Raytron Technical Review RESEARCH ARTICLE

Diffusion Phenomena at Metal-Metal Interfaces

Gao-Lei Xu¹ *

¹RAYTRON Group Technology Research Center, China

^*Corresponding author

Received: 2025-12 Accepted: 2026-02 Published: 03/2026

DOI: 10.1234/raytron.2026.WP-01-04

1. Introduction

1.1 Diffusionin Conductor role

Diffusionin Conductor ：

（vs）

MEDIA TODO

Figure Fig. 1 Dual Role of Diffusion Schematic (Formation vs Degradation)

Key：

Manufacturing：to （can ）
：to （can ）

1.2 as Diffusionimportant

2. DiffusionFundamentals

2.1

（）：

J = -D (∂C/∂x)

(1)

（）：

∂C/∂t = D (∂²C/∂x²)

(2)

animation，showing

0:30

VIDEO TODO

Video 1 Diffusion Process Animation Showing Concentration Distribution Over Time

2.2 temperature

Diffusion：

D = D₀ exp(-Q/RT)

(3)

Arrhenius，

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Figure Fig. 2 Arrhenius Plot, Diffusion Coefficients of Various Metals

2.3 Diffusion

three Main：

Diagram placeholder

MEDIA TODO

Figure Fig. 3 Three Diffusion Pathways Schematic

Diffusion：

D_eff = (1-f)D_L + f · δ · D_GB

(4)

：f = Interface，δ = Width（~0.5 nm），D_L = Diffusion，D_GB = Diffusion

3. Diffusion Mechanisms

3.1 Mechanism

Mechanism（）：

animation

0:20

VIDEO TODO

Video 2 Vacancy Diffusion Mechanism Animation

3.2 Diffusion

Harrison：

Harrison

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Figure Fig. 4 Harrison Classification Schematic

3.3 InterfaceDiffusion

Interfaceas Diffusion：

D_interface ≫ D_lattice

(5)

4. Intermetallic Compounds

4.1 Mechanics

meets Conditions，：

ΔG_reaction = G_IMC - (x · G_A + y · G_B) < 0

(6)

Cu-Al

MEDIA TODO

Figure Fig. 5 Cu-Al System Free Energy Diagram

4.2 Mechanics

：

x² = kt

(7)

：x = Thickness，k = （temperature ），t =

：

k = k₀ exp(-Q/RT)

(8)

4.3

in System ，same ：

SEM

MEDIA TODO

Figure Fig. 6 Multiphase Growth Layer Structure SEM Micrograph

(Cu-Al)：

Cu | Cu₉Al₄ | CuAl | CuAl₂ | Al

5. Kirkendall

5.1

two different Diffusion，in Diffusion：

Kirkendallanimation

0:30

VIDEO TODO

Video 3 Kirkendall Effect Formation Animation

：

v_K = (D_A - D_B) (∂C/∂x)

(9)

v_KKirkendallspeed 。

5.2 5.3 Strategy

KirkendallSEM

MEDIA TODO

Figure Fig. 7 Kirkendall Void SEM Micrograph

6. SpecificDiffusion

6.1 - (CCA)

Diffusion：

D_{Al in Cu} (400°C): 1.8 × 10⁻¹⁴ m²/s
D_{Cu in Al} (400°C): 3.2 × 10⁻¹⁴ m²/s
IMC: CuAl₂ (θ)
: 8 μm²/h

：

CCA（vs）

MEDIA TODO

Figure Fig. 8 CCA Processing Window Chart (Temperature vs Time)

6.2 - (CCS)

Diffusion：

D_{Fe in Cu} (500°C): 2.1 × 10⁻¹⁸ m²/s
D_{Cu in Fe} (500°C): 5.0 × 10⁻¹⁸ m²/s
: significant
Mechanism:

： Diffusion Interfacestable。

6.3 - (NCC)

DiffusionCharacteristics：

D_{Ni in Cu} (400°C): 3.2 × 10⁻¹⁸ m²/s
D_{Cu in Ni} (400°C): 2.1 × 10⁻¹⁸ m²/s
IMC: （）
Mechanism: Diffusion+

advantages：IMC。

6.4 - (SCC)

DiffusionCharacteristics：

D_{Ag in Cu} (400°C): 4.5 × 10⁻¹⁶ m²/s
D_{Cu in Ag} (400°C): 6.0 × 10⁻¹⁶ m²/s
IMC:
Special: 780°C

7.

7.1 Processtemperature Control

7.2 management

Optimization：

t_process = t_required + safety_margin

(10)

7.3 Diffusion

for Keyapplications，can ：

Diagram placeholder

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Figure Fig. 9 Diffusion Barrier Structure Schematic

7.4 Strategy

：

Al Si：reduces AlDiffusion，IMC
Cu Fe：CuDiffusion，stableInterface
Al Mg：，Surface

8.

