Technical Paper

Journal of the HKPCA / Issue No. 38 / 2010/ Q468

Active Optical Flex Module for Ultra-ShortActive Optical Flex Module for Ultra-ShortDistance Data ApplicationsDistance Data Applications

Active Optical Flex Module for Ultra-ShortActive Optical Flex Module for Ultra-ShortDistance Data ApplicationsDistance Data Applications

Abstract

In this paper, we present active optical flex module

aimed for mobile device applications. The module

utilizes flexible optical waveguides on electrical

printed circuit board for data links between high-

speed interface devices, such as application

processor-to-camera or display module. In this

study, flexible and rigid-flex optical electrical

circuit boards are used in product emulator

designed for in-device optical data links with

aggregate data rate up to 120Gbps (12-ch x

10Gbps).

This paper describes design and fabrication of OE-

R/F HDI engine boards with flexible optical

interconnect FPCs (OE-FPC); optical coupling

m e t h o d s ; c h i p e m b e d d i n g a n d a s s e m b l y

procedures; optical engines with 850-nm vertical

cavity surface emitting lasers, photo-detectors, and

multichannel integrated ICs. Characterization

results are given on optical waveguides, I/O

couplers, transceiver units, thermal simulations

and on selected l i fe-t ime performance and

r e l i a b i l i t y t e s t s . L o w - l o s s a c r y l a t e - b a s e d

waveguides (~ 0.05dB/cm at 850nm) are used for

OFPCs. Eff icient light in/out coupling structures

including integrated and embedded 45 mirrors are

characterized to meet the target link budget of 10-

15dB at 5-10Gbps data rate with bit-error-rate (BER)

of 10-12. With the chip set used in the test vehicle,

the power consumption is approx. 1350mW for the

bi-directional 12-channel 10Gbit/s/ch optical link

( T x + R x ) . T h i s i s 1 1 2 . 5 m W p e r l i n k o r

11.25mW/Gbit/s/link. Bending loss measurements

shows that the optical slide flex can be bent down

to 2mm with insignif icant increase in loss.

Repeatable folding tests indicate of high bend

O

M.Immonen* , J.Wu, P.Chen, Y.H.Luo, H.He, W.Huang, M.Ma, J.X.Xu, T.Rapala-VirtanenMEHK, a member of TTM Technology Inc

Active Optical Flex Module for Ultra-ShortDistance Data Applications

endurance and stability. These results show that

our active optical flex module could replace

electrical slide or hinge flex and provide high-speed

data links free of electromagnetic noise inside

typical mobile device conf igurations.

Optical PCBs, optical FPCs, optical

waveguides, mobile devices, smart phones

Mobile devices, especially smart phones, have

become increasingly feature rich and computing

capable devices. They are equipped with megapixel

cameras, high-resolution displays, audio I/Os; they

support high-speed internet, high-def inition videos,

3D graphics and gaming, video calling, mobile TV

and wide array of RF functions e.g. 3G mobile

broadband, Wi-Fi, Bluetooth, WLAN, GPS, and GSM

(Fig. 1) [1]. Applications are run simultaneously

and in multiple windows. These features demand

high computing power and data throughput inside

and in/out of the devices. To respond the increasing

computing needs and bandwidth requirements,

mobile phone processors have become available

with clock rates of 11.5GHz and above [2].

Flexible printed circuits (FPC) are used in electrical

devices to connect functional modules in locations

where flexible or bendable interconnects are

advantageous. For instance, FPC through the

connection hinge is used to connect upper block of

a foldable mobile phone (with a display part) to

lower block (with a host processor i.e. applications

engine or modem). Conventionally, 30 to 60 or

more signals are transferred in parallel traces

through the hinge. At high frequencies, copper

traces suffer from emissions and electromagnetic

Keywords:

Introduction

www.hkpca.org 69

Technical Paper

interference (EMI), particularly as degraded signal

integrity and crosstalk. FPCs that are long (typ. up

to 25cm) and thin (typ. 0.15-0.2mm thick)

structures, pose challenges to the EMI design.

Moreover, structural impacts, e.g. irregular shape,

increases EMI problems and unstable signal

integrity [3].

