Magnetics Business & Technology - November/December 2020 - 16

TECH TIPS

Embedding Magnetics Achieves Reliability & Consistency, Part 2 of 3
By Jim Quilici, SCC America

Part 1 of this series provided an overview of embedded magnetic construction. Part 2 looks at the implementation of an Ethernet media filter with the embedded technology. Ethernet has
been the leading conduit for Local Area Networks (LANs) since
the late 1980s. It has evolved from 10 Mb/s data rates exceeding 100 Gb/s . Readers familiar with the technology know that
behind every wire-line Ethernet port reside 8 or more transformers. Their function is to couple the signal from the transceiver
IC to the twisted pair cable and filter noise. Figure 1 shows
a typical schematic for a Gigabit Ethernet (GbE) media filer.
GbE is the data speed available on most laptops and personal
computers.

Figure 1, GbE Media Filter Schematic
Historically, the transformers in these modules have been hand
wound and hand soldered into their respective packages. This
is amazing when one considers that over 500,000 Ethernet
ports are shipped each year. In recent years, automatic winding technology has advanced to the point where many of the
transformers in the lower speed Ethernet applications can be
wound automatically. Even still, dressing the wires and soldering them into the packaging is still mostly done manually. For
higher speed applications, like 5GbE and 10GbE, the windings
are often implemented using twisted wire to carefully control the
differential impedance of the wire. Twisted wire and the small
core sizes add complexity that defies automatic winding.

Given that the windings and interconnects are implemented
through photolithography, reliability issues associated with wire
insulation and solder are eliminated. Also, the RF performance
characteristics are defined by design and can be tested on a
sample lot basis. Testing merely requires probing with and LCR
meter to verify inductance, winding resistance and detect opens
and shorts. This can be done very effectively with a flying probe
or bed-of-nails tester, which is equipment commonly found on a
PCB fabrication line.
The schematic shows 4 data channels, each channel has a
Common Mode Choke (CMC) and Isolation transformer. The
isolation transformers are usually implemented with high permeability (≥ 5000) Manganese Zinc (MnZn) ferrite material. The
CMCs provide noise filtering and are generally implemented
with lower permeability (800 to 1500) Nickle Zinc (NiZn) material. Those familiar with these materials know that the permeability can be diminished when pressure forces are applied.
When the ferrites are embedded into the FR-4 substrate panel,
they are encapsulated with a low shrink epoxy to minimize
stress on the ferrites. The top surface of the panel is then
planarized (sanded flat) to provide a flat surface for lamination. After the ferrites cores are embedded into the panel, the
subsequent steps follow standard PCB processes. Copper foil
is laminated to the top and bottom surface of the panel. Lamination pressures can exceed 150 PSI, however as long as the
cores are encapsulated, they will be protected from the lamination pressure and there will not hold residual pressure once the
panels are removed from the press. Once copper foil is applied,
interconnecting vias are drilled and plated. Photolithography is
then used to image and etch windings on the top and bottom
surfaces. Solder mask or polyimide cover-lay is applied to provide voltage isolation between the conductive windings.
Figure 2 compares GbE media filters that are hand wound
and fabricated with embedded magnetics. As noted in part
1, embedded magnetics generally requires the use of larger
core elements, compared to the hand wound construction. A
close look at the photo gives one a sense of the difference in
core sizes. In this particular implementation, the transformers
are implemented on a 6.35 mm OD cores. The cores are too
large to allow all 8 transformers to fit within two PCB layers. To
match the footprint of the conventional wire-wound design, two
embedded magnetic boards are laminated together, each layer
containing 2 of the 4 channels.

Embedded magnetics eliminates most of the manual content.
Devices are fabricated with automatic and batch processes.
They are arrayed in a PCB panel format, so every time a technician handles a panel, they are touching hundreds of devices.

16

Magnetics Business & Technology * November/December 2020

Figure 2, Comparison of Wire Wound and Embedded Magnetics

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Magnetics Business & Technology - November/December 2020

Table of Contents for the Digital Edition of Magnetics Business & Technology - November/December 2020

Magnetics Business & Technology - November/December 2020
Editor’s Choice/ Features
Paragraf Strides toward Commercializing Graphene- Based Hall Effect Sensors for Highly Precise Magnetic Measurement
Magnetics-Based Turboexpander Enables Energy Recovery from Pressure Letdown Stations in Natural Gas Pipelines
Buildup of Littoral Naval Forces Brings New Attention to Underwater Magnetic Sensing
Tech Tips
Embedding Magnetics Achieves Reliability & Consistency, Part 2 of 3
Product News
Industry News
Events Calendar/ Advertising Index
Visions
Magnetics Business & Technology - November/December 2020 - Magnetics Business & Technology - November/December 2020
Magnetics Business & Technology - November/December 2020 - Cover2
Magnetics Business & Technology - November/December 2020 - 3
Magnetics Business & Technology - November/December 2020 - Editor’s Choice/ Features
Magnetics Business & Technology - November/December 2020 - 5
Magnetics Business & Technology - November/December 2020 - Paragraf Strides toward Commercializing Graphene- Based Hall Effect Sensors for Highly Precise Magnetic Measurement
Magnetics Business & Technology - November/December 2020 - 7
Magnetics Business & Technology - November/December 2020 - Magnetics-Based Turboexpander Enables Energy Recovery from Pressure Letdown Stations in Natural Gas Pipelines
Magnetics Business & Technology - November/December 2020 - 9
Magnetics Business & Technology - November/December 2020 - 10
Magnetics Business & Technology - November/December 2020 - 11
Magnetics Business & Technology - November/December 2020 - 12
Magnetics Business & Technology - November/December 2020 - 13
Magnetics Business & Technology - November/December 2020 - Buildup of Littoral Naval Forces Brings New Attention to Underwater Magnetic Sensing
Magnetics Business & Technology - November/December 2020 - 15
Magnetics Business & Technology - November/December 2020 - Embedding Magnetics Achieves Reliability & Consistency, Part 2 of 3
Magnetics Business & Technology - November/December 2020 - 17
Magnetics Business & Technology - November/December 2020 - 18
Magnetics Business & Technology - November/December 2020 - 19
Magnetics Business & Technology - November/December 2020 - Product News
Magnetics Business & Technology - November/December 2020 - 21
Magnetics Business & Technology - November/December 2020 - 22
Magnetics Business & Technology - November/December 2020 - 23
Magnetics Business & Technology - November/December 2020 - Industry News
Magnetics Business & Technology - November/December 2020 - 25
Magnetics Business & Technology - November/December 2020 - 26
Magnetics Business & Technology - November/December 2020 - 27
Magnetics Business & Technology - November/December 2020 - Events Calendar/ Advertising Index
Magnetics Business & Technology - November/December 2020 - 29
Magnetics Business & Technology - November/December 2020 - Visions
Magnetics Business & Technology - November/December 2020 - Cover3
Magnetics Business & Technology - November/December 2020 - Cover4
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