Technical Paper

Embedded Magnetics Technology Overview

Jim Quilici Radial Electronics

Abstract

Ethernet application use 8 of these small

Inductors and transformers are basic building

transformers to interface the transceiver silicon to

blocks in electronic systems. Their functions

the transmission cable. Typically these devices are

include energy storage, signal f iltering, voltage

integrated into the Ethernet RJ45 connector or

isolation, and impedance matching. Applications

packaged in a discrete module that is sits on the

range from power conversion, communications,

system board between the connector and the

and electronic sensors. Embedded magnetics

transceiver. The transformers provide the functions

provides

fabricating

of voltage isolation, impedance matching and

transformers and inductors. Ferrite elements are

f iltering. To date, these small transformers have

embedded into an FR4 substrate and the device

def ied automation. While they can be made cheaply

windings are realized using printed circuits

with off shore labor, the performance and reliability

techniques. In comparison, the vast majority of

can vary greatly due to manual assembly.

a

new

approach

for

small transformers and inductors, used today in communications and power conversion, wound manually one at a time or by using semi-automatic equipment. Manual soldering and assembly is often employed to install these devices into their respective packages. The advantage of the embedded approach is that it uses a batch assembly process that is highly automated. With automation comes improved consistency and r e l i a b i l i ty. T h i s p a p e r d e s c r i b e s t h e d e v i c e composition and fabrication on a printed circuit line.

18

Background

Figure 1 Wound toroid transformer

Magnetic components are used in all types of

In most applications, the magnetic components are

communication and RF systems. Transformers

used in conjunction with an Integrated Circuits (IC).

provide voltage isolation and impedance matching

Over the years, the silicon industry has worked to

while inductors are used for energy storage and in

bring failure rates below the range of 50 parts per

f ilter circuits. In many cases, these components are

million (ppm). In comparison, a manually wound

wound on small cores made of ferrite or other

component typically has failure rates in the range

magnetic materials. The toroid shape is the most

of 1000 ppm. The high failure rate is primarily

eff icient for establishing a magnetic reluctance

attributed to nicks and kinks in the wire that occur

path through the core material and transferring

during winding. The f ine gauge wires can also

electromagnetic energy between the windings. The

become brittle and stressed when the wound

small size of many RF transformers and inductors

component is installed and soldered into its

has def ied automation and generally these devices

respective package. Consistency is another issue.

are hand wound. Figure 1, show a typical

Wound transformers and inductors have secondary

transformer used for LAN communications. The

circuit elements; such as inter-winding capacitance,

Journal of the HKPCA / Issue No. 44 / 2012/ Q2

Technical Paper

leakage inductance and winding resistance. These

Blind Vias

Imaged Windings

secondary "parasitic" elements often determine key performance parameters, such as bandwidth, Conductor Edge 3D Circuit

insertion loss and return loss of a device. With manual construction, the wires are manually dressed around the magnetic core. It is diff icult to

Ferrite Core

control the parasitic elements in a consistent manner. Performance variations can be signif icant, depending on the person who wound the device or

Plastic Substrate

Epoxy Fill

Figure 3 Cross Section View, Blind Via (BV) Construction

even the time during a work-shift, in which the Of the two, PTH is the more straight forward

device was fabricated.

approach. Winding layers are disposed on the top

Construction

and bottom surface of the substrate and vias are

Embedded magnetics can provide consistency and

mechanically drilled to interconnect the layers.

reliability unmatched by manual construction.

Given a required inductance, the designer begins

Rather than hand winding each device, RF

by selecting the ferrite core material and geometry.

transformers and inductors are fabricated using

Ferrite cores come in many shapes and sizes, yet

printed circuit technology. This allows devices to be

the toroid is the most eff icient shape and is

manufactured using a high degree of automation

available in a variety of diameters, from multiple

and in batch processes. The windings are def ined

manufacturers. To begin the design, the designer

by

must consider the window area of the core and the

p h o t o l i t h o g r a p h y,

making

them

highly

number of vias that can be f it within that area.

repeatable.

Another consideration is the height of the core and There are actually a couple of alternatives for

the aspect ratio of the interconnecting vias. Most

producing the embedded magnetic device, each

PCB subcontractors can produce vias with 1:12

having varying degrees of complexity and cost. The

aspect ratios. If the designer plans to use a 0.25mm

differences are primarily in the fabrication of the

(0.010") via, the height of the inserted core will be

windings and the vias used to connect the top and

limited to about 3.05mm (0.120") in order to keep

bottom

basic

the aspect ratio of the via within a practical limit.

classif ication is associated with the vias. Either

Often the designer is trying to achieve a specif ic

plated-through-hole (PTH) or blind-vias (BV) may

inductance, which is a function of the core size and

be employed. Figures 2 and 3 depict cross sections

number of windings. So some analysis is required to

of the two construction techniques.

determine the optimum core size and via design.

winding

layers.

The

most

Imaged Windings

Once the windings have been designed and artwork created, substrates may be prepared. For PTH, this Ferrite Core

can be achieved by simply routing cavities into FR4 sheet stock. The ferrite cores are inserted into the cavities and encapsulated with epoxy. Ferrite materials are sensitive to the shrinkage forces

FR-4 Substrate

Plated Through Hole Vias

imposed by the epoxy, which will reduce their 1

Epoxy Fill

Figure 2 Cross Section View, Plated Through Hole (PTH) Construction

p e r m e a b i l i ty . Fe r r i t e m a t e r i a l s w i t h h i g h e r permeability (>5,000) are more sensitive to epoxy stress and exhibit greater degradation. Low perm

www.hkpca.org

19

Technical Paper

materials, which are commonly used for f ilters and

100,000 vias. The drill time is signif icant and is the

power applications, are less affected by epoxy

dominant factor for determining the product cost.

shrinkage. The point is that the designer must account for the reduction in permeability when

With the blind via approach, one has the option of

selecting the core size. This may require winding

using either laser drilling or mechanical drilling.

some samples and characterizing their reduction in

Lasers can drill blind vias order or magnitude faster

inductance with different epoxy encapsulates.

than mechanical drills. Yet even with mechanical

There are a wide variety of epoxy encapsulates that

drilling the depth is only a few mills and drilling

are suitable for the application. A low shrink epoxy

time is much lower than the PTH design. Blind vias

should be selected. The designer should also

can also be implemented at a smaller diameter and

consider

expansion

higher density than the PTH. The challenge with the

coeff icient and the devices process and operation

BVA process, however, is in producing the bottom

temperatures. Generally, the more stable materials

half windings. This requires implementation of a 3-

are loaded with silica or another f iller material. This

dimensional (3D) circuit. There are a number of

reduces the shrinkage and expansion coeff icient,

ways in which the 3D windings can be fabricated.

making the construction more stable under thermal

One method utilizes molded plastic substrates. The

gradients. In the case of the PTH construction, the

substrate should be fabricated from a high

material should also have hardness suff icient for

temperature polymer, like Liquid Crystal Polymer

drilling the center via array. After the cores have

(LCP) or Nylon 66. Substrates can also be milled

been inserted and the epoxy encapsulation is cured,

from FR4 or G10 sheet stock. The substrates are

the panel should be planarized, cleaned and

plated with 1/2 oz copper and coated with an

prepped for lamination.

electro-deposited (ED) photo-resist. The cavity can

the

material's

thermal

then be imaged with either contact or projection For transformers, voltage isolation is often a key

photolithography. Depending on the depth of the

requirement. FR4 has a withstanding voltage

cavity, there will be some diffraction as the light

>13kV/mm (500 V/mil). For telecommunications, a

enters and images the cavity sidewalls and bottom

transformer usually needs to withstand over

surface.