Industry a l l iances, notably Mobi le Industry

Processor Interface (MIPI) [4], are working to

replace parallel bus interfaces in mobile phones by

high-speed serial links (Fig. 2). Serial data links

offer reduced pin count, number of transmission

lines, suppressed interference and lower-power

consumption. The supported data rate, for instance,

at the MIPI Camera Serial Interface 2 (CSI-2) and

D-PHY is 800Mb/s to 1Gpbs per lane, and is scalable

up to 4Gbit/s with four lanes [4].

Multi-gigabit data links for future display and

camera modu les a re in t roduc ing spec t rum

components that overlap cellular frequencies. They

are prone to EM coupling to/from positioning and

mobile antennas located in close proximity.

Figure 1. Explosion of functions and technology development

in handheld devices [1].

Figure 2. High Speed interfaces in mobile devices with

indications to selected MIPI Serial interface specifications.

Also, connector emissions lower signal quality and

impedance control [5]. Another concerns result

f r om con t i nues m in i a t u r i z a t i on , i n c r ea sed

component density, and number of high-speed chip

packages as increasing thermal management

challenges.

For that, another solution under investigation is use

of optical interconnects for inter-module connections

inside mobile devices. Potential of immunity to

e lect ro-magnet ic in ter ference, lower power

consumption per bandwidth (mW/Gbps), and low

heat dissipation are crit ical factors in high

performance computer applications, but also, key

performance metrics in portable devices with limited

source power and form-factor constrained packages.

Recent publications on optical data links for mobile

handhelds propose concepts are based on polymer

optical f ibers (POF) [6,7], optical waveguides on

FPC (O-FPCs, OE-FPCs, Rigid/Flex PCBs) [8-10],

optical waveguides part of sliding covers [11], and

active cables with optoelectronic devices [12].

Although f ibers can provide reliable and mature

solution, they suffer from limited bending radius

(typ. > 15mm) and need for fairly large connectors,

limiting their use at small device applications.

Recent studies provide results on propagation and

bending loss [8,12], high-speed signal transmission

characteristics [8-11], bending, sliding and folding

endurance [8,9], and optical link design [7].

Prev ious resul ts have been obta ined using

evaluation test vehicles that poorly mimic actual

product designs. Particularly, feasible interconnect

schemes to integrate optical FPCs for a high-speed

interface, has not been reported. Moreover, little

attention has been paid on system-level reliability

tests e.g. lead-free reflow soldering, thermal shock,

Technical Paper

Journal of the HKPCA / Issue No. 38 / 2010/ Q470

or mechanical shock loading conducted according

to IPC/JEDEC specif ications set for the mobile

handsets.

In our previous works, we showed results

integration of optical waveguides and out-of-plane

couplers on low-cost electrical bases [13]. Through

collaboration, we demonstrated optical full-duplex

4-channel 10Gbit/s data link embedded on rigid-

PCB with surface mounted optoelectronic Tx/Rx-

modules for high frequency CPUs with optical off-

c h i p I /Os i n h i gh pe r fo rmance compu te r

applications [14].

In this paper, we show results on development of

active optical flex module, which is a Rigid/Flex

high density interconnect (HDI) board with optical

waveguides and board embedded optical active

components. Flexible and rigid-flex (R/F) optical

electrical circuit boards are used in the high-end

smart phone product emulator designed for in-

device optical data links with aggregate data rate

up to 120Gbps (12-ch x 10Gbps). This paper

describes design and fabrication of OE-R/F HDI

engine boards with flexible optical interconnect

FPCs (OE-FPC); optical coupling methods; chip

embedding and assembly procedures; optical

engines with 850-nm vertical cavity surface

emitting lasers (VCSEL), photodetectors (PD), and

multichannel integrated ICs. Characterization

results are given on optical waveguides, I/O

couplers, transceiver units, thermal simulations

and on selected l i fe-t ime performance and

reliability tests. Further results on OE-R/F HDI

board assemblies, system reliability tests, coupling

analysis, verif ication of thermal modeling results

are reported in the conference and in the follow-on

papers.

