IPC-7095C-2013.pdf - 第147页
with the help of special microscopes such as the endo- scope, peripheral solder joints can be viewed. SnAgCu sol- der joint microstructures are multiphase microstructures and the surface of the solder joints appears roug…
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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with the help of special microscopes such as the endo-
scope, peripheral solder joints can be viewed. SnAgCu sol-
der joint microstructures are multiphase microstructures
and the surface of the solder joints appears rough. Figure
8-22 shows a typical SnAgCu BGA solder joint. This is
much different from typical SnPb BGA solder joints which
usually have a shiny surface. Ball
8.5.5.6 Transition Lead-Free Technologies The transi-
tion from a total tin/lead soldering system to a total lead-
free soldering system is not going to happen overnight.
There may be an interim phase where tin/lead and lead-free
solders will co-exist on board assemblies. This transition
phase entails that the impact of lead in the SAC solder be
evaluated for impact on solder joint yields and reliability.
The likely lead-free board assemblies during this transition
phase are listed in Table 8-5.
The first possible lead-free board assembly listed above is
‘‘forward compatibility.’’ The board assembly soldering
process for forward compatibility assemblies has been con-
verted to lead-free technology with a change in the solder
paste composition and the reflow soldering profile to match
this change. However, some components, such as BGAs
that are soldered on the board, will still have tin/lead sol-
der due to the component supplier’s lead-free roadmap
having a conversion date later than the board assembler’s
conversion date. This results in the tin/lead solder joints of
the BGA-type components being ‘‘contaminated’’ by the
lead replacement metals in the lead-free solder paste.
The second possible lead-free board assembly listed above
is ‘‘backward compatibility.’’ Backward compatibility sce-
narios arise when component suppliers introduce the lead-
free components, but not all board assemblers that use
these components have converted their board assembly
lines to lead-free technology. These assemblers will still be
soldering the lead-free components with eutectic tin/lead
solder paste using tin/lead reflow soldering profiles. A tin/
lead component would obviously be preferable in this case,
but the component suppliers may, due to economic reasons,
not want to carry two component line items, one tin/lead
and one lead free, for the same device. The solder joints
formed with this combination of tin/lead solder paste and
lead-free solder ball will have a mixed composition. When
soldering a Ball Grid Array package with SnAgCu lead-
free solder balls using tin/lead solder paste, two different
scenarios arise based on the reflow profile used. The two
reflow profiles compared are shown in Figure 8-23 with the
total lead-free reflow profile also shown for comparison.
The tin/lead reflow profile, which is illustrative of the pro-
file used today for tin/lead assemblies, does not exceed the
melting point of the BGA’s SnAgCu solder ball. When a
reflow profile is unable to make lead-free solder ball melt
or dissolve, the solder joint yield and/or solder joint fatigue
life may suffer. The tin/lead solder paste deposited on the
lands of the solder balls melts but the SnAgCu solder balls
are still not molten. In such a material combination, the
lead may likely diffuse through the grain boundaries of the
solder ball. How high the lead from the tin/lead solder dif-
fuses up the SnAgCu solder ball will depend on how high
the reflow temperature gets and for how long the tin/lead
solder is molten. As shown in Figure 8-24, which depicts a
micrograph of a cross-section of a SnAgCu solder ball of a
BGA package soldered to a board using a reflow profile
which is unable to melt or dissolve SnAgCu solder ball, the
resulting solder joint microstructure is inhomogeneous.
This deleteriously impacts the solder joint reliability.
Yield impact on such solder joint is also deleterious due to
two reasons. One is due to the poor self-alignment of the
BGA during reflow soldering because the solder ball does
not become molten. This creates a potential for open joints
when the component is misaligned to some extent during
or after the Placement process step. Secondly, the lack of
‘‘ball collapse’’ may cause open solder joints from a lack of
contact between the solder paste deposit and the solder
ball.
Figure 8-22 Endoscope Photo of a SnAgCu BGA Solder
Ball
Table 8-5 Types of Lead-Free Assemblies Possible
Definition
Component
Termination/
BGA Ball Solder Paste
Board
Surface
Finish
Forward
compatibility
contains lead lead-free
leaded or
lead-free
Backward
compatibility
lead-free 63Sn37Pb
leaded or
lead-free
Total
lead-free
lead-free lead-free lead-free
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Hence, for better solder joint yields and solder joint reli-
ability, the backward compatibility reflow soldering profile
depicted in Figure 8-23 should be used. During this reflow
profile, the SnAgCu solder ball also melts, and lead from
the molten tin/lead solder paste mixes thoroughly with
molten SnAgCu solder ball and generates a homogeneous
structure of a lead-rich phase in the tin matrix. Such a
microstructure is shown in Figure 8-25.
Moreover, since the SnAgCu solder ball melts and col-
lapses, the self-aligning process and coplanarity reduction
also occur, thereby enhancing solder joint yields of the
BGA.
8.5.5.7 Ball Replacement Due to limited availability of
components with SnPb balls, some functional designs may
require ball replacement of the BGA component to allow
common processing with tin lead production assemblies.
This practice, while commonly employed, has drawbacks
due to the additional thermal cycles required for the ball
removal and subsequent replacement. Additionally, the
resulting interface alloy at the component to ball attach-
ment site may not achieve the desired or expected eutectic
properties. The process, however, does allow the use of
limited component types to comply with tin lead use strat-
egies for performance, contract mandates, and/or compat-
ibility with legacy or production designs that are suscep-
tible to the additional temperature excursion required for
lead free production processes. The sphere replacement
IPC-7095c-8-23
Figure 8-23 Comparison of Reflow Soldering Profiles for Tin/Lead, Backward Compatibility and Total Lead-Free Board
Assemblies
300
250
200
150
100
50
0
0
50
100
150 200 250
300
350 400
Tmelting for SnAgCu
Tmelting for SnPb
SnAgCu Balls do NOT Melt
Not good for backward compatibility
SnAgCu BAlls Melt
Will be OK for backward compatibility
Typical Lead Free Solder (SnAgCu) Profile
Temperature, Deg C
Time, Seconds
Figure 8-24 Micrograph of a cross-section of a BGA
SnAgCu solder ball, assembled onto a board with tin/lead
solder paste using the standard tin/lead reflow soldering
profile. The SnAgCu solder ball does not melt; black/grey
interconnecting fingers are lead-rich grain boundaries; rod
shape particles are Ag3Sn IMCs; grey particles are Cu6Sn5
IMCs.
Figure 8-25 Micrograph of a cross-section of a BGA
SnAgCu solder ball, assembled onto a board with tin/lead
solder paste using a backward compatibility reflows sol-
dering profile. The SnAgCu solder ball has melted.
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No reproduction or networking permitted without license from IHS
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