16 Processing and Reliability of Solder Interconnections …
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PoP or PoPoP components—e.g., to minimize warpage—while also maintaining a
satisfactory yield for the solder joints of all other devices. In the event that there is a
suspicion that the PoP or PoPoP materials set and geometry causes the component
to be susceptible to excessive warpage or damage due to process peak temperatures,
hold times, and ramp rates, it is necessary to perform the “up-front” studies to understand these potential degradation mechanisms as part of developing a PWA soldering
process.
The one aspect that must be considered with respect to stack packages and the
PWA assembly process pertains specifically to the single-pass methodology. In this
case, the PWA process must also reflow, for the first time, the first-level solder joints
between individual packages. At the present time, alternative Pb-free solders may
be used for the first level interconnections that may differ from the second-level
solder ball compositions. The range of melting temperatures of these alternative
solders (Section A) must be understood in order to be assured that first-level solder
balls, or solder balls-plus-(dip) paste, will reflow during the second-level assembly
process. This point becomes particularly important in the case of high reliability
PWAs having second-level interconnections made with the traditional eutectic SnPb alloy. The reduced temperature profile may not be sufficient to melt the higher
Sn, first-level solder balls. Running a Sn-Pb second-level assembly process “hotter”
than normal to alleviate this discrepancy, poses a risk of thermal damage to the flux
as well as to other components on the PWA.
16.2.3.3 Cleaning Considerations
The consumer electronics industry uses processes that are based upon no-clean -or
low-residue tacky fluxes in the solder paste. These flux technologies are utilized for
the assembly of first-level interconnections as well as the final soldering step used
to make all second-level interconnections to the PCB. Therefore, cleaning processes
are not always required in this manufacturing environment.
On the other hand, PWAs for high-reliability systems are required to have flux
residues removed by a suitable cleaning process. This action avoids potential reliability concerns posed by electromigration or general corrosion mechanisms. Cleaning
processes are also recommended to establish pristine surfaces that will optimize both
the flow and adhesion of conformal coatings, encapsulants, and underfills.
By-and-large, stacked packages can accommodate a range of cleaning “systems”—defined as the cleaning materials and the equipment. The cleaning materials,
which are selected based on the type of flux residues, have no greater propensity
to attack PoP and PoPoP materials than they would degrade the more traditional
plastic encapsulated microcircuits. The gaps between individual packages do not
present a particular challenge to current cleaning systems. Because stacked packages have a higher profile than do BGAs, passive devices, etc., it is prudent to
confirm that high-pressure jets do not cause physical damage the first- or second-level
interconnections.
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PoP or PoPoP components—e.g., to minimize warpage—while also maintaining a
satisfactory yield for the solder joints of all other devices. In the event that there is a
suspicion that the PoP or PoPoP materials set and geometry causes the component
to be susceptible to excessive warpage or damage due to process peak temperatures,
hold times, and ramp rates, it is necessary to perform the “up-front” studies to understand these potential degradation mechanisms as part of developing a PWA soldering
process.
The one aspect that must be considered with respect to stack packages and the
PWA assembly process pertains specifically to the single-pass methodology. In this
case, the PWA process must also reflow, for the first time, the first-level solder joints
between individual packages. At the present time, alternative Pb-free solders may
be used for the first level interconnections that may differ from the second-level
solder ball compositions. The range of melting temperatures of these alternative
solders (Section A) must be understood in order to be assured that first-level solder
balls, or solder balls-plus-(dip) paste, will reflow during the second-level assembly
process. This point becomes particularly important in the case of high reliability
PWAs having second-level interconnections made with the traditional eutectic SnPb alloy. The reduced temperature profile may not be sufficient to melt the higher
Sn, first-level solder balls. Running a Sn-Pb second-level assembly process “hotter”
than normal to alleviate this discrepancy, poses a risk of thermal damage to the flux
as well as to other components on the PWA.
16.2.3.3 Cleaning Considerations
The consumer electronics industry uses processes that are based upon no-clean -or
low-residue tacky fluxes in the solder paste. These flux technologies are utilized for
the assembly of first-level interconnections as well as the final soldering step used
to make all second-level interconnections to the PCB. Therefore, cleaning processes
are not always required in this manufacturing environment.
On the other hand, PWAs for high-reliability systems are required to have flux
residues removed by a suitable cleaning process. This action avoids potential reliability concerns posed by electromigration or general corrosion mechanisms. Cleaning
processes are also recommended to establish pristine surfaces that will optimize both
the flow and adhesion of conformal coatings, encapsulants, and underfills.
By-and-large, stacked packages can accommodate a range of cleaning “systems”—defined as the cleaning materials and the equipment. The cleaning materials,
which are selected based on the type of flux residues, have no greater propensity
to attack PoP and PoPoP materials than they would degrade the more traditional
plastic encapsulated microcircuits. The gaps between individual packages do not
present a particular challenge to current cleaning systems. Because stacked packages have a higher profile than do BGAs, passive devices, etc., it is prudent to
confirm that high-pressure jets do not cause physical damage the first- or second-level
interconnections.