Characterization of Ultra-Thin Flexible Ceramics for High ... · PDF fileCharacterization of...

PACKAGING RESEARCH IN ELECTRONIC ENERGY SYSTEMS

2016 Electronics Packaging Symposium & Workshop

Points of contact: [email protected], [email protected], [email protected]

Xin Zhao Bo Gao

Douglas C. Hopkins

North Carolina State University

Characterization of Ultra-Thin Flexible Ceramics for High-Density, 3D-Stackable Substrates

for Wearable Power Electronics

Presented at the 2016 Electronics Packaging Symp & Heterogeneous Integration Workshop, Binghamton, NY, Oct 6-7, 2016



Points of contact: [email protected], [email protected], [email protected]

Douglas C. Hopkins, Ph.D.Director – PREES Laboratory

FREEDM System CenterNorth Carolina State Univ.1791 Varsity Dr., Suite 100Raleigh, NC 27606-7571

[email protected]

www.prees.org

WelcomePackaging Research in Electronic Energy

Systems (PREES)

Feb 2016

CIRCUIT DESIGN BEYOND THE SCHEMATIC



Multiphysics Simulation, Test & Assembly

Heavy wire & ribbon (Al, Cu) bonding research with:

Hesse Mechatronics –

fully automated BJ939

Manual high-precision Pick ’n Place (≥0403 chips)

Torch-TP39V with video assist

Thermal Imaging Flir

Hand Soldering and Re-work station

3D Optical Profiler (1.7um resolution)ACS VisionMaster

Electrical circuit analysis, CAD layout, and thermo-mechanical analysis with:

COMSOL & ANSYS (MultiPhysics simulators),

AutoCad, SolidWorks, Q3D, PLECS, SPICE

Wet Bench for cleaning& “formulation”

High End Electrical Test:• Tektronix 371A Power Curve Tracer • Oscilloscopes (2GHz/5GS/s)• Arb Funct Gen., Multimeters (6.5 digit),

Power supplies (kV&HC),

• ≤500A & ≤40kV probes



Full Processing Including Cu Systems

3-Zone Rapid Deep Infrared Furnace (150˚C - 1000˚C)(curing, sintering, thick film, glass sealing, controlled atmosphere-Cu)

Vacuum Oven (0˚C – 250˚C)(curing, drying, soldering, controlled

atmosphere; Cu)

5-Zone Reflow Oven (0˚C – 450˚C)Sakama - (curing, solder reflow, polymer thick-

film, controlled atmosphere; Cu)

4-axis Robotic Dispenser EFD – w/ 3 Valves (10µm)

(3D printing; thick film & PTF, and solder & sinter inks)

UV Curing Oven & Wands

3D Printer - Thermoplastic MakerBot Rep2X (200˚C – 290˚C)

3D scanner

Class 1000 Flow Hood



Flexible PCB for Ultra Dense VRMs By Bo Gao

1. Stackable for FREE– Eliminates board to board connectors.2. Flat surface for GaN– Lower stress caused by CTE mismatch, simple heat sink design.3. No module substrate– FPC itself can be folded back to form pads.

A 12V Input, 100A Output High Density VRM

0.8kW/[email protected]/100A 1kW/[email protected]/100A

1.6kw/[email protected]/100A

10mm*25mm*10mm

3-ph sync. buckFrom simulation:

≤97%@2.5MHz, ≤90%@10MHz



Objective

Investigate new ultra thin ceramics (ThinStrate by ENrG, Inc.) for power electronics applications

OUTLINE• Wearable Power Electronics• Ultra-thin Flexible Ceramics• Electrical and Thermal Characterization of 3 mol% Yttria Stabilized

Zirconia (3YSZ)• Pre-stress analysis of 3YSZ substrate with thick copper layers• Circuits Topology design Corresponding to Specific Requirements• Applications in Power Electronics• Summary



Wearable Power ElectronicsSi WBG power semiconductors (GaN, SiC)

• Higher Breakdown Voltage• Higher Operating Temperature• Higher Thermal Conductivity• Higher power density (smaller die)

• High Voltage Devices• Less heatsink volume• Less structural complexity• Smaller Size

Challenge of Wearable Power Electronics• Suitable Substrate with flexibility• Substrate with enough thermal conductivity

Possibility for Wearable Power Electronics Module

From Traditional to Wearable

Power Module



Ultra-thin Ceramics3YSZ Ceramic Substrates with 40 µm & 20 µm Thickness

Properties of 3YSZ substrate

Thermal Conductivity

CTE (@RT) 8.2 ppm/°

Dielectric Constant 26-28

Breakdown Voltage 2.3kV (20µm)3.5kV (40µm)

Density 6.05 g/cm2

Surface Roughness: 24-26 nm (rms)

