Thin Cu Stress Testing

Project leader:Fujitsu Advanced Technologies LimitedFacilitator: Hiko Nakamura

Feb. 27-28, 2013

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Problem statement

• Thickness of plated copper in a small through hole or in a through hole with a high aspect ratio of a PWB may be critical to securing adequate reliability for temperature cycling.

• The reliability of thin Cu has been carried out using ATC. However, it takes long time to get the result.

• Prediction of thin Cu reliability by FEM simulation also is required thin Cu mechanical properties.

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Objectives

• To characterize material property of plated thin Cu

• To study the relationship between material properties and thickness for plated Cu by mechanical stress testing and FEM simulations

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Project Goal

• Propose how to measure stress-strain property and creep property of thin Cu

• Find out the similarities and differences between temperature cycling test and FEM simulation with the above properties for thin Cu (in order to decide adequate thickness for designated life time of a PWB)

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Scope:Do and Do not in this project

Do

･ How to measure mechanical properties of thin Cu

･ Prediction of lifetime for stress test by using FEM

･ Prediction of lifetime for Accelerated Temperature

Cycle test by using FEM

Do not

･ Find key parameters for better reliability on Cu

plating process

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Approach: How to measure thin Cu

• Try a stress test for a thin Cu to confirm whether it can be used as alternative of plated through hole.

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Cu plated sheet (20,30,50um in thickness)

Cut into a test sample (a dumbbell shape)Do stress test (tension test)

Mechanical property(σ-ε)

[Stress-strain test]

Temperature dependency Thickness dependency

Mechanical Property

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Temperature dependency Thickness dependency

Mechanical Property(in detail)

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Crystal orientation

Bulk Copper Electroplated Copper(50μm)(In subsurface region) (The inside region)

Images processed by changing the CI area less than 0.3 into black color

Many defects in grain boundary per volume Lower Young’s modulus?Will reflow stresses change crystal orientation and Young’s modulus?

Mechanical Property

Thickness[mm] Temp[K] Young’s Modulus [GPa]

Yield stress [MPa]

Work-hardening exponent

50

298 115 249 0.116

323 122 248 0.113

348 91 241 0.121

373 90 215 0.109

398 49 166 0.190

30 298 61 213 0.134

20298 45 208 0.120

348 42 192 0.102

10 298 29 1) － －Bulk copper 298 136 2) 60 3) 0.350 4)

References1) Kinji Tamakawa, Kazuhiko Sakutani, Hideo Miura： Journal of the Society of Materials Science Japan Vol.56 No.10,

(2007), pp.907-9122) Yoshihara Mae : “A story about Copper and Aluminum”, JIS, (2000), p.11 (In Japanese)3) M. F. Ashby, D. R. H. Jones： ENGINEERING MATERIALS, (2004), p.944) A. Misra, X. Zhang, D. Hammon, R.G. Hoagland： Acta Materialia 53, (2005), p.224

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Approach:How to measure thin Cu

[Low cycle fatigue test]

Samples must have a stiffness which can be pulled and pushedagainst repeated shear stress cycles.

We are trying to make samples as follows.(1)An hourglass shaped plastic sample (2)An hourglass shaped plastic sample with thin plated Cu

　　　　　　 (Cu thickness: 15-20μm, 40μm, 60μm)

Do a low cycle fatigue test

Desirable result?

(1) (2)

Do the low cycle fatigue testObserve X-section of thin Cu

Specimen: Hourglass shaped plastic sample with thin plated CuFatigue test: Amplitude 120mm, Ramp rate 240mm/sec

Fatigue test specimen20μm in thickness

Shape and dimensions

Preliminary low cycle fatigue test(tension & compression force)

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1mm 0.5mm

Surface of specimen(after Fatigue test)

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Fatigue test result (number of cycles vs. load reduction ratio)

Fatigue test result

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Hysteresis loop of load and displacement in Fatigue test

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Fatigue test result

• Design a test board• Build an FEM model of the test board• Estimate the relationship

between the life time of PTH

and the thickness of plated Cu

by FEM simulations• Prepare some boards and

do ATC,

and compare FEM results

Approach: Life time estimation for ATC

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Meshed model

Solver : ANSYS ver. 14.0 Element type : hexahedron 20 nodes (SOLID186) Modeling : one half of a model (51,468 nodes)

Thermal load : trapezoid wave (398K(15min) - 233K(15min))

Young’s modulus : electroplated = 29 GPa bulk = 136 GPa Creep strain characteristics : Garofalo law

)24067

- ( exp] )03.0([sinh 105 2.219ss T

Comparison of electroplated copper and bulk copper

Preliminary thermal fatiguesimulation of through hole via

Whole model Copper portion

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Electroplated copper(Elast-creep analysis)

Bulk copper(Elastic analysis)

Contour map of total strain in y-direction after the 15 minuteduration at 125C heated from 25C in the first cycle of ATC

Simulated results

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Relation between y-direction total strain vs. accelerated temperature cycle

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Simulated results (maximum point)

Electroplated copper get plastically strained in ramp-up period

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Further study

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• From the observation in page 9, effects of reflow soldering stress on electroplated copper should be considered.

• From the observation in page 19, ratchet type loading can be also used for fatigue test of thin electroplated copper.

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A plan for further experiments

© HDP User Group International, Inc.• We would like to propose our plan by the end of May.

Tension & compression forces Ratchet forces

Stress-strain characteristics Fatigue test

Thickness dependency at RT

Temperature dependency Decide forces

No reflow stress

Thickness dependency at RT

Temperature dependency

Reflow stress 2x (ex.)

Fatigue test specification

Design Test Board

Fabricate Board FEM modeling

ATC testing FEM simulation

Analysis, consideration and final report

Number of samples andtest conditions should bediscussed among projectmembers and contributors

draft

Promising deliverables

• A stress test method of thin Cu• Stress-strain characteristics of thin Cu• Observations of the tested samples by SEM• Creep characteristics of thin Cu• Comparison between the mechanical fatigue of

actual ATC and FEM simulation with the obtained material properties

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Project Execution Plan

Project Task Plan Actual

Project plan draft(Fujitsu) 4/2012 5/2012

Preliminary testing 5/2012 7/2012

Detailed description and schedule 6/2012 9/2012

Thin Cu stress test and X-sectional observation,and trials of Low cycle fatigue test

10/2012-3/2013

10/2012-

Low cycle fatigue test of thin Cu, FEM simulations,and actual ATC test of a PWB

4/2013-2/2014

Final report 3/2014

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Team Members and Resources

• Fujitsu Advanced Technologies – Project leader

(Samples preparation, ATC, Report)• Hitachi Chemical

(Samples preparation, material property (TCE))• Shibaura Institute of Technology [non-member]

(Methodology, Stress test, observation, FEM simulation)

• HDP User Group (Hiko Nakamura) - Project facilitator

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