The optical electrical Rigid/Flex high density

Experimental

interconnect (OE-R/F HDI) test vehicle (TV) used

as a mobile device product emulator is illustrated in

(Fig. 4). Photographs of the reference product are

shown in (Fig. 3).

The TV contains integrated and free-standing

opt i ca l in te rconnec ts rea l i zed us ing board

embedded polymer waveguides and optical flex

cables. Each test site contain optical engines with

up to 12 bi-directional (12+12) 10Gbps optical data

channels in 250 m pitch. The engines contain the

optoelectronic devices for O/E/O conversion and

optical ICs, i.e. vertical cavity surface emitting

laser (VCSEL), photodiode (PD), laser diode driver

(LDD), and transimpedance/l imit ing amplif ier

µ

31

a.

b.2

c.

d.

e.

f.g.

Figure 3. Reference product (1) engine board (bottom side),

(2) slide assembly with main interconnection FPC, (3)

lower enclosure. (a) Application processor, (b) 5Mp camera

module, (c) slide flex, (d) interconnection flex, e) VGA

camera module, (f), and (g) FPCs to LCD IC and digitizer [15].

Figure 4. Schematic of the OE-Rigid/Flex HDI test vehicle

with two rigid OE Modules (Board1 and 2), optical engine

test sites (1-5), and optical flex and R/F connection

portions (Opt1a- Opt5).

www.hkpca.org 71

Technical Paper

(TIA/LA) supplied from Philips and Iptronics.

Aggregate data rate in 12-ch arrays is 120Gbit/s or

40Gbit/s/mm. The concept is down-scalable for 1x1

or 1x4 arrays, but the current design can be used as

well for other mobile computing or portable

internet devices e.g. laptops or tablets.

The selected reference product represents a high-

end mobile device with state-of-the art technical

specif ications [15]. The reference was used to

def ine dimensions, board materials, stackup design,

FPC layering and approximate component locations,

and for specifying performance and life-time

requirements for the optical emulator.

Printed circuit board (PCB) layers of the OE-R/F HDI

TV board units are detailed in (Fig. 5). The flex part

is composed of an optical layer with a core layer

and surrounding claddings, an electrical layer with

a PI base f ilm covered by a Cu foils (flexible copper

clad laminate, FCCL). Integrated out-of-plane

micromirrors are fabricated for light beam path

conversion in/out of the waveguide cores. For the

optical layer, we used optical material with selected

proper t ies deta i led in Tab le 1. Wavegu ide

fabrication was carried out with the process

described in [8]. VCSEL and PD are butt-coupled to

optical waveguides without using micro-optical

components.

accelerated stress test and isothermal annealing

tests. Optical waveguide transmission loss, mirror

coupling eff iciency, and optical alignment tolerance

are measured using insertion loss measurement

and cut-back methods at 850 nm. The physical

characteristics (e.g. dimensions, surface roughness,

alignments) of the fabricated optical structures in

PCBs and FPCs are qual i f ied using opt ica l

microscopy, confocal laser scanning microscopy

(CLSM) and scanning electron microscopy (SEM).

Bending and folding endurance tests are executed

using separate test vehicles shown in (Fig. 6).

1

2

3

5

10

4

6

7

9

8

Figure 5. PCB layers in the optical electrical Rigid/Flex HDI

(OE R/F HDI). 1: Solder mask, 2: Copper, 3: FR4, 4: Contact

pad, 5: Optical waveguide layer, 6: FCCL, 7: prepreg, 8: FR4,

9: copper, 10: solder mask.

The l i fe-t ime performance and rel iabi l i ty is

investigated using bending and folding endurance

tes t , so lder re f low, therma l shock , h igh ly

Results

Prior concept demonstrator assembly, we qualif ied

the critical parts of the OE-R/F HDI of their physical

characteristics and performance. The cross-section

mic rographs o f the fabr i ca ted wavegu ides

embedded in rigid FR4 and PI/Cu FPC are shown in

(Fig. 7.a) and (7.b), respectively. Waveguide

channels with micro-mirrors and layer-to-layer

optical vias is shown in (Fig. 8).

To optimize the design, the waveguide core

line/space (L/S) parameters were varied of core

Figure 6. Test vehicles for life-time tests.