Poisson Ratio 0.32

Young's Modulus 207 GPa

Tensile Strength 248 MPa @RT

Bend Strength 1.2 GPa

• Suitable for Metallization• High tensile strength and bending strength• Good thermal conductivity compare with organic dielectric materials with

similar thickness• Light for lower weight profile



Electrical Characterization – Leakage CurrentLeakage Current Measurement • Electrode by E-beam Deposition

• 200 nm Ti layer• Electrode Dia. 12 mm• 25 oC ~ 175 oC

High Voltage Setup for leakage current

measurement of power semiconductor devices

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

0 500 1000 1500 2000 2500 3000 3500

40 micron 3YSZ Reverse Characterization

25C

75C

125C

175C

• Suitable for low temperature (< 100oC) and low voltage (< 100V) applications

• Wearable power electronics modules

Leak

age

curre

nt

Voltage



Electrical Characterization –– Penn State

• Samples are loaded with stepped voltage

• Stopped when conduction path is observed in the sample

• No leakage current value can be obtained due to the setup

• Limitation of leakage current is ~ 1.3 mA connector

• 20 sample points are measured for each thickness

Weipull Plot to calculate the Breakdown Voltage

• Solid red lines represent the 95% confidence bounds

• Black lines stand for ideal breakdown strength distributions

• Dielectric Strength• 288 kV/mm for 20 µm• 196 kV/mm for 40 µm

240

250

260

270

280

290

300

310

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Die

lect

ric

Stre

ngth

, MV

/ m

Observation

0

50

100

150

200

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Die

lect

ric

Stre

ngth

, Mv

/ m

Observation

-4

-3

-2

-1

0

1

2

5.55 5.6 5.65 5.7 5.75

Ln(1

/(1-F

))

Ln(Breakdown Strength (MV / m))

-4

-3

-2

-1

0

1

2

5.05 5.1 5.15 5.2 5.25 5.3 5.35 5.4

Ln(1

/(1-F

))

Ln(Breakdown Strength (MV/ m))

25˚C, 40µm25˚C, 20µm



Electrical Characterization –Dielectric Strength & Breakdown Voltage v. Temperature – Penn State

• Same measurement is applied at different temperature• Dielectric Strength and Breakdown Voltage decreased at higher temperature

(25 oC ~ 150 oC)



Electrical Characterization – Dielectric Constant and Loss

• Measured from 100 Hz to 1 MHz• Temperature -65 oC ~ 250 oC• Dielectric loss increased at higher temperature, exceeded 1 at 75 oC• Dielectric constant increased at higher temperature, the difference

of curves for 20 µm and 40 µm is due thickness variations



Thermal Characterization – 3-Omega MethodThermal Conductivity Measurements – 3-Omega Method V+ V-

I-I+

• Kelvin connection to GOLD line deposited on the ceramic

• Gold line length, 8mm, width 0.1mm• Define input current at different frequency• Measure output voltage at corresponding 3ω

frequency

3 12 / V (dR/ dT)dT V=3 21 / / (4 )K V dR DT lR=

Increase in temperature of heater line per unit

power vs frequencySimulation and

Experiment are very good

Thermal Conductivity decreases with higher temperature



Thermal Characterization – TTR MethodThermal Conductivity Measurements – Transient Thermo-reflectance (TTR) method

Measurement Setup• Laser Beam A applied at the sample surface• Laser Beam B is applied to measure the transient thermal reflectance signal• Both beams are focused at the same spot on the sample surface• Si photodiode collects the reflected laser beam B• Oscilloscope records the reflected laser beam B after amplification• Si detector input the TTR signal into the Oscilloscope

The result by TTR method is 2.85 W/mK, verifying the measurement results by 3-omega method



Thermal Characterization – Simulation

Parts Dimension / mm3

Heat Capacity/

J/KgK

Thermal Conductivity /

W/m·K

Thermal Resistance

/ oC/WSiC

Diode 4××4××0.4 690 250 1E-4

Solder 4××4××0.05 230 50.9 6.14E-5Cu

plate 12××12××0.127 385 383.8 2.30E-6

3YSZ 12××12××0.02 (0.04) 157 2.3 6.04E-5

12.08E-5Al2O3 12××12××0.254 880 23 7.67E-5AlN 12××12××0.254 780 170 1.04E-5

20µm – 3YSZ 40µm – 3YSZ

10mil - AlN 10mil – Al2O3

Substrate Junction Temperature / K

20µm – 3YSZ 346

40µm – 3YSZ 368

10mil - AlN 323

10mil – Al2O3 355

• Bottom Temperature is fixed at 25 oC• Power density on the device surface is defined as 1000 W/cm2

Simulation comparison between 3YSZ ThinStrate v. traditional power substrate for 2 kV applications

