IPC-7095C-2013.pdf - 第145页
• Mechanical, electrical and thermal properties • Reworkability • Compatibility with lead (during transition period) • A vailability from suppliers • Cost • Patent concerns T able 8-4 compares the composition selected by…
assembly consideration for BGAs are described next with
a discussion on the transition technologies that occur when
converting from a tin/lead to a lead-free package and
assembly concluding this section.
8.5.5.1 Lead-Free Alloy Selection Ideally, the lead-free
alloys selected should be drop-in replacements for the cur-
rently used tin/lead (Sn-Pb) alloys. Drop-in replacement
alloys do not require any significant change in the materi-
als, equipment and processes for package and board assem-
bly. Drop-in replacement does exist, but the cost of these
drop-in replacements are not on parity with that of alloys
that require higher process temperatures (e.g., SAC305)
within the list of potential lead-free solder alloys that are
currently available.
The seminal work to select the best lead-free solder alloys
in existence was the three year study undertaken by the
National Center for Manufacturing Sciences (NCMS). This
study culminated in a report that covered the evaluation of
over 79 lead-free solder alloys.
Table 8-3 shows some of the common lead-free solders that
were evaluated by the NCMS group. These are listed
according to their melting points. The overwhelming
majority of these alloys are tin-rich alloys (>90%Sn) with
Sn respectively forming binary or ternary systems with
other elements such as Bi, Zn, Sb, Ag, and Cu. The melt-
ing points and the advantages and drawbacks for these
alloy systems as well as other potential alternatives are
listed in the table.
The binary and ternary tin-rich alloys, except the Sn-Zn
system, noticeably have 30-40°C higher melting points
than eutectic tin/lead solder (melting point, mp = 183°C).
Alloys with comparable melting points while possessing
the desirable physical and mechanical properties, suitable
for SMT assembly have to extend to quarternary alloys.
Some consortia around the world have selected alloys from
the Sn-Ag-Cu family as the lead-free solders of choice. In
determining this final choice of the alloy family as well as
the particular alloy compositions, many factors were con-
sidered and evaluated. These included:
• Melting temperature
• Wettability to common component substrate and board
surface finishes
• Compatibility to common fluxes, particularly no-clean
fluxes
• Component and board reliability
Table 8-3 Common Solders, Their Melting Points, Advantages and Drawbacks
Alloys or Alloy
Systems
Melting
Point (°C) Advantages Drawbacks
Sn95Sb5 240 Good fatigue resistance.
Higher toxicity than lead; high melting
temperature; 8°C pasty range; poor wetting;
low tensile strength.
Sn99.3Cu0.7 227
Low cost in comparison to other lead-free
solders; not prone to fillet lifting in the absence
of lead.
Reduced wettability in air, but adequate in an
inert atmosphere.
Sn96.5Ag3.5 221
One of the primary choices by NCMS study;
used for many years in certain applications;
fatigue properties are similar to tin/lead solders
for some accelerated reliability
test results.
Poorest wetting in reflow soldering among
high-tin alloys; though wettability still adequate
for most board assembly operations.
SnAgCu 217-220
Better creep resistance than tin/lead solders;
fatigue properties are better than tin/lead
solders for some accelerated reliability test
results. Optimum pasty range for tombstone
control.
Prone to fracture in high shock applications.
Some compositions are patented.
SnZnBi 191-199
Closest in melting point to tin/lead alloys;
better strength than tin/lead solders; fatigue
properties are better than tin/lead solders for
some accelerated reliability test results.
Very susceptible to oxidation and corrosion
but small amount of Al could alleviate these
problems; requires special fluxes and solder
processes for achieving acceptable manu-
facturing yields.
Sn91Zn9 199
Sn63Pb37 183 Most widely used solder alloy. Contains lead.
Sn62Pb36Ag2 179
Higher tensile strength, pasty range for
anti-tombstone, greater creep resistance
Contains lead, more expensive.
Bi58Sn42 139
One of the alloys down-selected by NCMS;
presently used in low temperature applications.
Melting point is too low for computer
applications; susceptible to formation of low
melting ternary phase by lead contamination.
In52Sn48 118 One of the lowest melting point solders.
