16 Processing and Reliability of Solder Interconnections …
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apart. Solder joints are particularly weak in tension, with premature failures often
occurring in the IMC layer.
Underfill materials are expected to fill the gap. Therefore, they are designed
to have a CTE value that closely matches that of the solder alloy so that the two
expand/contract to the same degree upon heating/cooling temperature cycles. In
order to achieve this goal, underfills combine a filler material, typically alumina or
silica particles, with the organic matrix. The effect is to lower the CTE value to one
similar to the solder. The tradeoff is an increase of elastic modulus, which causes the
underfill to be less compliant and thus, less able to relieve residual stresses.
16.3.3 Reliability Studies
16.3.3.1 Mechanical Shock and Vibration
As noted above, the early implementation of stacked packages took place in the
consumer electronics community. The PoP and PoPoP components provided a means
to further miniaturize hand-held products while, at the same time, increasing their
functionality. As a “lesson-learned” from the introduction of BGA packages, dropshock was the foremost reliability concern for stacked components in these products.
The evidence to this effect came in the form of the multiple investigations, two of
which are cited in the reference section [28, 29].
The solder interconnections of stacked packages are more susceptible to damage
in mechanical shock and vibration environments than are those of other surface
mount devices. Stacked packages have a greater mass and higher center-of-gravity
than those latter packages, which places more loads particularly so on the secondlevel (bottom) solder joints made to the PCB. Encapsulation may be necessary to
mitigate the detrimental effects caused by shock and vibration. Interestingly, PoP
and PoPoP components have relatively modest footprints so that board bending has
a lesser effect on the solder joints than it would for the larger area-array packages.
Solder alloys and surface finishes are the two materials whose physical and
mechanical properties generally apply across the wide range of assembly geometries, attachment methodologies, as well as shock and vibration environments. Their
effects are discussed, below.
Solder Alloy Effects
As use of stacked packages and, primarily PoP, grew with time, so too were there
changes in supporting technologies that affect their reliability. New Pb-free solders
were developed to replace the SAC305, SAC396, etc. alloys (which had replaced
eutectic Sn–Pb solder) in order to improve drop shock performance of area-array
packages, which included stacked packages. Those new solders were the low-Ag
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apart. Solder joints are particularly weak in tension, with premature failures often
occurring in the IMC layer.
Underfill materials are expected to fill the gap. Therefore, they are designed
to have a CTE value that closely matches that of the solder alloy so that the two
expand/contract to the same degree upon heating/cooling temperature cycles. In
order to achieve this goal, underfills combine a filler material, typically alumina or
silica particles, with the organic matrix. The effect is to lower the CTE value to one
similar to the solder. The tradeoff is an increase of elastic modulus, which causes the
underfill to be less compliant and thus, less able to relieve residual stresses.
16.3.3 Reliability Studies
16.3.3.1 Mechanical Shock and Vibration
As noted above, the early implementation of stacked packages took place in the
consumer electronics community. The PoP and PoPoP components provided a means
to further miniaturize hand-held products while, at the same time, increasing their
functionality. As a “lesson-learned” from the introduction of BGA packages, dropshock was the foremost reliability concern for stacked components in these products.
The evidence to this effect came in the form of the multiple investigations, two of
which are cited in the reference section [28, 29].
The solder interconnections of stacked packages are more susceptible to damage
in mechanical shock and vibration environments than are those of other surface
mount devices. Stacked packages have a greater mass and higher center-of-gravity
than those latter packages, which places more loads particularly so on the secondlevel (bottom) solder joints made to the PCB. Encapsulation may be necessary to
mitigate the detrimental effects caused by shock and vibration. Interestingly, PoP
and PoPoP components have relatively modest footprints so that board bending has
a lesser effect on the solder joints than it would for the larger area-array packages.
Solder alloys and surface finishes are the two materials whose physical and
mechanical properties generally apply across the wide range of assembly geometries, attachment methodologies, as well as shock and vibration environments. Their
effects are discussed, below.
Solder Alloy Effects
As use of stacked packages and, primarily PoP, grew with time, so too were there
changes in supporting technologies that affect their reliability. New Pb-free solders
were developed to replace the SAC305, SAC396, etc. alloys (which had replaced
eutectic Sn–Pb solder) in order to improve drop shock performance of area-array
packages, which included stacked packages. Those new solders were the low-Ag