16 Processing and Reliability of Solder Interconnections …
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fillets are important as they reduce the residual stresses that can develop at the edge of
the packages after curing as well as under shock or temperature cycling environments.
An alternative approach is to fill all of the gaps at one time by a single pass of the
dispensing needle. The advantage is reduced process time. However, this technique
requires a greater control of the underfill material’s flow properties (e.g., temperature,
flow rate, etc.) because a larger quantity of material is being dispensed along the edge
of the stacked package. An uncontrolled dispensing process can quickly lead to voids
in the gaps, poor fillet development, and a “mess” of accumulated underfill material
on the PCB.
16.2.5.2 Conformal Coatings
The use of conformal coatings on stacked packages has not been extensively explored
in the electronics community. The primary application of conformal coatings is in
high-reliability electronics. When this point is coupled with the slow introduction of
PoP and PoPoP technology into military, space, and satellite system, there has been
only a limited need for such studies, that is, until very recently. Several resources
provide excellent reviews of the four basic types of conformal coatings: (1) acrylics,
(2) urethanes, (3) silicones, and (4) parylene, including attributes versus drawbacks,
material properties, and application methods [16, 17].
The primary concern is the extent to which the conformal coating fills the gaps
between the first- and second-level interconnections. Reliability degradation can arise
if the coating completely bridges the gap between the two packages, or between the
bottom package and the PCB. The high CTE value of conformal coatings vis-à-vis
the solder alloy, coupled with the confined geometry created by the bridged gap,
can result in significant residual stresses in the package and interconnections, alike.
At elevated temperatures, expansion of the conformal coating can push apart the
interconnections due to very high tensile loads. If the conformal coating simply flows
into the gap, but does not bridge it, the solder joints do not experience significant
residual stresses.
Fortunately, the gaps are sufficiently large in current PoP and PoPoP components
that the conformal coating does not completely fill them. This point is illustrated in
Fig. 16.9, which is a photograph of a PoP component having a urethane conformal
coating over it. The orange arrow shows the first-level solder joint gap; it is not filled
with coating material.
The selection of a conformal coating material is not particularly sensitive to a
presence of PoP or PoPoP components on the PWA from the assembly perspective,
unless there is the risk of bridging a gap. Then, parylene coatings are the proper
alternative because their thickness is limited to less than 125 μm and thus, would
not even the smallest gap. Otherwise, the introduction of underfills can mitigate the
potential risks associated with bridging the gap by a conformal coating.
The method of application must consider the higher profile of stack packages. For
example, dipping and hand brushing methods are not recommended because those
approaches can result in puddling around some of the package sides. Puddles lead to
489
fillets are important as they reduce the residual stresses that can develop at the edge of
the packages after curing as well as under shock or temperature cycling environments.
An alternative approach is to fill all of the gaps at one time by a single pass of the
dispensing needle. The advantage is reduced process time. However, this technique
requires a greater control of the underfill material’s flow properties (e.g., temperature,
flow rate, etc.) because a larger quantity of material is being dispensed along the edge
of the stacked package. An uncontrolled dispensing process can quickly lead to voids
in the gaps, poor fillet development, and a “mess” of accumulated underfill material
on the PCB.
16.2.5.2 Conformal Coatings
The use of conformal coatings on stacked packages has not been extensively explored
in the electronics community. The primary application of conformal coatings is in
high-reliability electronics. When this point is coupled with the slow introduction of
PoP and PoPoP technology into military, space, and satellite system, there has been
only a limited need for such studies, that is, until very recently. Several resources
provide excellent reviews of the four basic types of conformal coatings: (1) acrylics,
(2) urethanes, (3) silicones, and (4) parylene, including attributes versus drawbacks,
material properties, and application methods [16, 17].
The primary concern is the extent to which the conformal coating fills the gaps
between the first- and second-level interconnections. Reliability degradation can arise
if the coating completely bridges the gap between the two packages, or between the
bottom package and the PCB. The high CTE value of conformal coatings vis-à-vis
the solder alloy, coupled with the confined geometry created by the bridged gap,
can result in significant residual stresses in the package and interconnections, alike.
At elevated temperatures, expansion of the conformal coating can push apart the
interconnections due to very high tensile loads. If the conformal coating simply flows
into the gap, but does not bridge it, the solder joints do not experience significant
residual stresses.
Fortunately, the gaps are sufficiently large in current PoP and PoPoP components
that the conformal coating does not completely fill them. This point is illustrated in
Fig. 16.9, which is a photograph of a PoP component having a urethane conformal
coating over it. The orange arrow shows the first-level solder joint gap; it is not filled
with coating material.
The selection of a conformal coating material is not particularly sensitive to a
presence of PoP or PoPoP components on the PWA from the assembly perspective,
unless there is the risk of bridging a gap. Then, parylene coatings are the proper
alternative because their thickness is limited to less than 125 μm and thus, would
not even the smallest gap. Otherwise, the introduction of underfills can mitigate the
potential risks associated with bridging the gap by a conformal coating.
The method of application must consider the higher profile of stack packages. For
example, dipping and hand brushing methods are not recommended because those
approaches can result in puddling around some of the package sides. Puddles lead to