Optical waveguidesOE FPC

Mirrors

Copper wiring

VCSEL/

PD site

VCSEL/

PD site

2L FPC (reference)

Unit Core Clad Remarks

Tg C 120-130 ~150 DVE

Td C 350 320 in Air

CTE ppm/ C 50-70 50-70 a1

ppm/ C 140-170 140-170 a2

Elastic Modulus GPa ~3 ~3 RT

~0.02 ~0.02 Above Tg

Tensile Strength MPa 20-40 60-80 RT

Elongation % 2-3 5-10 RT

Refractive index - 1.586 1.551 830 nm [i]

Thickness m 50 30 and 72

O

O

O

O

µ

Notes: [i]: Prism coupling method

Table 1. Selected material properties.

Technical Paper

Journal of the HKPCA / Issue No. 38 / 2010/ Q472

FR4L/S 25/75

100 mµ

25 mµ

PI Copper

L/S 50/200

a)

b)

Figure 7. Optical waveguides embedded for OE-R/F HDI

board in a) rigid board b) flex FPC. Dimensions of the

waveguides: a) L/S 25/75 m, pitch 100 m (100Gbps/mm)

b) L/S 50/200 m, standard 250 m pitch (40Gbps/mm).

µ µ

µ µ

width from 25 to 70 m, and space from 30 to

200 m. Channel pitch varied from 62.5 m (high

density) to 250 m (standard pitch). Topographical

investigations using CLSM revealed uniform and

well-def ined waveguide channels (Fig. 9). Optical

layers were fabricated on PCBs with production

panel size of 508 x 457mm (20"x16"). All of the

optical layer processing steps (lamination, curing,

photodef inition, development) were conducted in

Class100 and Class1000 clean-room premises at

our manufacturing site in Shanghai.

Micrographs of the micro-mirrors fabricated as out-

of-plane beam couplers are shown in Fig. 10.

Critical mirror parameters, namely angle, flatness,

surface roughness and positional accuracy were

characterized. The measured mirror angle variation

was 45 1 . Average of the surface roughness (Ra)

was 75nm measured of a surface area of ~30 x

30 m . The results indicate of the process

capability to fabricate uniform flat planes usable for

micro-optical couplers. Yet, the peak-to-valley of

the surface roughness (R ) was in a range of few

hundreds of nanometers. With further optimizing,

we expect to reduce also Rz value well below

hundred nm.

The transmission loss of waveguides obtained using

µ

µ µ

µ

±

µ

O

2

O

z

the insertion loss measurement and the cut-back

method was 0.1-0.17dB/cm at 850nm (Fig. 12).

Insertion loss measurement setup is shown in (Fig.

11.a) Waveguides with mirror I/Os were measured

with the modif ied setup shown in (Fig 11.b). The

measurements were carried out using an 850nm

VCSEL source with a one meter-long-multimode

f iber (MMF) output aligned and butt-coupled to the

waveguide core. At the output, the light was

col lected with MMF and transmitted to the

photodetector connected with the power meter.

The measurement losses were approx. 4dB. These

include scattering losses, Fresnel back-reflection

losses, NA and geometrical mismatch losses, and

misalignment losses at interfaces. The measured

waveguide channels were rectangular 50 m x 50 m

of core size with NA=0.33. MMFs with 50/125 m

core/clad diameters and NA=0.2 were used at both

ends of the waveguides. In order to qualify

overf illed launch and collect conditions at the

measurement, MMF size and NA should better f it to

those of the waveguides. Also, the current

measurement results are obtained without index

matching. Use of index matching fluid at the in/out

coupling interfaces reduce scattering and Fresnel

losses, and thus signif icant ly improves the

accuracy and repeatability of the measurement,

especially, when characterizing channels with very

low losses. With the above modif ications in the

current measurement procedure, we expect to

report loss values < 0.1dB/cm.

The loss measurements for the samples with mirror

I/Os were not f inished by the time of the paper.

Based on the physical qualif ication results and

previous results [8,13], we expect mirror loss

values approx. 1-2dB.

