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The perfect integration of a product within
the customer’s assembly process can
be just as critical to the success of that
product as the electrical performance.
Mini-Circuits and many RF/microwave
component manufacturers build surface-
mount components that are soldered
onto customers’ PC boards using a reflow
process. Following reflow, the board
must be cleaned to remove solder balls,
flux, salt deposits and other debris. While
a number of cleaning methods exist, the
industry – and especially manufacturers
of equipment for military and other hi-rel
applications – have gravitated towards
aqueous wash. A conformal coating is
then applied to protect the circuit from
moisture and other adverse environmental
conditions. This process poses unique
challenges for the integration of surface
mount parts into customer assemblies,
particularly components with open and
semi-sealed case styles.
This article presents a proprietary
technique Mini-Circuits has developed
to adapt non-sealed surface mount
components for customer assembly
processes, preserving high reliability after
aqueous wash and conformal coating.
Reliability Risks of Open and Semi-Sealed
Surface-Mount Parts
Mini-Circuits’ surface-mount parts may
be categorized into three basic package
types:
Plastic encapsulated MMIC devices.
Examples include QFN and SOT-89 case
styles. A semiconductor die is wirebonded
to bonding pads and sealed in a plastic
enclosure.
Figure 1: plastic encapsulated MMIC
devices.
Ceramic, hermetically sealed cavity
packages for hi-rel requirements. MAC-
series mixers, for example incorporate a
semiconductor die on GaAs wirebonded
to baluns embedded in an LTCC multi-
layer substrate. The die is hermetically
sealed in a controlled nitrogen
atmosphere.
Open or semi-sealed PCB component
assemblies such as VCOs, mixers, filters
and diplexers. Circuitry is either fully
exposed or shielded in a non-sealed case
or “can”.
Plastic encapsulated and hermetically
sealed components are generally well-
suited to hi-rel assembly processes
without any special adaptations. Open
and semi-sealed components, however,
are susceptible to performance shifts and
even catastrophic failure when exposed
to the conditions of aqueous wash and
conformal coating processes. During
aqueous wash, components are fully
immersed in de-ionized water at high
temperature which can ingress into the
unit package and become trapped. After
aqueous wash, moisture along with any
contaminants trapped inside the case of
semi-sealed units can lead to dendritic
growth, shorting, corrosion and a variety
of other known reliability risks.
Open component assemblies are less
problematic than semi-sealed assemblies
with respect to trapped moisture, but
for some more sensitive open structures
such as filters and diplexers, the
customer’s conformal coating can change
Adapting Non-Sealed Surface Mount Parts for
Hi-Rel Customer Assembly Processes
Jonid Xhakollari, Manufacturing Process Engineer, Mini-Circuits
38 l New-Tech Magazine Europe