IPC-7095C-2013.pdf - 第140页
The dynamic warpage of the PCB and/or FCBGA stack also leads to varying shapes of solder joints that are accept- able from a quality standpoint. Figure 8-14 illustrates examples of acceptable solder joints. Most BGA sold…
mismatch creates a generalized ‘‘bi-metallic strip’’ effect
when the BGA package is heated and cooled during the
SMT Reflow Soldering process. This effect creates a
‘‘dynamic’’ warpage of the package. The FCBGA package
may become concave in shape, with negative (-) warpage,
when heated to the SMT reflow temperature, above 220°C,
but then become less warped when cooled down after exit-
ing the reflow soldering oven. Similarly, the PCB also
exhibits dynamic warpage behavior and may go from a
being relatively flat to becoming convex or concave (as
shown in Figure 8-12).
The FCBGA package and board warpage can affect the
solder joint formation. Various solder joint defects can
occur during SMT reflow soldering due to the increase in
PCB and/or BGA stack warpage, under un-optimized SMT
process. These include Head-on-Pillow (HoP), Head-on-
Pillow Open which is similar to HoP but the solder ball
does not make any contact with the solder on the PCB
land, Non-wet Open (NWO) where there is no solder on
the PCB land and Solder Bridging. Figure 8-13 depicts the
case of a severely warped PCB and BGA stack causing
these solder joint defects across the ball array of the pack-
age. SMT process optimized solutions are listed in Section
9.4.
Figure 8-11 Missing Solder Ball
IPC-7095c-8-12
Figure 8-12 Example of Dynamic Warpage of Flip Chip BGA Packages and PCBs
At Room Temperature / Before
Reflow Soldering
PCB
Relatively Flat
PCB
Positive (+) Warpage
Convex in Shape
PCB
Negative (-) Warpage
Concave in Shape
When Heated to Reflow Temperatures
(OR)
FCBGA Package
Positive (+) Warpage
Convex in Shape
FCBGA Package
Negative (-) Warpage
Concave in Shape
FCBGA Package
Negative (-) Warpage
Concave in Shape
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IPC-7095c-8-13
Figure 8-13 Example of a Severely Warped BGA Package and PCB After Reflow Soldering in an Un-Optimized SMT Process
Die
Package Substrate
Board
Non-Wet Open
(NWO)
Head-on-Pillow
(HoP)
Solder bridging Head-on-Pillow
(HoP) Open
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The dynamic warpage of the PCB and/or FCBGA stack
also leads to varying shapes of solder joints that are accept-
able from a quality standpoint. Figure 8-14 illustrates
examples of acceptable solder joints. Most BGA solder
joints are convex solder joints, since the smallest angle (in
blue in the top left photo of Figure 8-14) of the solder joint
surface tangent with respect to either pad surface is ≤90° at
all points along the solder joint surface.
PCB and/or BGA stack dynamic warpage can lead to some
solder joints being stretched into a columnar shape. Figure
8-15 shows an acceptable columnar solder joint. For
columnar solder joints, the solder joint surface is normal to
either side of the PCB land.
8.4.7 Failure Signature-6: Mechanical Failure Mechani-
cal stress caused by board flexing from in-circuit testing is
not uncommon in PCB assembly. As the size of the BGA
becomes larger, the stress experienced by corner joints
becomes more significant. Even probing beneath and sur-
rounding the BGA is required. Mechanical stress caused by
probe pins and vacuum force is sometimes ignored. It is
important to note that excessive monotonic stress induced
by mechanical stresses will lead to solder joint failures.
Since the weakest interface is the one that breaks, this fail-
ure signature could be different. The crack could be within
the BGA ball or at the PCB or package interface or within
the PCB as a lifted land (pad ‘‘cratering’’). Figure 8-16
shows two examples of a lifted corner land caused by
excessive mechanical stress. This defect is also termed as
pad cratering.
Pad cratering can result in a failure of the joint due to elec-
trical opens. The initial crack weakens the joint mechani-
cally. As it progresses, the crack may intercept electrical
traces causing an open. This is shown in Figure 8-17. Fail-
ure by this signature is enhanced by the higher tempera-
tures of lead-free reflow temperatures, harder laminates,
and increasing land or trace densities.
The robustness of the BGA joints against mechanical stress
is a function of several factors:
• Location of the BGA
• Thickness of the PCB
• Stack Up
• Land Size
Figure 8-14 Examples of Acceptable Convex Solder Joints with Solder Joint Surface Tangents Shown in the Top Left Photo
Figure 8-15 Example of an Acceptable Columnar Solder
Joint
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• Stiffening Mechanism
• Solder Volume
• Solder Mechanical Properties
• Solder Creep Properties
• Quality of solder Joint Interfaces
As a remedy, some designers especially in the cell phone
industry had adopted larger corner lands, elongated lands,
and underfill to enhance robustness. From the assembly
perspective, enforcing proper fixturing and handling are
keys to avoid solder joint damage.
8.4.8 Failure Signature-7: Insufficient Reflow This fail-
ure signature occurs when the BGA ball does not receive
sufficient heat for the solder to reach a temperature above
liquidus temperature of the solder. Insufficiently reflowed
solder joints, when they occur, are typically found under-
neath the center portion of the BGAs, because those areas
heat the slowest during soldering process, and would be the
most prone to not reaching adequate soldering tempera-
tures.
Sometimes, insufficient reflowed solder joints occur due to
the existence of a feature on a component, such as a cam
on a socket, depriving the heat from that area during the
reflow process. Figure 8-18 below illustrates an example of
insufficiently melted solder joints due to this reason.
8.5 Critical Factors to Impact Reliability
8.5.1 Package Technology
Area array components
come in a variety of styles and materials. The majority of
the commercial array devices utilize plastic encapsulation
and a reinforced rigid organic substrate interposer material
for packaging. For package-to-board interconnect, a metal-
lized land or alloy sphere (ball) is employed. Land grid
array (LGA) packaged ICs are often specified when pack-
age height is an issue, while the ball grid array (BGA) uses
small alloy spheres for the interconnect system. The con-
tact alloys furnished on the majority of the plastic based
BGA is a Sn/Pb (eutectic) or a Sn/Ag/Cu (lead-free) com-
position. Area array packages using a ceramic based sub-
strate interposer may be supplied with high-lead solder ball
or solder column, e.g., Pb90/Sn10. A growing number of
area array package variations adapt nonreinforced film
dielectric for the substrate and a diverse combination of
encapsulation materials. Miniature fine-pitch (FBGA) and
die-size package (DSP) are also widely used (especially in
portable or hand-held electronic products) and many higher
power applications incorporate an in-package heat spreader
or heat-spreading layer (see Section 4).
Long-term reliability of the solder attachment of the area
array package, when soldered to a conventional printed
board, is of primary concern. The difference in the
Figure 8-16 Two Examples of Pad Cratering (Located at
Corner of BGA)
Figure 8-17 Pad Crater Under 1.0 mm Pitch Lead-Free Sol-
der Ball. Crack in Metal Trace Connected to the Land is
Clear; However, the Pad Crater is Difficult to See in Bright
Field Microscopy.
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Provided by IHS under license with IPC
Not for Resale
No reproduction or networking permitted without license from IHS
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