Materials mechanical toughness to withstand

pressing conditions was evaluated of physical

cross-section samples, and by comparing the

µ µ

µ

www.hkpca.org 73

Technical Paper

measured IL values before and after pressing.

Optical layer sustained undamaged during the

pressing conditions (total time 185min, T =195 C,

P =2,4 MPa). Increase in IL was < 0.035dB/cm.

Peak

max

O

The results of the excess loss in -looped

waveguides are shown in (Fig. 13). The results by

the supplier indicate that with current waveguide

materials, the minimum bending radius is 2.0mm

with 0.5dB excess loss penalty. Since the OE-FPCs

are designed for dynamic interconnections inside a

phone, the optical FPC should possess excellent

flexural and sliding characteristics at the small

radius of curvature. Particularly, in slim slide phone

conf igurations specif ication for bending radius is

below 1mm. We aim to obtain R down to 1mm by

new core/clad design.

á

bend

45O

Gold

mirror

WG

core

Beam

pathOptical via

Optical via

Figure 8. Waveguide channels with micro-mirrors and filled

optical vias.

a) b)

Figure 9. CLSM 3D profiles of the waveguide channels.

Waveguide L/S a) 50/50 m, b) 70/30 m.µ µ

a) b)

Figure 10. a) CLSM 3D profile of a mirror facet,

b) SEM micrograph of the waveguides with micro-mirrors.

Figure 11. Measurement setup for transmission loss of a)

waveguides b) waveguides with 90 out-of-plane couplers.O

a)

b)

Figure 12. Transmission loss results at 850nm.

Figure. 13 Excess loss vs. bending radius at 850nm.

Results of the folding endurance tests are given in

Table 2. for the free-standing optical flex and OE-

FPC (FPC with copper and opt ica l layers),

respectively. In both tests, the insertion loss was

measured in intervals during the test using 850 nm

source.

Technical Paper

Journal of the HKPCA / Issue No. 38 / 2010/ Q474

Folding test conducted by the supplier was carried

out for the sample with the waveguides of length

L=50mm. Folding was angled from initial position

to 180 , with bending radius of 2mm and with the

cycle frequency of 60 times per minute. The test

was carried out at room temperature (T=23 C).

Variation in the insertion loss during the repetitive

folding of the optical flex up to 1 x 10 times was

measured to be less than 0.1dB. For the OE-FPC

with 12 m copper layer in contact with the optical

layer, the change in the IL was less than 0.25dB.

Physical damage e.g. microcracking, was not

observed during the tests for either design. These

results indicate of a very high bending endurable

for the optical material, but also for OE-FPC

structure with copper layer.

I t i s known, tha t tempera tu re con t ro l i n

optoelectronic devices is very critical to its

performance and reliability. Heating in a GaAs laser

limits the gain of the active medium, shifts the

output wavelength, affects the polarization state of

a mode, and shortens the laser lifetime. Since the

developed active optical flex module contains

active components embedded in the board, we

conducted thermal analysis to evaluate package

temperature and heat dissipation distribution

across the components during operation. A 3-D

model was constructed us ing CFD-software

(Flomerics Flotherm 8.2). The simulation was

carried out as steady-state worst-case analysis for

4L (2+W+2) OE-PCB containing embedded 1x12

VCSEL/PDs and LDD/TIA ICs and with 1x1 VCSEL

reference (Fig. 15). Different thermally enhancing

O

O

7

±

µ

±

structures, namely via matrixes, thermal interface

materials (TIM), Cu plate varying in volume and

geometry, were used as variables in the simulations.

Simulation results indicated that under natural

convection conditions, T of each device exceeds

max. allowable operation temperatures (Fig. 14.a).

Thermal via matrix alone did not provide enough

thermal conductivity (Fig. 14.b). With copper plane

in contact with the backside of the ICs, the

minimum thermal requirements were met for the

embedded optical engine unit (Fig 14.c). When the

IC backside is fully contacted by the thermal vias

reaching to the top layers of HDI package,

additional vias do not lower thermal resistance and

the copper plane geometry becomes a main

contributor to heat transfer. By increasing copper

volume, thermal performance was improved.

J

Table 2. Folding endurance test results.

Optical flex OE-FPC Remarks

Folding Shift in Shift in # of cycles:

("U"-bending) insertion loss: insertion loss: 1 000 000

< 0.1 dB < 0.25 dB Angle: 180

Radius:

No No R=2mm

destruction, destruction,

delamination delamination

± ±O

a)

TIA

LDDVCSEL

PD

Optical layer

12.45mm20mm

b)

TIA

LDDVCSEL

PD

Optical layer

Thermal vias

(20x13)

12.45mm 20mm

c)

TIA

LDD

VCSEL

PD

Optical layer

Cu plate

(20x13x0.1)

Figure 14. Thermal modeling of the embedded 12-ch x 10Gbps

optical engine with 850-nm VCSELs, PD, LDD and TIA.

Simulation results for a) no thermal structures, T = 133.9 C,

b) thermal via matrix, T = 121.0 C, c) Cu plate

T = 84 C.

J,VCSEL

J,VCSEL

J,VCSEL

O

O

O

www.hkpca.org 75

Technical Paper

With the chip set used in the current test vehicle,

the power consumption is approx. 1350 mW for the

bi-directional 12-channel 10Gbit/s/ch optical link

(12-ch Tx + 12-ch Rx, one-way) in typical operation

conditions. This converts 112.5mW per single

10Gbps link or 11.25mW/Gbit/s/link. The thermal

design and simulations were done for worst-case

scenarios with total power dissipation of 2200mW

(LDD: 1200mW with VCSEL, and TIA: 1200mW with

PD P for VCSEL is 360mW and for PD 120mW).

The power levels - up to 3.5W - are in line with

typical hand held application. Maximum junction

temperatures (T ) specif ied for VCSEL, PD and the

mated ICs (LDD and TIA) are 85 C, 100 and 95 ,

respectively. Due to most strict requirements, the

package design is done to f it the VCSEL thermal

requirements. Thermal conductivities (W/mK) used

f o r t h e m a t e r i a l s w e r e : F R 4 : 0 . 3 ; C u

traces/vias/planes: 385, waveguides: 0.2, TIM-A:

0.68, TIM-B: 0.29, Silicon: 118, and GaAs: 48. Full

results of the study will be published in the future.

We showed results on optical flex and rigid/flex

printed circuit boards developed for optical active

flex module aimed to provide optical data links

between high-speed interface devices, e.g. camera

or LCD module and application processor, inside a

mobile device.

Optical waveguides were successfully fabricated on

diss, max

J

O O OC C

Conclusions

polyimide copper flex PC (OE-FPC) and embedded

in conventional PCB board (OE-PCB). The measured

transmission loss of waveguide channels was

0.1-0.17dB/cm at 850nm. The minimum bending

radius of the optical waveguide flex was 2.0mm

with < 0.5dB excess loss. Optical layer sustained

undamaged under pressing conditions used for

multilayer OE-PCB fabrication, verif ied by increase

<0.035dB/cm in the insert ion loss. Folding

endurance tests showed that shift in the insertion

loss was < 0.1dB, and < 0.25dB, for optical flex

and for the OE-FPC with 12 m copper layer in

contact with the optical layer, respectively.

Thermal modeling results conducted for optical

act ive f lex module wi th embedded VCSEL,

photodiodes, and their mating optical ICs (LDD and

TIA), showed signif icant differences in thermal

resistances depending on copper volume and

thermal structures. The thermal design needs

further improvements to lower thermal resistance

of the package, especially, when also the electrical

components are taking into account.

We acknowledge Dr. Atsushi Takahashi at Hitachi

Chemical for the materials and characterizations, Dr.

Tingyun Wang at Shanghai University for the

waveguide loss measurements, and M.Sc. Juha

Karppinen at Helsinki Univ. of Technology for his

contr ibut ion for the thermal model ing and

simulations.

1. T.Rapala, "HDI PCBs: Application of Mobile

Devices - Trends and Applications",

, 20.2.2009.

2. Snapdragon QSD8X50 and QSD8672 Series

Chipsets, Technical Features, Qualcomm.

Retrived 31.5.2010.

3. I.Kelander , "Modeling for High-Speed

± ±

µ

Phys. Soc.

of Hong Kong

et al.

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