Magnetics Business & Technology - September/October 2020 - 14

TECH TIPS

Embedding Magnetics Achieves Reliability & Consistency, Part 1 of 3
By Jim Quilici, SCC America
Magnetic components are widely used in conjunction with
semiconductor devices. For telecommunication applications, transformers couple signals into transmission lines and
antennas.They provide impedance matching, level shifting,
voltage isolation and common mode filtering. In power applications, transformers and inductors are used for energy storage,
level shifting, isolation and noise filtering. Over decades, the
semiconductor industry has made continual improvements in
reliability, to the point where failure rates today are in the 1
part per million and are now trending towards parts per billion.
In contrast, magnetic components have lagged behind their
semiconductor counterparts and often exhibit failure rates in the
range of 50 to 1000 ppm. This fact often positions the magnetics as the "weak link" in system reliability.
When fabricating the small transformers and inductors used in
data communications, it is often difficult to employ automatic
winding. A large percentage of magnetic components coming
out of Southeast Asia are still manually wound. Such high level
of manual content introduces failures in the form of shorts due
to nicked wire insulation and opens due to kinked wires and
cold solder joints. Even when automatic winding is employed,
there is still a considerable amount of manual content required
to dress, terminate and solder the wires to the package pins.
Manual construction can also result in relatively large performance variance from device-to-device.

Figure 2, Embedded Magnetic Cross Section
In this construction, cavities are routed into a FR-4 (fiberglass)
substrate and ferrites cores are inserted and encapsulated
with a low shrink epoxy. After embedding the ferrite cores, the
fabrication follows standard PCB processes. Copper foil is
laminated to the top and bottom surfaces and interconnecting
vias are drilled and plated between the layers. The inductive
windings are then imaged and etched onto the top and bottom
surfaces. Using photolithography to define the windings assures accurate placement and highly consistent performance.
Secondary parameters; like winding resistance, inter-winding
capacitance, leakage inductance and voltage isolation, can be
managed by controlling the width and separation between the
conductors. The end product is essentially a solid state device
and has reliability consistent with a PCB. Table 1 summarizes
the features and benefits.
Table 1, Features and Benefits of Embedded Magnetics
Features
Benefits
Fabricated on a
Printed Circuit
Board line
Windings are
defined by
photolithography

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*
*
*
*
*
*
*

Vertical
Integration

Figure 1, Embedded Magnetic Transformers
Embedded magnetics advances the reliability and consistency of magnetic components. Figure 1 shows an example
of embedded magnetic transformers that have been step and
repeated on a PCB panel. Figure 2 depicts a cross section.

Magnetics Business & Technology *

September/October 2020

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*

Utilizes automated and batch processes
Portable process that leverages existing capital equipment
investments
Low labor content
Reliability consistent with PCB technology
Highly consistent, performance defined by circuit artwork
RF testing can be done on a lot sample basis
Conductors can be shaped and spaced to control resistance,
inductance, capacitance and voltage isolation
Performance can be accurately modeled and optimized with an RF
simulator
Multiple transformers and inductors can be arrayed and stacked to
minimize the device footprint
The magnetic device is essentially a PCB upon which other
passive and active surface mount devices can be placed

Of course, among these benefits there are also limitations. The
primary being that embedded magnetics generally requires
the use of larger ferrite core sizes, compared to wire-wound
construction. Figure 3 shows a section of the artwork for a
transformer. Inductance is determined by the ferrite material
properties and the number of windings that are applied. The
number of windings is limited by the "window area" of the core.
Placement of inter-connecting vias in the window-area requires
certain design rules to be followed for manufacturability. Via
hole diameters are determined by the aspect ratio (panel thickness/drill diameter) and what can be plated in volume produc-

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