IPC-7095C-2013.pdf - 第144页
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-Fre…
factors that control stand-off are land size, available solder
volume, and the weight of the component. The lower the
weight, the smaller the land size; the larger the solder vol-
ume, the higher the stand-off.
8.5.3 PCB Design Considerations Another influence on
reliability is the geometry of the solder joints as well as the
solder land metallization. Solder masks can have a negative
influence if they are used for solder mask-defined (SMD)
lands with the solder mask on the metallization lands
affecting the solder joint geometries. Stress concentrations
created by the SMD solder joint geometries can be the ori-
gin of solder joint failures and reduced reliability. More
than that, the solder mask shape and thickness could influ-
ence the reliability of the solder joint. Figure 8-19 shows
crack due to stress concentration at the solder mask.
For equal solder joint height, increases in fatigue life by
factors of about 1.25 to 3 can be anticipated with the use
of nonsoldermask-defined (NSMD) vs. SMD lands with the
larger improvements for solder joints with the more severe
loading conditions. Surface finish also plays a critical role
in BGA solder joint reliability. HASL, a commonly used
surface finish may be too thick or too thin. Insufficient sol-
der thickness may be consumed as intermetallic, which is
unsolderable. Immersion gold over electroless nickel is
prone to the black pad defect which leads to brittle interfa-
cial solder joint failures under mechanical and/or thermal
stress. The ‘‘black pad’’ defect is thought to be caused by
excessive corrosion of the nickel during the gold plating
process.
Laminate cracking is also a possible failure mechanism
under BGA solder joint lands. Such failure is thought to be
caused by thermal mechanical stress during reflow and or
subsequent mechanical stresses on the joints. Via-in-pad or
via next to land may cause drainage of solder. This is gen-
erally not recommended. Via-in-pad is being tried by some
companies with successful results. However, such an
approach should be considered only by companies with
extensive internal resources to validate reliability of solder
joints with via in pad technology.
Microvias are becoming more common in BGA lands.
Most of the BGAs will have voids whenever microvias are
used. Studies showed that most voids are not a reliability
risk to initiate a crack, however, they reduce the joint area
and will shorten the time to failure when a crack is propa-
gated. Figure 8-20 shows a failure after reliability testing
where the void was so large the ball collapsed.
8.5.4 Reliability of Solder Attachments of Ceramic Grid
Array
Ceramic CTE is about 6 ppm/°C; the CTE of
organic-based PCBs is in the range of 16-20 ppm/°C. Thus,
a global CTE-mismatch of about 10-14 ppm/°C exists
between ceramic components and organic printed boards.
To compensate for the large global CTE-mismatch, ceramic
components typically require solder columns to function
reliably in most applications. Since the corner joints are
loaded more than other solder joints (they are farthest from
the neutral point or DNP), they fail first.
The solder columns, which currently are only used for
ceramic Grid Array Components (GACs), are 10Sn/90Pb
columns with lengths of 1.27 mm to 2.29 mm that are
either cast onto the CGA or are wire soldered to both the
CGA and the substrate with near-eutectic tin/lead solder.
With all other conditions being equal, the ratio of CBGA
solder joint fatigue lives of three column heights, 0.41 mm
[16 mils], 0.76 mm [30 mils], 2.29 mm [90 mils] is 1:4:45.
The height of the solder columns is limited by the require-
ment that the column aspect ratio (height-to-diameter) does
not produce slender columns thus changing the loading
conditions; cast columns can accommodate larger aspect
ratios.
8.5.5 Lead-Free Soldering of BGAs This section covers
various aspects of board assembly of BGAs using lead-free
solders. A description of various available lead-free alloys
and their selection is described first. Board design and
Figure 8-19 Solder Mask Influence
Figure 8-20 Reliability Test Failure Due to Very Large Void
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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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