn-1Ag showed
similar performance to 63Sn-37Pb in shock and
vibration followed by dye and pry at up to 1500g, and
also during scans from 100-2000Hz and 5 minutes at
resonant frequency.53
No-clean, air-reflow SMT process development
57Bi-42Sn-1Ag manufacturing is very similar to
63Sn-37Pb. The same equipment is used, and in
most cases, identical processes.
This section
describes the unique design, materials, and process
requirements for volume, no-clean, air-reflow 57Bi42Sn-1Ag SMT manufacturing.
Design
Existing 63Sn-37Pb design rules can be used for
57Bi-42Sn-1Ag manufacturing with few exceptions.
This section describes two design areas where there
are differences: BGAs and backside components.
BGAs: The recommended ball metallurgy for area
arrays is 57Bi-42Sn-1Ag, but 58Bi-42Sn or Sn-AgCu balls may be used. The use of non-melting, highSn BGA balls reduces the performance of the joints
by degrading the near-eutectic microstructure.34 The
use of 58Bi-42Sn or 57Bi-42Sn-1Ag balls improves
joint microstructure and mechanical properties and
should be used whenever possible.
• Collapsible balls: When 57Bi-42Sn-1Ag or 58Bi42Sn balls are used, they will collapse during
reflow. Current 63Sn-37Pb pad/stencil designs
should be used.
• Non-collapsible balls: High temperature balls
will not collapse during reflow. Bend tests on
57Bi-42Sn-1Ag assemblies revealed that correct
solder paste volumes for BGAs with non-melting
balls are critical to achieving performance
comparable to 63Sn-37Pb. Stencil apertures
should be adjusted to print additional solder paste
in order to achieve acceptable joint geometries.
Aperture designs must print 9000-9500 mil3 of
solder paste for 1.27mm pitch BGAs in order for
joints to be fully convex, but volumes of 40006000 mil3 can still result in BGAs performing
comparably to 63Sn-37Pb.54,55 Design rules must
be determined for other area arrays.
Backside components: The allowable weight for
backside components in double-sided reflow is a
function of surface tension. Since the surface tension
of 57Bi-42Sn-1Ag is less than 63Sn-37Pb, the
allowable weight of backside components will be
lower. TSOP-40, 0603, and 0805 components have
been successfully soldered on the backside, but
design rules for other components need to be
determined.
Materials
This section describes changes to materials for 57Bi42Sn-1Ag manufacturing.
Solder Paste: The key to making 57Bi-42Sn-1Ag
feasible for production is having a flux vehicle that
activates at lower temperatures and meets the
requirements for standard surface mount processes.
Most commercially available fluxes have been
formulated for 63Sn-37Pb and can activate at
temperatures as high as 150°C. This activation
temperature is above the melting point of 57Bi-42Sn1Ag and therefore most existing 63Sn-37Pb flux
vehicles cannot be used.
HP has worked with solder vendors for many years to
develop flux vehicles for 58Bi-42Sn and other low
temperature alloys.56-59 Several pastes are now
commercially available and perform similarly to
existing 63Sn-37Pb pastes. 57Bi-42Sn-1Ag solder
pastes should be held to the same printing and
performance standards as any paste material.
Although the qualification process for these materials
is the same as any new paste, there are a few areas
that deserve mention:
• No-clean and water-clean: Most of the process
development on this material was done for noclean surface mount, however, Bi-Sn powders
have successfully been used in water-clean
formulations.
The use of water-clean flux
vehicles may produce better wetting. If waterclean formulations are used, ensure that the
temperature of the wash and dry processes remain
below 85°C.
• Misprint and stencil cleaning:
Cleaning
experiments showed good performance when
cleaning stencils with the current machine
configuration and chemistry, however, poor
results were observed when cleaning misprinted
boards as residues were visible after cleaning.
Additional cleaning time, modification of
machine parameters, or increased saponifier
concentration may be necessary to improve
cleaning results for misprinted boards with
current chemistries.
• Solder impurities: Small amounts of incidental
exposure to lead or lead impurities in the paste
should not cause concern. Contamination studies
found that lead content of <0.3%wt does not
significantly degrade the mechanical properties of
57Bi-42Sn-1Ag even when aged at 100°C. The
maximum allowable lead impurity level should be
0.1%wt.60
• Metal load: 57Bi-42Sn-1Ag pastes may have a
higher mass percent of flux vehicle than 63Sn37Pb pastes in order to minimize oxidation of
bismuth (Bi). This modification may improve
shelf life, printing, tack, solder balling, and
wetting. The metal loading percent may be lower
than existing 63Sn-37Pb pastes and can range
from 89.0%wt to 90.5%wt, depending on the
vendor, and should be optimized in production.
• Viscosity: 57Bi-42Sn-1Ag powder may be more
sensitive to variations in chemistry and exposure
than 63Sn-37Pb powder, and may exhibit changes
in viscosity over time. Viscosity should be
carefully monitored and controlled.
