484
P. Vianco and M. Neilsen
The post-soldering cleanliness requirements may affect the choice of single-pass
versus two-pass (PoP) or three-pass (PoPoP) assembly methodologies. The singlepass approach is preferred because both first- and second-level solder joints are made
in a single reflow step. The tacky flux or tacky paste has a short residence time on
the parts prior to reflow so that after reflow, residue removal is relatively easy for the
subsequent cleaning step. Conversely, the multiple pass process requires that flux
residues be cleaned from the individual stacked components shortly after making
each set of first-level interconnections. Otherwise, there is the possibility of a longtime interval developing between making the first-level interconnections and the
second-level reflow soldering step. The flux residues become more tenacious, which
increases the difficulty of removing them in the post-reflow (second-level assembly)
cleaning step.
A second challenge to removing flux residues at the stacked package level is the
need for special fixtures to hold the individual PoP or PoPoP units in a multiplepass process. Trays or other fixture methodologies must secure the components to
prevent damage during the cleaning machine. Otherwise, the package structures and,
in particular, the bottom package solder balls, become susceptible to damage or loss,
which reduces subsequent assembly yields at the PWA level.
16.2.3.4 Rework
The difficulty of performing a manual assembly of PoP and PoPoP parts predisposes
rework operations to only high-reliability products. The rework of stacked package
components was discussed by McCormick et al. [13]. A critical finding from that
work was that the existing package cannot be readily salvaged, whether the defect
is traced to a first-level or second-level interconnection. Overall miniaturization,
together with the fine pitch of both first- and second-level interconnections, simply
adds too much risk of ancillary degradation to the individual package structures to
warrant their re-use except in the most extenuating circumstances (e.g., prohibitively
high component cost).
The premise that rework leads to the scrapping the pre-existing package(s) places a
significant cost burden on stacked package use on high-reliability electronics because
piece-parts typically have very high unit values. High unit costs are due to short-run
(low-volume) manufacturing contracts and the need for extensive testing, particularly
when up-screening test regimens are applied to COTS components. Therefore, the
best approach to this scenario is simply to optimize process development up to firstarticle production. This method assures the process engineer of acceptable assembly
yields the high-reliability assembly.
P. Vianco and M. Neilsen
The post-soldering cleanliness requirements may affect the choice of single-pass
versus two-pass (PoP) or three-pass (PoPoP) assembly methodologies. The singlepass approach is preferred because both first- and second-level solder joints are made
in a single reflow step. The tacky flux or tacky paste has a short residence time on
the parts prior to reflow so that after reflow, residue removal is relatively easy for the
subsequent cleaning step. Conversely, the multiple pass process requires that flux
residues be cleaned from the individual stacked components shortly after making
each set of first-level interconnections. Otherwise, there is the possibility of a longtime interval developing between making the first-level interconnections and the
second-level reflow soldering step. The flux residues become more tenacious, which
increases the difficulty of removing them in the post-reflow (second-level assembly)
cleaning step.
A second challenge to removing flux residues at the stacked package level is the
need for special fixtures to hold the individual PoP or PoPoP units in a multiplepass process. Trays or other fixture methodologies must secure the components to
prevent damage during the cleaning machine. Otherwise, the package structures and,
in particular, the bottom package solder balls, become susceptible to damage or loss,
which reduces subsequent assembly yields at the PWA level.
16.2.3.4 Rework
The difficulty of performing a manual assembly of PoP and PoPoP parts predisposes
rework operations to only high-reliability products. The rework of stacked package
components was discussed by McCormick et al. [13]. A critical finding from that
work was that the existing package cannot be readily salvaged, whether the defect
is traced to a first-level or second-level interconnection. Overall miniaturization,
together with the fine pitch of both first- and second-level interconnections, simply
adds too much risk of ancillary degradation to the individual package structures to
warrant their re-use except in the most extenuating circumstances (e.g., prohibitively
high component cost).
The premise that rework leads to the scrapping the pre-existing package(s) places a
significant cost burden on stacked package use on high-reliability electronics because
piece-parts typically have very high unit values. High unit costs are due to short-run
(low-volume) manufacturing contracts and the need for extensive testing, particularly
when up-screening test regimens are applied to COTS components. Therefore, the
best approach to this scenario is simply to optimize process development up to firstarticle production. This method assures the process engineer of acceptable assembly
yields the high-reliability assembly.