Indium supplies are limited; melting point too
low for computer applications; susceptible to
corrosion.
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• Mechanical, electrical and thermal properties
• Reworkability
• Compatibility with lead (during transition period)
• Availability from suppliers
• Cost
• Patent concerns
Table 8-4 compares the composition selected by three con-
sortia. The compositions are very close to each other and
behave very similarly in the reflow soldering process. Fur-
ther, solder alloy suppliers usually state a tolerance of ±
0.2% by weight for each elemental component of the sol-
der, which is consistent with the ANSI J-STD-006 specifi-
cation. When taking this into consideration, the alloy com-
positions below all overlap.
8.5.5.2 Recommendations for Alloys with Ag Content
Around 1%
• Solder joints must reach a minimum of 235°C with a TAL
of at least 60 seconds.
• Reflow profiles for some PCAs may need to be modified
when BGAs with low Ag alloys are used
• At a minimum, reflow profiles may need to be validated
using metallography, requiring additional engineering
effort
Along with careful oven profiling, metallography should be
performed (with statistically significant sample sizes) to
validate that components with low Ag solder ball alloys are
forming proper joints.
The number of thermocouples used to validate a profile
should be significantly increased so that the low-Ag com-
ponent can be shown to meet the conditions given above,
and that all other components meet the existing specifica-
tions (no overheating).
The 1% Ag alloys appear to be incompatible with many
current industry lead-free assembly specifications for
SAC305 that require a minimum reflow peak temperature/
TAL of 235°C/60 sec, while current specifications are
likely adequate for other alloys. The actual limit in Ag
content at which significant change to the lower limit of the
reflow profile is needed has not been precisely identified. It
appears to be between 1% and 2.3% Ag. Other variables
may impact this value, so caution is warranted for any
alloy with less than 3% Ag.
Reliability of Solder Joints – Industry studies consistently
indicate better drop performance for low Ag alloys than for
near-eutectic SAC. Results depend on the solder land fin-
ishes in addition to alloy composition. The exact reason for
the improvement is still being debated. It may be due to
reductions in the stiffness and yield strength of the bulk
solder ball. It is known that changes to the intermetallic
layer composition and/or structure of the solder joint can
be expected. A reduction in under cooling is required to
solidify the BGA ball.
The published data indicates that the fatigue life of low Ag
alloys is lower than the higher Ag counterparts. However,
the existing models are very product-specific. There are
currently no agreed-to acceleration models available for
these alloys, making product life predictions difficult. The
existing study results available relate to various end-
product conditions, thus understanding how those low Ag
alloys perform relative to eutectic tin/lead is not easy.
Managing the Change to a Low Ag Ball Alloy – Due to
the potential impacts on reflow process window and reli-
ability, changing from a near-eutectic SAC ball alloy to one
with less than 3% Ag should be considered to be a change
in form, fit, or function. In this case a change in part num-
ber for any BGA component, where the only change is ball
alloy, is considered appropriate.
8.5.5.3 Board Design Considerations Board Design for
BGA Assembly with Lead-Free Solders is generally very
similar to that for presently used tin/lead solders. The same
Design for Manufacturability (DfM) rules and guidelines
should be applied for lead-free boards as those used to tin/
lead (SnPb) boards. These include consideration for com-
ponent orientation, soldering, via holes, solder mask,
repairability and testability. Some of these are described
further below.
BGA Land Pattern Designs – As in the case of tin/lead
BGA soldering, the preferred BGA Land style for SnAgCu
soldering is the nonsoldermask-defined design as opposed
to the soldermask-defined land style since it allows the
maximum flexibility for the PCB designer, and less stress
points are introduced to the solder joint by the solder mask.
