IPC-7095C-2013.pdf - 第122页
much higher than that of solder joints after 1X reflow . For example, the mean value of %voiding in solder joints after 3X reflow is about two times the %voiding level in solder joints after 1X reflow . This is because the …
result in higher levels of voids in these solder balls after the
SMT reflow soldering process.
Reflow time/temperature profile, solder paste formulation,
solder paste amount, component and PCB contamination/
oxidation and multiple reflow are the parameters believed
to impact the voiding level in solder joints after assembly.
However, these parameters may impact BGA voids by dif-
ferent levels. The details of each parameter’s impact to sol-
der joint voiding are discussed as follows.
7.7.1.1 Solder Paste Formulation Flux used to assemble
BGA components tends to generate lower voiding in solder
joints than solder paste. The reason is the oxide of the sol-
der powder in solder paste reacts with the solder-paste-flux
and generates outgassing in reflow process, which leads to
more voiding in BGA solder joints.
BGA solder joints assembled with water soluble solder
paste tend to produce higher voiding than with no-clean
solder paste. This is because the flux formula in water
soluble and that in no-clean paste is different. The water
soluble solder-paste-flux is more aggressive than the
no-clean solder-paste-flux, and reacts faster and more com-
pletely with the oxides on the solder powder. This reaction
leads to more outgassing in the reflow soldering process.
For no-clean lead-free paste, the voiding level in solder
joints with different types/manufacturers of no-clean paste
does not make a big difference. Even though the pastes
vary in flux solvent and metal content, the voiding level is
not significant in solder joints (<1%).
Solder powder type in solder paste also impacts the void-
ing level after board assembly. The finer the solder powder,
the more the surface area to volume ratio of the solder
powder particles and hence the higher their oxide content.
There is more outgassing which is caused by more oxide
reacting with flux in the reflow soldering process.
7.7.1.2 Solder Paste Volume Addition of solder paste
applied to lands on PCBs may result in more voiding in the
solder joints. This is because the more paste applied, the
more flux solvent is present, which leads to more outgas-
sing from reacting with solder powder oxide and solder
ball oxide. Solder paste volume will become more essential
to voiding generation since ball size is getting smaller, and
the ratio of paste volume to solder ball volume is getting
larger.
7.7.1.3 Component/PCB/Solder Paste Contamination/
Oxidation
Solder ball and land surface contamination
may lead to potential process voids generation in the SMT
process. The contamination may present a nonsolderable
surface on the PCB lands which may keep macrovoids
attached to the lands and may generate volatile gases at
soldering temperatures if the contamination decomposes.
Solder powder in the solder paste may oxidize if the paste
is exposed to air or humidity conditions. In the reflow pro-
cess, flux reacts with the oxidation layer of solder powder
and generates volatile gases, which form voids in solder
joints. Finer sized powder has more surface area and leads
to a potential of more outgassing in the SMT process.
BGA solder balls exposed to elevated temperature, humid-
ity condition or ambient temperature long enough develop
a layer of oxide crust formed outside of the solder balls.
This oxide crust can prevent voids from escaping from the
molten ball during reflow soldering.
Compared to solder ball oxidation, the oxide of the PCB
land surface does not impact voiding level solder joints
significantly. This may be because compared to solder
powder surface area, the land surface is much smaller and
flux may not react with oxide on the land surface com-
pletely.
7.7.1.4 Reflow Profile Reflow peak temperature and
time above liquidus (TAL) are two key parameters consid-
ered to impact voiding in reflow profile. Some manufactur-
ers believe that increasing peak temperature and prolonging
time above liquidus will let the voids in solder joints
expand and grow. However, the test results show increas-
ing peak temperature and prolonging time above liquidus
only bring limited impact to voiding level in solder joints.
For example, increasing peak temperature from 235°C to
250°C and TAL from 35 sec to 85 sec, the mean value of
voiding level only increased less than 1%.
There are two types of conventional oven profile, ramp
soak spike (RSS) and ramp to spike (RTS) profiles. The
RSS profile is recommended to control voids in solder
joints by some paste suppliers, because the soak stage may
allow the flux to function well to get rid of voiding. How-
ever, the RSS profile and RTS profile do not have a big
difference in voiding level. For example, if the RSS and
RTS with the same peak temperature and the same time
above liquidus (TAL) are used to assemble the BGAs,
using the same solder paste, the voiding level will be very
similar. The mean value of %voiding in solder joints is
(<1%).
