16 Processing and Reliability of Solder Interconnections …
497
The use conditions should include any next-assembly, transportation and handling
environments.
The complex construction of stacked packages and the multiple materials
(including underfills) used in them, requires that a careful survey be made of their
properties and thus, possible limitations, vis-a-vis the test parameters. This precaution
will prevent an over-testing event, which generates failure modes that are irrelevant to
the actual wear-out behavior of the interconnection structures. The consequence can
be a lower-than-realistic reliability of the solder joints due to a high rate of premature
failures.
Otherwise, in general, current aging test practices, whether for temperature
cycling, vibration, or mechanical shock assessment, are suitable for PWAs having
PoP and PoPoP components. In the case of temperature cycling and the use of daisy
chain links, there is an added complexity of monitoring some, or all, of the first- and
second-level interconnections in a stacked package. Although the current suppliers
of “dummy” components offer several varieties of daisy chain patterns, all signals
must eventually pass through the second-level interconnections. If the latter fail first,
there is no information gained on the longevity of the first-level interconnections.
This scenario exemplifies the need for careful, post-test failure analysis to determine
the source of the electrical open.
16.3.2 Underfill, Conformal Coatings, and Encapsulants
When considering long-term reliability, conformal coatings, encapsulants, and underfills can have a role that is active or passive in nature. Encapsulating foams and underfills actively mitigate against the effects of severe vibration and shock by mechanically anchoring the individual packages to one-another and to the PCB. Conformal
coatings perform this role, but to a lesser degree because they are present only as
thin layers that do not completely cover the sides of, or fill the gaps between, the
individual packages.
A photograph was shown in Fig. 16.9 in which a PoP component had been coated
with a 125 μm thick urethane layer. The method of application was spraying. The
coating was present on the top surface of the upper package and on the bottom fillet of
the underfill. The coating was discontinuous between these two locations. In fact, the
coating did not cross the gap between the top and bottom packages (orange arrow).
Therefore, in the case of high-profile stacked packages, conformal coating provides
minimal, if any, additional mechanical attachment function.
16.3.2.1 Materials Properties
On the other hand, under temperature cycling conditions, all three of these materials
can potentially affect solder joint reliability. The material properties that determine
the magnitude of this effect are:
497
The use conditions should include any next-assembly, transportation and handling
environments.
The complex construction of stacked packages and the multiple materials
(including underfills) used in them, requires that a careful survey be made of their
properties and thus, possible limitations, vis-a-vis the test parameters. This precaution
will prevent an over-testing event, which generates failure modes that are irrelevant to
the actual wear-out behavior of the interconnection structures. The consequence can
be a lower-than-realistic reliability of the solder joints due to a high rate of premature
failures.
Otherwise, in general, current aging test practices, whether for temperature
cycling, vibration, or mechanical shock assessment, are suitable for PWAs having
PoP and PoPoP components. In the case of temperature cycling and the use of daisy
chain links, there is an added complexity of monitoring some, or all, of the first- and
second-level interconnections in a stacked package. Although the current suppliers
of “dummy” components offer several varieties of daisy chain patterns, all signals
must eventually pass through the second-level interconnections. If the latter fail first,
there is no information gained on the longevity of the first-level interconnections.
This scenario exemplifies the need for careful, post-test failure analysis to determine
the source of the electrical open.
16.3.2 Underfill, Conformal Coatings, and Encapsulants
When considering long-term reliability, conformal coatings, encapsulants, and underfills can have a role that is active or passive in nature. Encapsulating foams and underfills actively mitigate against the effects of severe vibration and shock by mechanically anchoring the individual packages to one-another and to the PCB. Conformal
coatings perform this role, but to a lesser degree because they are present only as
thin layers that do not completely cover the sides of, or fill the gaps between, the
individual packages.
A photograph was shown in Fig. 16.9 in which a PoP component had been coated
with a 125 μm thick urethane layer. The method of application was spraying. The
coating was present on the top surface of the upper package and on the bottom fillet of
the underfill. The coating was discontinuous between these two locations. In fact, the
coating did not cross the gap between the top and bottom packages (orange arrow).
Therefore, in the case of high-profile stacked packages, conformal coating provides
minimal, if any, additional mechanical attachment function.
16.3.2.1 Materials Properties
On the other hand, under temperature cycling conditions, all three of these materials
can potentially affect solder joint reliability. The material properties that determine
the magnitude of this effect are: