NOVEMBER 2025 I DESIGN007 MAGAZINE 41
Via Filling
Filled vias can improve
routing density (Figure 3)
and board assembly, and
aid with electrical and ther-
mal performance. Via fill
should only be used if abso-
lutely required. The bene-
fits of via-in-pad technology
include increased capture
pad, solder mask clarity/
registration, and elimina-
tion of "pad cratering."
There are two types of
via fill in common use—
conductive and non-conductive—and care must be
taken to assure the correct fit to the application as
both have pros and cons.
• Conductive fill: Copper filling ensures electrical
continuity and mechanical strength in stacked
vias. Microvias may be plated shut, filled sepa-
rately, or filled at the same time as the through-
holes (if applicable), which is dependent on
the type of bath and rectifiers utilized.
• Non-conductive fill: Epoxy-based via fills
are later overplated and planarized for thick-
ness and flatness control, and for component
mounting. This is not a preferred process
over copper filling from a reliability stand-
point and the epoxy fill of microvias must be
processed in a special vacuum chamber to
prevent air entrapment.
Conclusion
UHDI advances beyond traditional HDI by integrating
ultra-thin dielectrics, low-loss substrates, fine copper
F i g u re 2 : M i c rov i a c o m p a r i s o n .
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foils, stacked microvia structures, and embedded
component technologies. These innovations deliver
exceptional interconnect density, superior high-fre-
quency performance, and highly compact form fac-
tors. As the push for smaller, faster, and more inte-
grated systems continues, progress in UHDI materi-
als and layer architectures will play a pivotal role in
shaping the future of electronic packaging. DESIGN007
Anaya Vardya is president
and CEO of American
Standard Circuits. He is
co-author of The Printed
Circuit Designer's Guide
to…Fundamentals of RF/
Microwave PCBs and Flex
and Rigid-Flex Fundamentals.
He is the author of Thermal Management: A Fabri-
cator's Perspective, The Printed Circuit Designer's
Guide to DFM Essentials, and The Companion Guide
to Flex and Rigid-Flex Fundamentals.
F i g u re 3 : M i c rov i a i m p rove d ro u t i n g d e n s i t y.
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