7.7.1.5 Reflow Atmosphere (Nitrogen or Oxygen)
Atmospheres with lower oxygen content produce less
amount of oxidation of the solder ball and PCB surfaces
during the reflow soldering process. Less oxides will result
in less process voids since reduction of oxides creates
water vapor which is one source of macrovoids and lack of
an oxide crust on the other surface of the balls when mol-
ten; this oxide crust can prevent voids from escaping from
the molten ball during reflow soldering.
7.7.1.6 Multiple Reflow Cycles Sometimes, the same
BGAs will go through multiple reflow cycles due to being
located on double sided boards or due to rework. Voiding
level of BGA solder joints after multiple reflow cycles is
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much higher than that of solder joints after 1X reflow. For
example, the mean value of %voiding in solder joints after
3X reflow is about two times the %voiding level in solder
joints after 1X reflow. This is because the multiple reflow
cycles give extra time for existing voids to grow and
expand, or for small voids to coalesce together to form
bigger voids, when the solder joints are molten. This
growth and coalescence mechanisms have been confirmed
through analysis of 3D CT X-ray images before and after
each SMT reflow cycle.
7.7.1.7 Land Surface Finish The impact of land finish
type on process voiding is still not clear. But co-deposited
volatile organic compounds from surface finish plating
operations is believed to generate more process voids in
BGA solder joints. Although there has been much debate
on the impact of different land finishes, the voiding level of
these different finishes does not vary too much.
BGA voiding has become a focus in electronic industry due
to quality concern. For quality issue, some SMT process
parameters are essential to the voiding level in the BGA
solder joints. For example, if the component is exposed to
high humidity conditions or elevated temperatures, the sol-
der balls will have a layer of oxide crust, which could lead
to more voiding trapped in solder joints in reflow soldering
process. The other SMT process parameters which can sig-
nificantly impact voiding levels include multiple reflow
and solder paste formulation. Inspection of voiding levels
of BGA components is very important in order to under-
stand the percent, size and location of voids caused by the
SMT process.
There is no data showing a relationship between the BGA
void and reliability. Size of BGA voids is insufficient to
define acceptance criterion for fatigue life of the solder
joints. Macro process voids observed in X-ray images may
not cause any significant reliability impact, but microvoids
which cannot be seen in X-ray images may deleteriously
impact reliability. Large voids away from the solder joint to
component land interface do not impair the solder joint
fatigue life, but small voids at the interfaces can be
‘‘zipped’’ through once a crack starts. Location of voids
within the solder joint is as much, if not more, important
than void size.
7.7.2 Process Control Criteria for Voids in Solder
Balls
There is a continuing need for process development
and control to accommodate changing technologies. As
BGA land sizes, solder ball sizes and land pitch continue to
decrease, the dimensional parameters used on the produc-
tion floor need to change. New materials and processes
may be required to meet quality and reliability goals.
Voids will likely be encountered during various stages of
product life from development through manufacturing.
Maintaining a minimum acceptable standard is necessary to
assure that the product meets customer expectations,
product-life and reliability requirements. Manufacturers
need to use process control and continuous product
improvement techniques for void control. Readily available
statistical process control and process improvement tools
may be used.
A change in frequency and size of voids should indicate a
need for process control as well as improving the process
and materials. A baseline can be used to determine the need
for process adjustments to control the frequency and size of
voids. In addition, a void size limit could be established.
Size is determined in relationship to the ball. Thus a void
size larger than 25% of the solder ball’s cross-sectional
image diameter is approximately 6% of the total projected
area variation (see Figure 7-46). Any such process control
limits should be set with customer agreed-to contractual
commitment.
When there is more than one void per solder ball, the
dimensions of the voids will be added to calculate the total
voiding in that solder ball.
The equations for determining percentages are as follows
and all relate to the diameter of the ball or the contact sur-
face.
Void diameter percentage
Dvoid
Dball
x 100 = void percentage
Void Area
Contact or Ball Area
x 100 = void percentage
* Area is determined using the diameter and Pi (πr
2
).