• Storage: 57Bi-42Sn-1Ag pastes have a shorter
shelf life than 63Sn-37Pb pastes due to the
oxidation of bismuth (Bi). Strictly follow the
vendor recommendations.
PCBs: Any laminate material can be used in this
process because of the low temperatures, and OSP is
the recommended finish.
• Laminates: The product boards and test vehicles
used for this work were 0.016” and 0.062” thick
FR-4. It is also possible to use alternative
laminates, such as FR-1, FR-2, CEM-1, or
CEM-3.61 57Bi-42Sn-1Ag process temperatures
are well within the range tolerated by these grades
of materials.62 FR-1 and CEM-3 boards were
exposed to an aggressive reflow profile of 90
seconds preheat at 130°C and 75 seconds over
reflow to a peak temperature of 185°C. Although
there was slight discoloration, the warpage of the
boards was 0.5%, which meets IPC-A-610
standards.63
• Organic solderability protection (OSP): OSP
consistently shows the best mechanical and
process results, except in peel tests, and should be
used whenever possible.36 Peel tests have shown
that 57Bi-42Sn-1Ag on OSP over copper (Cu) has
approximately 50% of the absolute strength of
63Sn-37Pb on a similar surface, however, no
related failure mechanisms, such as delamination
or interfacial failures in thermal cycling or shock
and vibration, have been observed.64
• Other lead free surface finishes: Immersion Ag,
electrolytic Ni-immersion Au, and immersion Sn
all produce acceptable wetting when used as a
PCB metallization. Thick coatings of Ag or Au
should be avoided to minimize changes to solder
joint metallurgy because excess Ag (>3%wt) or
Au (>3µin) may reduce reliability.55,65
Components: The compatibility of low temperature
materials and temperature sensitive components with
this process is an obvious advantage. Component
metallization recommendations and other notes are
listed below:
• 57Bi-42Sn-1Ag and 58Bi-42Sn are the preferred
finishes. These finishes provide good wetting and
are most compatible with the solder joint
metallurgy.
• As with PCB metallizations, immersion Ag,
electrolytic Ni-immersion Au, and immersion Sn
provide acceptable wetting. Thick coatings of Ag
or Au should be avoided.
• The process temperatures for 57Bi-42Sn-1Ag are
lower than the ideal soldering temperature for NiPd and Ni-Pd-Au, but these finishes can still
produce acceptable wetting.66,67
• The interfacial adhesion strength of 63Sn-37Pb
and 57Bi-42Sn-1Ag are both lower for Alloy 42
surfaces than Cu, however the silver (Ag) makes
57Bi-42Sn-1Ag solder comparable to 63Sn37Pb.2,68
• If Sn-Ag-Cu or other high temperature balls are
used on area arrays such as BGAs, coplanarity is
critical. The JEDEC specification for PBGA
coplanarity in MS-034B is 0.2 mm, but that may
be too large for the non-collapsing case. It may
be necessary to enforce a tighter coplanarity
requirement.
Process
Many 63Sn-37Pb processes can be transferred
directly to 57Bi-42Sn-1Ag manufacturing, but there
are areas which deserve some attention:
Printing: 57Bi-42Sn1-Ag solder pastes should be
held to the same printing and performance standards
as any paste material. The print process for 57Bi42Sn1-Ag is identical to 63Sn-37Pb.
Paste rework and PCA bake out: If a partial
assembly (components already soldered on one side
of the PCB) requires paste rework, the board can be
washed and dried in an oven. Current baking
guidelines for electronic assemblies recommend 12
hours at 125°C. These baking conditions are too
harsh for 57Bi-42Sn-1Ag joints. Baking conditions
should not expose the solder joints to temperatures
above 90°C. The bake out time needs to be validated
for each product, but normally should be 12-15 hours.
Component placement: Alignment of the pad, solder
paste deposit, and component lead are important for
57Bi-42Sn-Ag.
Less component self-alignment
occurs during soldering because of the lower surface
tension of the molten metal. It is recommended that
the placement tolerance be maintained such that
FP/XFP parts meet IPC-A-610, Class 3 requirements
of < 25% off of the pad. Most modern equipment
easily meets this requirement. Also, designers should
adhere to alignment best practices, such as matching
land pattern, component and stencil aperture
centerlines, and placing fine-pitch components as
close to the center of the board as possible.
Reflow: It is critical to follow vendor guidelines to
achieve proper flux activation. Additional reflow
profile development may be necessary and here are
some guidelines:
• Preheat: Excess heating prior to reflow promotes
oxidation, solvent drying, and activator
breakdown, which can result in solder balls and
wetting problems.
57Bi-42Sn-1Ag requires
modest temperatures to achieve good wetting, and
wicking can be a symptom of unnecessarily
aggressive thermal profiles. Tests on currently
available fluxes have shown good performance
when the preheat time was set to 60 seconds and
the preheat temperature was between 120°C and
130°C. Increasing the preheat time has not shown
any improvements in wetting, and potentially
increases wicking and voiding.
