16 Processing and Reliability of Solder Interconnections …
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4. The top package is located on the bottom package. Although the flux has a high
degree of tackiness, care must still be taken to prevent excessive movement of
the top package that could lead to solder joint defects.
The steps (1)–(3) would simply be repeated one more time for the PoPoP case.
The selection of a dip flux versus dip paste is based upon several factors. The dip
flux is used when additional solder metal is not required for the finer pitch, first-level
interconnections. The dip flux is preferred in the event that warpage causes the two
packages to displace closer together, which increases the risk of bridge defects due to
excessive solder metal. The dip paste, which provides additional solder to the joints,
is used to compensate for warpage that separates the two packages from one-another.
The latter phenomenon leads to a solder joint open.
16.2.3 Assembly Methodologies
16.2.3.1 Stacked Packages
There are two general assembly processes for stacked packaging technology. The
first process assembles the individual packages into a PoP or PoPoP component,
first, using multiple solder reflow steps. This method is often referred to as the twopass process for PoP components, or three-pass process for PoPoP components.
Returning to Fig. 16.4, the unit illustrated in Fig. 16.4d would next be placed into
a reflow process to form the first-level interconnections. Steps (1)–(4) would be
repeated, including another reflow step, to create the PoPoP component. Then, the
now-assembled PoP or PoPoP component would be pick-and-placed on to the PCB
with other surface mount components and passed through a final reflow process to
create the second-level interconnections.
The two- or three-pass process is not the preferred approach for the following
reasons: First of all, the multiple exposures to soldering temperatures increases the
likelihood of, not only damage to the internal structures of individual packages (e.g.,
die attach, passivation layers, etc.), but also increases the likelihood of warpage that
can cause solder joint open or bridging defects. Secondly, the stacked package is
more susceptible to handling damage in the tray as well as during the pick-and-place
process. Third, the multiple reflow steps lengthen the manufacturing time.
The second and preferred process is illustrated in Fig. 16.5. It is referred to as
the single-pass process and is represented here with the dip flux approach. The same
procedure applies to a dip paste. The steps (1) through (4) are the same as in Fig. 16.4.
The exception is that the bottom package has already been placed on the PCB with
printed solder paste to create the second-level interconnections. The remaining steps
are described below:
5. The PoP assembly is placed into the reflow furnace to make both the first-level
(top package-to-bottom package) and second-level (bottom package-to-PCB)
solder joints—hence, the term, single-pass process.
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4. The top package is located on the bottom package. Although the flux has a high
degree of tackiness, care must still be taken to prevent excessive movement of
the top package that could lead to solder joint defects.
The steps (1)–(3) would simply be repeated one more time for the PoPoP case.
The selection of a dip flux versus dip paste is based upon several factors. The dip
flux is used when additional solder metal is not required for the finer pitch, first-level
interconnections. The dip flux is preferred in the event that warpage causes the two
packages to displace closer together, which increases the risk of bridge defects due to
excessive solder metal. The dip paste, which provides additional solder to the joints,
is used to compensate for warpage that separates the two packages from one-another.
The latter phenomenon leads to a solder joint open.
16.2.3 Assembly Methodologies
16.2.3.1 Stacked Packages
There are two general assembly processes for stacked packaging technology. The
first process assembles the individual packages into a PoP or PoPoP component,
first, using multiple solder reflow steps. This method is often referred to as the twopass process for PoP components, or three-pass process for PoPoP components.
Returning to Fig. 16.4, the unit illustrated in Fig. 16.4d would next be placed into
a reflow process to form the first-level interconnections. Steps (1)–(4) would be
repeated, including another reflow step, to create the PoPoP component. Then, the
now-assembled PoP or PoPoP component would be pick-and-placed on to the PCB
with other surface mount components and passed through a final reflow process to
create the second-level interconnections.
The two- or three-pass process is not the preferred approach for the following
reasons: First of all, the multiple exposures to soldering temperatures increases the
likelihood of, not only damage to the internal structures of individual packages (e.g.,
die attach, passivation layers, etc.), but also increases the likelihood of warpage that
can cause solder joint open or bridging defects. Secondly, the stacked package is
more susceptible to handling damage in the tray as well as during the pick-and-place
process. Third, the multiple reflow steps lengthen the manufacturing time.
The second and preferred process is illustrated in Fig. 16.5. It is referred to as
the single-pass process and is represented here with the dip flux approach. The same
procedure applies to a dip paste. The steps (1) through (4) are the same as in Fig. 16.4.
The exception is that the bottom package has already been placed on the PCB with
printed solder paste to create the second-level interconnections. The remaining steps
are described below:
5. The PoP assembly is placed into the reflow furnace to make both the first-level
(top package-to-bottom package) and second-level (bottom package-to-PCB)
solder joints—hence, the term, single-pass process.