Component Placement Location on the PCB – Since
SnAgCu solders require higher reflow temperatures for
component soldering, the placement location of large, tem-
perature sensitive BGA components may need to be
addressed carefully. The regions near the edges of the
board are, depending on the board size, thickness and layer
count, typically 5-15°C higher in temperature than those in
the central locations. Since large packages are more prone
to moisture and thermal stress induced defects when sub-
jected to higher reflow temperatures, such packages, if pos-
sible, should be confined to the central regions of the
board. Other factors, such as trace routability and density,
Table 8-4 Comparison of Lead-Free
Solder Alloy Compositions in The Sn-Ag-Cu
Family Selection by Various Consortia
Consortium % Sn % Ag % Cu
IDEAL 95.5 3.8 0.7
Solder Products Value Council 96.5 3.0 0.5
iNEMI 95.5 3.9 0.6
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may make it necessary to place large BGAs at the edges of
the board. In such cases, the reflow soldering process win-
dow will be narrowed to keep the maximum temperature
the BGA components are exposed to below acceptable lim-
its.
8.5.5.4 Reflow Soldering Considerations Reflow sol-
dering is commonly done in an IR convection oven with
heated air convection. Despite the higher reflow tempera-
tures necessary to melt the SnAgCu (SAC) solders when
compared with the tin/lead solders, new equipment may
not be necessary for lead-free reflow soldering. The same
ovens as used before for tin/lead solders can be used with
the obvious increase in the settings for the various heating
zones in the reflow oven. As the PWB mass increases the
ability to limit the maximum reflow temperature, delta T
across the PWB and reflow time becomes more difficult
without extended zone ovens.
The environment in the oven can be either air or inert, such
as nitrogen. For lead-free soldering, to minimize the oxida-
tion of the materials on the board assemblies during the
high temperature reflow operation, an inert atmosphere can
be helpful. A lot of the criteria depends on the solder paste
and the solder paste manufacturer as well as the thermal
mask of the electronic assembly. Some board surface fin-
ishes, such as organic solderability preservatives (OSP) on
copper may require an inert atmosphere during reflow sol-
dering to attain acceptable solder joint yield levels.
A reflow solder profile should be developed for all board
assemblies. Since SAC solders require higher reflow sol-
dering temperatures, it is important to determine the maxi-
mum temperature at various different locations on the
board. Component temperatures may vary because of mate-
rial type, surrounding components, location of part on the
board, and package densities.
To avoid moisture and thermo-mechanical stress induced
failures on plastic components, it is best to measure the
temperature of the component body and check to ensure
that it doesn’t exceed the maximum temperature it is rated
for. Hence, thermocouples, which are generally used to
measure the temperatures during reflow profiling, should
be attached at the solder joint as well as the body of the
various components during reflow profiling of board
assemblies. Large components generally have greater than
5°C difference between the leads/solder balls and the mold-
ing compounds of the component.
A typical SAC reflow profile is compared with a tin/lead
reflow profile for a BGA solder joint in Figure 8-21 below.
Four different regions of the reflow profile are shown: the
Preheat region during which the low melting volatile ingre-
dient in the solder paste vehicle are evolved; the Flux Acti-
vation region which enables the temperature to start equili-
brating across the board and to start to activate the flux; the
Reflow region where the solder melts, wets the land surface
and forms the solder joint; and finally the Cool down stage,
where the solder solidifies and the board assembly exits the
oven to be cooled down by forced air blown down on the
board by fans.
The profiles illustrated in Figure 8-21 are termed ‘‘FAT’’
(flux activation time) profiles since they have a soak zone
before the solder is reflowed. Alternatively, ‘‘ramp’’ pro-
files can also be developed which contain a continuous
ramp from the preheat zone to the reflow soldering zone.
These ramp profiles increase the throughput of the board
assemblies in the reflow ovens. But, care should be taken
to avoid overheating of components, particularly on the
edges of the boards.
8.5.5.5 Appearance of BGA Lead-Free Solder Joints The
BGA package body obscures its solder joints. However,
IPC-7095c-8-21
Figure 8-21 Comparison of a Lead-Free (SnAgCu) and Tin/Lead (SnPb) BGA Reflow Soldering Profiles
Tin-Lead Solder Profile
Reflow @ 183°C
Peak Temp. = 205 to 220°C
Lead Free Solder Profile
Reflow @ 217°C
Peak Temp = 235 to 245°C
257
227
197
167
137
107
77
47
17
0.0
0.6
1.2
1.8 2.4 3.0 3.6 4.2 4.8
5.4
6.0
Temperature
Time in Minutes
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