Once the relationship of the void diameter to the ball image
diameter is known, the actual numbers and size relation-
ships become important. A 25% difference in diameters is
IPC-7095c-7-46
Figure 7-46 Example of Voided Area at Land and Board
Interface
Solder Outline
Void Outline
0.25 d
d
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not significant at the larger ball image sizes but becomes
more important when the land diameter shrinks to accom-
modate a smaller ball or smaller BGA pitch. This is shown
through the determination of the area connected to the land
after the void has been determined.
Image of a BGA ball 600 µm with a 25% void =
(Dvoid)
2
= 22500 µm
2
(Dball)
2
= 360000 µm
2
= 6.25% area taken up by the void.
Comparing area of ball image with area of void image
leaves 265073 µm
2
remaining (πr
2
).
Image of a BGA ball 300 µm with a 25% void =
(Dvoid)
2
= 5625 µm
2
(Dball)
2
= 90000 µm
2
= 6.25% area taken up by the void.
Comparing area of ball image with area of void image
leaves 66268 µm
2
remaining (πr
2
).
Appendix A provides recommendations for process
improvement regarding the occurrence of voids in the ball
at incoming (Type A and B voids) as well as those voids
that are discovered after the assembly has gone through the
attachment processes (Type C, D and E voids).
See Appendix A, Table A-1 to Table A-3 for corrective
action indicators.
7.7.3 Process Control Criteria The accept/reject criteria
for BGA assemblies is established by J-STD-001 and IPC-
A-610. Those documents provide the final accept/reject cri-
teria used in contractual agreements. The process correc-
tive actions identified in the following sections are intended
to establish continuous process improvement based on the
size of the voids, their location, and method of determina-
tion of the void occurrences.
Void clarification is also defined as to whether the voids
occur prior to attachment to the mounting structure, or after
the assembly has taken place. This useful information can
be correlated to reliability conditions based on the end-use
environment. Using the size limitation structure a process
can be established that helps to meet the customers’ defined
acceptability conditions (see Appendix A).
7.8 Solder Defects
7.8.1 Solder Bridging
Solder bridging is unacceptable.
Electrical testing, optical inspection (endoscope) or X-ray
inspection is necessary to detect solder bridging. Poor sol-
der paste printing, inaccurate placement, manual ‘‘tweak-
ing’’ after placement, and solder splattering during reflow
are typical causes of solder bridging. Solder balls too large
for the gap between the two substrates can also cause
bridging.
7.8.2 Cold Solder The reflow profile should reach tem-
peratures high enough to ensure that the solder melts com-
pletely and proper wetting of the land surface occurs. A
cold solder joint can reduce mechanical integrity and can
cause it to fail electrically or function intermittently. Opti-
cal inspection after cross-sectioning is the best way to
inspect for cold solder joints.
7.8.3 Opens Solder opens are also unacceptable. Electri-
cal testing, optical inspection (endoscope) or X-ray inspec-
tion is necessary to detect solder opens. Poor solder paste
printing, inaccurate placement and manual ‘tweaking’’ after
placement are typical assembly-related causes of solder
opens. Coplanarity and substrate solderability problems can
also cause opens. Excessive mechanical stress can also
cause solder joints to crack and create opens.
7.8.4 Insufficient/Uneven Heating A common process
problem is insufficient or uneven heating of the BGA. This
problem occurs more often during rework but can also be
seen in production when working with multilayer boards
with many ground or power planes. The problem can also
occur on double-sided boards when a shielded component
is on the backside near the location of the BGA. The prob-
lem results when a thermal conductor removes the heat
from the BGA before complete reflow can occur. The
X-ray image of this problem is characterized by a variation
in the size of the solder balls at different locations under
the package.
Insufficient heating is generally characterized in an X-ray
image by small partially reflowed solder balls in the center
or to one side of the package. Insufficient heating may also
be characterized by a jaggedness around the perimeter of
these solder balls; indicating that the solder partially
reflowed but not long enough to completely wet to the land
and collapse to a nice round ball. Misalignment of the sol-
der ball with respect to the land is also an indicator of
inadequate heating. The X-ray image of misalignment is
characterized by elongated solder balls which may or may
not have a consistent orientation.
X-ray inspection at a 45° angle is also a useful technique
to locate signatures associated with insufficient heating or
nonwetting. The solder ball should contact and completely
wet to the land forming a smooth pillar. Signatures associ-
ated with insufficient heating include incomplete wetting to
the land, or an image elongation in the solder connection
indicating that the solder ball and solder paste did not flow
together to form a single solder joint (see Figures 7-47 and
7-48).
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