8.1 Interface

temperature ：

MEDIA TODO

Figure Fig. 10 Service Life vs Temperature Curve

8.2 Thermal Cycling

Mechanism：

CTE
Diffusion
IMC
can

SEM

MEDIA TODO

Figure Fig. 11 Post-Thermal Cycling Interface Degradation SEM Micrograph

8.3 Prediction

Model：

IMC(t,T) = IMC₀ + √(2k(T) · t)

(11)

ExampleCalculation：

for 150°COperation CCA：

IMC₀ = 2.0 μm（）
k(150°C) = 0.002 μm²/h
25（219,000 h）：IMC = 2.0 + √(2 × 0.002 × 219000) = 4.1 μm

can Acceptance？——5 μm。

9. Conclusion

9.1 Key

Diffusionin
temperature Controlmanagement Diffusion Main
Cu-AlSystem Main
Kirkendallin can causes Failure
stablerequires Conditions

9.2 Design

for Manufacturing：reduces 、makes 、MonitoringIMCThickness、ProcessControl

for applications：temperature 、Thermal Cycling、Interface、Designcan Acceptance

9.3 future directions

research requires ：

Prediction Testingmethods
Diffusiontechnology
Interface Monitoring
based on PredictionModel

Figures

Dual Role of Diffusion Diagram (Formation vs Degradation)

Fig. 1 Dual Role of Diffusion Diagram (Formation vs Degradation)

Arrhenius Plot, Diffusion Coefficients of Various Metals

Fig. 2 Arrhenius Plot, Diffusion Coefficients of Various Metals

Three Diffusion Pathways Diagram

Fig. 3 Three Diffusion Pathways Diagram

Harrison Classification Diagram

Fig. 4 Harrison Classification Diagram

Cu-Al System Free Energy Diagram

Fig. 5 Cu-Al System Free Energy Diagram

Multiphase Growth Layer Structure SEM Micrograph

Fig. 6 Multiphase Growth Layer Structure SEM Micrograph

Kirkendall Void SEM Micrograph

Fig. 7 Kirkendall Void SEM Micrograph

CCA Processing Window Chart (Temperature vs Time)

Fig. 8 CCA Processing Window Chart (Temperature vs Time)

Diffusion Barrier Structure Diagram

Fig. 9 Diffusion Barrier Structure Diagram

Service Life vs Temperature Curve

Fig. 10 Service Life vs Temperature Curve

Post-Thermal Cycling Interface Degradation SEM Micrograph

Fig. 11 Post-Thermal Cycling Interface Degradation SEM Micrograph

Tables

Table 1 DiffusionCorrelationQuestion

Question	Reason	rear
IMCGenerationLong	Diffusion
Formation	Kirkendall Effect	ConductivityReductionLow
Interface Resistance	Composition Variation	PropertyDamageLoss
Delamination	IMC Stress	Failure

Table 2 Copper in DiffusionParameter

DiffusionSpecies	D₀ (m²/s)	Q (kJ/mol)	D at 400°C (m²/s)
Cu in Cu (Self- Diffusion)	2.0 × 10⁻⁵	197	2.1 × 10⁻¹⁶
Al in Cu	6.5 × 10⁻⁵	136	1.8 × 10⁻¹⁴
Ni in Cu	2.7 × 10⁻⁵	236	3.2 × 10⁻¹⁸
Fe in Cu	3.0 × 10⁻⁴	240	2.1 × 10⁻¹⁸

Table 3 Activation Energy Composition

Composition	Description	Typical Value (kJ/mol)
VacancyFormation	E_f	80-120
Vacancy Migration	E_m	60-100
TotalActivation Energy	Q = E_f + E_m	140-220

Table 4 Cu-AlSystem Intermetallic CompoundStability

Compound	ΔG_f (400°C, kJ/mol)	FormationOrder
CuAl₂ (θ)	-35	First
Cu₉Al₄ (γ)	-28	Second
CuAl (η)	-25	Third

Table 5 Cu-Al IMC Generation Normal Count

Temperature	k (μm²/h)	Formation 5 μm Required Time
300°C	0.5	50 h
350°C	2.0	12.5 h
400°C	8.0	3.1 h
450°C	30.0	0.8 h

Table 6 MetalSystem in Kirkendall Effect

System	ComparativelyFast Diffusionor	VoidPosition	StrictHeavyProcessDegree
Cu/Al	Al → Cu	AlSide	Significant
Cu/Ni	Ni → Cu	NiSide	in etc.
Cu/Zn	Zn → Cu	ZnSide	Significant
Ni/Al	Al → Ni	AlSide	StrictHeavy