• Peak temperature and time over reflow: Peak
temperatures should be 20-30°C above the
melting point of the solder.
The liquidus
temperature of 57Bi-42Sn-1Ag is 139°C. Tests
with several flux vehicles have shown acceptable
solder joints with peak temperatures between
160°C and 190°C. The recommendation is to use
a peak temperature of 170°C and time over reflow
of no more than 60 seconds, unless the vendor
recommends different parameters.
These
parameters can result in dramatically higher
throughput due to the increased conveyor speed.
• Cooling:
Faster cooling rates improve the
microstructure and solder joint appearance. Do
not exceed the thermal shock limit of 3°C/second.
• Nitrogen: This process was developed for reflow
in air. A nitrogen atmosphere produced slightly
better wetting, but not enough to justify a nitrogen
requirement.58
• Conveyor vibration and backside components:
Vibration should be controlled to ensure that
bottom side components do not fall off while
solder joints are at or above liquidus. Properly
maintained chain edge conveyors minimize
vibration, and the use of reflow carriers may
reduce the transmission of vibration to assemblies
while molten.
Inspection: IPC-A-610 acceptability requirements
apply to 57Bi-42Sn-1Ag. Wetting angle should be
the primary indication of acceptability of 57Bi-42Sn1Ag solder joints. Concave fillets should be used as
evidence of acceptable wetting. Non-wetted joints,
as shown by convex fillets or wetting angles greater
than 90 degrees, are defective. The surfaces of 57Bi42Sn-1Ag joints often have a dull or even grainy
appearance, but this appearance is merely cosmetic
and has no bearing on reliability.63 Other lead free
alloys require inspector retraining regarding joint
acceptability as well, so correct inspection should be
possible with 57Bi-42Sn-1Ag. The figures below
show the difference in appearance between
acceptable 63Sn-37Pb joints and 57Bi-42Sn-1Ag
joints.
temperature flux (cored wire is not currently
available). Profiles should be the same as the
original reflow profile. Component placement in
rework is as critical as the initial placement. Test
results show faster component removal and better
temperature control on adjacent components with
57Sn-42Bi-1Ag than with 63Sn-37Pb and another
low temperature alloy (43Sn-43Pb-14Bi). Also,
lower power-rated tools could be used.
Conclusion
Volume no-clean manufacturing with 57Bi-42Sn1Ag is not only feasible, but produces good yields,
increased throughput, and may be less expensive than
high temperature lead free processes because existing
equipment can be used.
The majority of 63Sn-37Pb processes can be
transferred directly to 57Bi-42Sn-1Ag manufacturing
with only minor modifications. Current industry
standards apply to 57Bi-42Sn-1Ag, including most
design rules and acceptability criteria.
Figure 1: Acceptable 57Bi-42Sn-1Ag joints (top)
and 63Sn-37Pb joints (bottom)
X-ray inspection: X-ray equipment, such as the
Agilent 5DX, will show almost identical results with
57Bi-42Sn-1Ag as with 63Sn-37Pb because the two
alloys have similar densities and other physical
properties.69
Depanelization: Test board images were separated
using a router. Keep-out distances of 0.040” and
0.065” were used on a 0.016” thick board. No
fractures or joint failures due to the routing process
were found. However, the effects of depanelization
on 57Bi-42Sn-1Ag solder joints have not been
studied in detail. 57Bi-42Sn-1Ag may be more
susceptible to damage during depanelization than
63Sn-37Pb because of the strain rate sensitivity. Use
conservative keep-outs, adequate fixturing and board
support, and single images when possible. Special or
critical components should be tested for
depanelization effects prior to product release.
57Bi-42Sn-1Ag requires a completely lead free bill
of materials, and is best suited for use with
temperature sensitive components and PCB materials.
It can be used in applications that do not require more
reliability than 63Sn-37Pb. Several solder pastes are
now commercially available and processes can be
implemented immediately.
Acknowledgements
This work would not have been possible without the
expertise, hard work, and support of many people
inside HP, including: Geary Chew, Anaida Classen,
Enid Davila, Judy Glazer, Jerry Gleason, Kristen
Gratalo, Greg Henshall, Bill Leong, Al Saxberg,
Valeska Schroeder, and Kris Troxel. We also greatly
appreciate contributions from Agilent (Randy White),
Kester Solder (Maureen Brown, Brian Deram, Dave
Torp, Greg Hayes, Senju Metal Industry Co., Ltd.
(Jeff Gaul, Richard Wulfert, Tetsuo Okuno), Indium
Corporation of America (Patrick Ryan, Bill
McCartny, N.-C. Lee), Amkor Technology (Craig
Colpo), Texas Instruments (Don Abbott), Agere
Systems (Kelly Mennell), Micron Technology (Jeff
Reeder), JST (Dave Huggins), and Molex (Radames
Negron).
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Rework: Rework should be performed with 57Bi42Sn-1Ag paste or wire with an appropriate low
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