1500Vrms of voltage stress. A laminate thickness of 0.10 mm (0.004") is generally suff icient to provide

This requires that the artwork be compensated to

the required isolation. FR4 pre-preg and foil are

correct for the diffraction and that the light source

applied to both the top and bottom surface. Vias

be highly collimated. In Radial's development work,

are then drilled, plated and the panels are then

we have found that the sidewalls of the cavity need

imaged and etched with the winding array. The

to have a slope greater than 10

benef it of this method is that it that it readily f its

suff iciently expose the cavity sidewalls. Imaging

within the capabilities of most PCB shops. The

has been successful down to a depth of 2mm

primary limitation is the aspect ratio of the vias.

(0.080").

O

in order

This dictates the via diameter and number of windings that may be applied within the core

Another method for fabricating the 3D circuit is to

window, for a specif ic core thickness. Also, with

cover the plated substrate with an etch-resist

high density designs, the drilling time and cost may

material and image the 3D surface with laser

be signif icant. If a transformer design has 24

ablation.

windings (48 vias) and is arrayed across a 609mm

imaged the 3D cavity using

(24") square panel, there can easily be over

plating, as the etch resist. The tin flash can then be

1

20

For etch resist, Radial has successfully

permeability is the measure of the ability of a material to support the formation of a magnetic within itself

Journal of the HKPCA / Issue No. 44 / 2012/ Q2

5 µ m of tin flash

Technical Paper

selectively ablated (removed) with a diode pumped

designer should select an epoxy with low a low

laser.

A spray etching system is then used to

expansion coeff icient. An advantage of the BVA

r e m o v e t h e e x p o s e d c o p p e r, w h i l e c o p p e r

approach is that the ferrite cores are inserted

underlying the tin flash remains intact. An

around a center pedestal. This reduces the

alternative is to use a polymer etch resist and Co2

encapsulating material to just a thin f ilm between

laser for the ablation. A simple spray enamel is

the ferrite core and cavity walls. The epoxy volume

suff icient as an etch resist. One has to be careful to

is much lower, compared to the PTH design. Since

use enamel that can withstand subsequent process

vias are not being machined into the f iller

temperatures

encapsulate,

and

the

devices

operating

it

is

practical

to

use

softer

temperatures. Otherwise, the etch resist will need

encapsulates. Softer materials apply less shrinkage

to be stripped after etching the exposed copper.

stress and buffer the ferrite cores from stress

Again, a spray etching system is used to remove the

forces

exposed copper. Figure 4a and 4b show 3D circuit's

coeff icients. In general, less encapsulate results in

fabricated using ED photo-resist and laser ablation.

a more stable design under thermal transitions and

caused

by

mismatches

in

thermal

less degradation of the ferrite permeability. Once the 3D circuit is imaged, the ferrite core can be loaded into the cavity and encapsulated with a

Once the cores are embedded in the 3D circuit and

low shrink epoxy. As with the PHT design, the

the top lamination is applied, the windings can be imaged, etched tied together with blind micro vias. Again, def ining the windings with photolithography produces consistent and reliable performance. The conductor traces can be shaped, to minimize secondary parameters like winding resistance, leakage inductance and inter-winding capacitance. Figure 5 shows a completed PTH device. This device is an Ethernet media f ilter and contains 8 transformer elements stacked in two layers. For comparison, the internal construction of a conventional hand wound device is shown. Close inspection of the photo will show that the winding

Figure 4a Imaged 3D circuit

pattern on the embedded magnetic transformer is physically

larger

than

the

hand-wound

counterparts. While automation provides much eff iciency, it also has its constraints. Windings applied to the embedded magnetic core must adhere to conventional PCB design rules. There are specif ic limitations for line width and spacing, which

must

be

followed

for

consistent

photolithography and etching. Similarly, there are practical limitations associated with drilling and plating vias. Imaging a 3D circuit also has its own set of limitations. In the end, one can always cram more wire around ferrite core with manual assembly, Figure 4b Laser ablated 3D circuit

compared

the

automated

PCB

process.