Thermal impedance of 3YSZ is comparable to AlN and Al2O3 for 2 kV applications



Mechanical Analysis - Pre-stress analysis

• ¼ symmetric applied to improve calc efficiency• Sn63Pb37 Solder Paste was set as 50 µm thick• Anand Constitutive Model was applied to describe

the viscoplastic behavior of solder paste

Copper Layer

SiC diode

Solder Paste

Ceramic

Temperature profile for soldering process, region between two red lines are for pre-stress analysis

Young’s Modulus

/Gpa

CTE/

10-6/oC

PoissonRatio

Heat Capacit

y/

J/

Density / Kg/m3

ThermalConductivity

/W/

SiC 450 4.5 0.142 690 3210 250

Sn63Pb37

Temperature

dependent24.5

Temperature

dependent

230 8400 50.9

Cu 117

Time

dependent

0.34 385 8920 383.8

AlN 330 4.6 0.24 780 3260 170

Al2O3

300 8.1 0.21 880 3690 23

3YSZ 200 8.2 0.23 157 6090 2.7

Materials Parameters for Pre-stress Simulations

Simulation setup for comparison between 3YSZ ultra-thin substrate with traditional power substrate for 2 kV applications



Mechanical Stress – Von Mises Stress Distribution

10mil AlN substrate 10mil Alumina substrate

20 µm 3YSZ substrate 40 µm 3YSZ substrate

Max Von Mises Stress• AlN – 141 Mpa• Alumina – 248 Mpa• 20 µm 3YSZ – 292 Mpa• 40 µm 3YSZ – 261 Mpa

Location• The interface between

Ceramic Layer and Copper Layer

Higher Stress concentration on the 3YSZ substrate• Small thickness• Large Stress gradient



Mechanical Stress – Shear Stress in Solder Layer



Shear Stress on solder layer• AlN – 0.3586 Mpa• Alumina – 0.2904 Mpa• 20 µm 3YSZ – 0.772 Mpa• 40 µm 3YSZ – 0.7612 Mpa

Location• The corner of SiC device• Front side for AlN and

Alumina Modules• Backside for 3YSZ

modules



Mechanical Stress – Shear Stress Ceramic Layer



Shear Stress on Ceramic layer• AlN – 33.8 Mpa• Alumina – 60.9 Mpa• 20 µm 3YSZ – 118 Mpa• 40 µm 3YSZ – 194 Mpa

Location• The corner of SiC device• Interface with Cu layers

Large shear stress in 3YSZ substrate, need to optimized metallization process for better stress management

Tensile stress of 3YSZ: 248 Mpa

Lower than the maximum allowed tensile stress



Circuit Design for general wearable power electronics

Most general topologies for power electronics half-bridge configuration

Previous Work on single switch configuration for flexible power electronics applications



3D Layout design for wearable power electronics

3D concept layout based on ultra-thin 3YSZ substrateLeft: with switching node floating;

Right: with switching node sitting on the substrate



Application Characterization for circuit selection

Properties of substrate required by different specific applications

2D – 2 dimensional 3D – 3 dimensionalHV – High Voltage HF – High FrequencyHC – High Current HP – High PowerHT – High Temperature



Summary

• 3YSZ ultra-thin ceramic substrate was introduced for its potential application to wearable power electronics

• Electrical Characterization indicated that the substrate can withstand high voltage, but considering its higher leakage current under high temperature and high voltage, its potential application mainly focuses on lower voltage level and lower temperature

• Thermal Characterization results showed that the thermal conductivity remains high at lower temperatures, such as for wearables

• Thermal simulations showed that the thermal performance of ultra-thin 3YSZ substrate is comparable to traditional substrate AlN and Al2O3

• Pre-stress analysis indicated that the substrate suffered higher shear strength, which requires optimization of metallization process for better thermal management

• General topologies for phase leg configuration are introduced, together with former works done by PREES on flexible power switches based on 3YSZ ultra-thin flexible substrate

• Two 3D topologies for single switches are proposed• Requirements of flexible substrate materials for different specific applications are

summarized



Acknowledgements

The authors would like to thank Texas Instruments for Laboratory support of such technical explorations, along with other PREES Sponsors, and ENrG for supplying materials.

Also, Haotao KE and Mingyu Yang for early design contributions.



Thank youQuestions?

Douglas C. Hopkins, Ph.D.Director – PREES Laboratory, FREEDM System Center

North Carolina State University1791 Varsity Dr., Suite 100; Raleigh, NC 27606-7571

[email protected] www.prees.org919-513-5929

Characterization of Ultra-Thin Flexible Ceramics for High ... · PDF fileCharacterization of...

Documents

Transcript of Characterization of Ultra-Thin Flexible Ceramics for High ... · PDF fileCharacterization of...