www.hkpca.org

21

Technical Paper

Consequently, embedded magnetics generally

ultimately def ines the inductance and the low

requires the designer use larger cores to achieve a

frequency

specif ic inductance value, compared to the hand-

performance is determined by all of the secondary

wound counterparts. Larger cores can have a slight

characteristics of the transformer, such as winding

cost premium, yet this is usually outweighed by the

resistance, leakage inductance, intertwining

eff iciencies of automation. The issue usually comes

capacitance, etc. These secondary parameters are

when displacing a hand wound component and

def ined by how the conductors are wound around

f itting the design into an established footprint and

the core. Circuit windings can be carefully crafted

package form-factor. When size is a constraint, the

to manage the capacitance and AC impedance of

designer usually has the option to stack and array

the windings. Leakage inductance can be controlled

the embedded magnetic devices on multiple PCB

by assuring that the windings cover the core and

layers.

minimize gaps, where electromagnetic energy may

cut-off

point.

High

frequency

escape. Inter-winding capacitance can be controlled by the material selection

and

spacing

of

the

conductors and vias. Similar to designing a high frequency PCB, all of these attributes can be modeled and optimized on an RF simulator. Before

committing

the

PCB

fabrication, performance can be evaluated at both the device and network level. Many applications Figure 5 Gigabit Ethernet Media Filter. Bottom view of a conventional device and one realized with embedded magnetics

require more than one transformer

or inductor, in the circuit network. It is practical for

Performance

the designer to model the interactions between

Consistency is one of the primary benef its of embedded magnetics. Once the circuit artwork is set; it can be step and repeated across the PCB panel. This level of repeatability cannot be matched with manual construction. The biggest variable will be the inductance per turn (A L) of the ferrite core material. This value can vary as much as ± 20% and will impact the device's open circuit inductance (OCL). If required, one can work with the ferrite supplier to get the cores graded to a tighter tolerance. The inductance primarily impacts low frequency performance. In most instances, the critical operation is at the higher frequency, at which the performance is primarily determined by the AC impedance of the windings and the circuit artwork. Figure 6 shows the schematic and frequency response of a typical transformer. If one thinks of a transformer as a band pass f ilter, the A L

22

Journal of the HKPCA / Issue No. 44 / 2012/ Q2

Figure 6 Transformer Equivalent Circuit and Typical Frequency Response

Technical Paper

different winding and other components in the

expect, the two channels on the bottom layer have

device network. The model can also be used to

shorter interconnects and exhibit better return loss

evaluate the device's performance on the system

performance. The measured curves are consistent

board. This is a big advancement from the status

with results from the RF simulation.

quo. Historically, it has not been practical to

Frequency, Hz

accurately model wound components, due to the

0.00 1.00E+06 -0.50

winding inconsistency.

1.00E+07

1.00E+08

1.00E+09

-1.00

Gigabit Spec Limit

-1.50

Stitching all of the windings together with vias may raise some concerns of discontinuities in the signal path. Test data shows that this is not an issue. Insertion loss and return loss data for the Gigabit Ethernet media f ilter is show in f igure 7. The plots

EM Channel 1

-2.00 -2.50

EM Channel 2

-3.00

EM Channel 3

-3.50

Em Channel 4

-4.00 -4.50

Insertion Loss, dB

-5.00

show 4 channels of data plotted against the industry spec limit. In the insertion loss plots, the

Frequency, Hz

pass band is smooth and doesn't exhibit the peaks

0 1.00E+06 -5

and valleys that would indicate discontinuity or impedance mismatch. Return loss is a primary circuit's characteristic impedance. In this case the

1.00E+09 Gigabit Spec Limit

-20

EM Channel 1

-25

EM Channel 2

-30

Ethernet media f ilter, the transformer network

-35

should have a 100 Ù impedance. The test data

-40

shows a return loss is less than -20dB out to

-50

Ethernet application. The media f ilter is a 4 channel

1.00E+08

-15

indication of the how well the device matches the

125MHz, which is more than suff icient for the

1.00E+07

-10

-45

EM Channel 3 EM Channel 4

Return Loss, dB

Figure 7 Insertion Loss & Return Loss Data for a Gigabit Ethernet module

device, implemented with 2 channels on 2 layers. In the return loss plot, differences in the different

Reliability

curve shapes are attributed to differences in the

Reliability testing has only been completed on pre-

circuit layout on the two different layers and the

production

interconnection to the I/O pads. As one would

accordance with standard requirements for

Test Name

Method

Conditions

Thermal Cycling Highly Accelerated Stress Test (HAST) Autoclave, Pressure Cook + Humidity

Mil-Std-883, method 1010 JEDEC Spec 22-A110 JEDEC JESD22-A102

Low Temperature Storage High temperature Storage Solder Reflow

Mil-Std-810 method 502.4 Mil-Std-810 method 501.4 JEDEC-J-STD-20

-40 C to 100 C, 20 min dwell time O 85 C, 85% relative humidity (RH) O 121 C, 15PSI for 96 O hours then 85 C, 85% RH O Storage -55 C unbiased O Storage 150 C unbiased O Max. Temp. 255 C O 265 C, 20 second dwell time 1500 Vrms

Hipot

O

O

samples.

Te s t i n g

was

done

Duration / Frequency 100 cycles

Sample Size 10

Pass

50 Hrs. 100 Hrs. 200 Hrs. 500 hours

10

Pass

10

Pass

100 Hrs.

10

Pass

100 Hrs.

10

Pass

3 passes

9

Pass

60 sec.

6

Pass

in

Result

Table 1 Embedded Magnetics Development Reliability Tests

www.hkpca.org

23

Technical Paper

computer and data communication equipment. Each Ethernet media f ilter has 4 channels. Prior to

5. Relatively low lead times that are consistent with PCB technology.

testing, the OCL of each channel on each sample was measured and recorded. After stresses testing,

Finally, the product is essentially a PCB, which

the OCL was tested again. Failures occur then there

allows vertical integration of other passive and

is a large drop in the OCL, an open or short circuit.

active components. Vertical integration can be used

All devices passed the stress testing, indicating

to actually reduce the overall system footprint. It

that the embedded magnetic components have

also provides short interconnects between devices,

reliability consistent with standard PCB technology.

which is benef icial for high frequency performance

Further testing is required to determine the stress

and minimizing I R loss in power circuits. Whether

parameters that cause failure.

building a device from hand wound components or

2

embedded magnetics, the material cost is very

Conclusion

s i m i l a r. T h e m a i n a d va n t a g e s o f e m b e d d e d

Embedded magnetics provides an automated and

magnetics come from reduced labor content, lower

batch process for fabricating small transformers

overhead,

and inductors. In the case of the Ethernet media

performance.

f ilter, a 609mm (24") square PCB panel format can easily accommodate over 800 devices. Rather than building one part at a time, they can be fabricated in batches. Automation and the PCB format allow manufacturers to reduce labor content, improve eff iciency at test and assembly. As demonstrated by the test data, embedded magnetic

provide

a

practical

method

for

implementing inductors and transformers. The fabrication uses standard process steps, is scalable and can be

transferred between different

manufacturing

locations.

While

this

paper

d e m o n s t ra t e s t h e E t h e r n e t a p p l i c a t i o n , t h e technology can be employed in other data communication and power applications. The following is a summary of salient benef its: 1. Automated batch process that allows the devices to be fabricated on a PCB assembly line with a fraction of the labor content used to make conventional devices. 2. Flexible design that can be tailored to a specif ic applications through circuit design and material selection. 3. Consistent performance that can be modeled and optimized. 4. Reliability consistent with PCB technology.

24

Journal of the HKPCA / Issue No. 44 / 2012/ Q2

high

yields

and

the

consistent