Table 7 Kirkendall Mitigation Method

Strategy	Mechanism	Effective Properties
Diffusion Barrier Layer	Barrier Fast Rate Diffusion	High
Gradient Interface	Reduction Low Concentration Gradient	Moderate
Temperature Control	Reduction Low Diffusion Rate	Moderate
Optimization Composition	Balance Diffusion Rate	Variable

Table 8 Processing Temperature Limitations

Material	MostHigh Processing Temperature	LimitationsFactor
CCA	400°C	IMCGenerationLong
CCS	600°C	SteelProperty
NCC	500°C	Ni Oxidation
SCC	400°C	Ag Softening

Table 9 Process Time Guide

Material	MostShort Time（Optimal Temperature）	Recommendation
CCA	30 min	45-60 min
CCS	15 min	20-30 min
NCC	20 min	30-45 min

Table 10 DiffusionBarrier LayerMaterial

Barrier Layer	Thicknessss	Effective Properties	Application
Ni	1-5 μm	Good	Cu/Al Interface
Cr	0.5-2 μm	Good	Cu/Al Interface
Ti	1-3 μm	in etc.	Each
W	0.1-1 μm	Excellent	High Temperature

Table 11 Service Temperature under Prediction IMCGenerationLong

Service Temperature	InitialIMC	10Year rear	25Year rear
75°C	2.0 μm	2.1 μm	2.2 μm
100°C	2.0 μm	2.2 μm	2.4 μm
150°C	2.0 μm	2.5 μm	3.0 μm
200°C	2.0 μm	3.5 μm	5.0 μm

Table 12 HotCycleProperty

CycleCycleCount	TemperatureScope	IMC Variation	Mechanism
1000	-40 to +125°C	+0.2 μm	StressAuxiliary Diffusion
500	-55 to +200°C	+0.5 μm	SignificantAccelerated
100	-65 to +250°C	+1.0 μm	StrictHeavy Degradation

References

Mehrer, H. Diffusion in Solid Metals and Alloys Springer-Verlag (2007)
Shewmon, P. G. Diffusion in Solids (2nd ed.) TMS (1989)
Glicksman, M. E. Diffusion in Solids: Field Theory, Solid-State Principles, and Applications Wiley (2000)
Philibert, J. Atom Movements: Diffusion and Mass Transport in Solids Les Editions de Physique (1991)
Kirkaldy, J. S., & Young, D. J. Diffusion in the Condensed State Institute of Metals (1987)
Bocquet, J. L., et al. Diffusion in Metals and Alloys Trans Tech Publications (1996)
Springer, H., et al. Intermetallic phase formation Acta Materialia 59 , 1586-1600 (2011)
Xu, L., et al. Interface evolution in CCA Materials Science and Engineering A 771 , 138613 (2020)
Peng, X., et al. IMC growth kinetics Journal of Materials Processing Technology 267 , 1-9 (2019)
ASTM International ASTM B566-04: Standard for Copper-Clad Aluminum Wire ASTM (2020)

Authoritative References

ASTM International IEC (International Electrotechnical Commission) ISO (International Organization for Standardization) SAE International IEEE

Gaolei Xu

Senior Materials Scientist

Credentials & Honors

• CTO, Raytron Group
• Zhejiang Provincial High-level Talent Special Support Program - Young Talent
• Shaoxing "Technology Vice President"
• Shaoxing Science and Technology Commissioner
• Member of National Technical Committee 243 on Heavy Metals (SAC/TC 243/SC2)

National Standards (Lead Author) View Official

GB/T 27671-2011 - Copper Profiles for Electrical Conductors
YS/T 1043-2015 - Silver-bearing Oxygen-free Copper Strip for Motor Commutators
YS/T 974-2014 - Copper and Copper Alloy Strip for Composite Contact Materials
YS/T 1082-2015 - Copper Strip for Lamp Lead Stents

Patents (Inventor) Search Patents

CN104959396A - Production Process of Copper Strip for Composite Contact Materials
CN106077125A - Production Process of Copper Profile for Magnetic Pole Coils
CN201410710206 - Conductive Material for High-speed Railway Traction Motors and Production Method
CN201310719717 - Method for Controlling Strip Shape of Copper Strip Blank by Continuous Extrusion
CN201310720126 - Device for Controlling Strip Shape of Copper Strip Blank by Continuous Extrusion
CN201310376884 - Five-in-one Copper Strip Edge Treatment Equipment for Transformers
CN201420184755 - Continuous Extrusion Die Flow Promotion Device
CN201320761640 - Continuous Extrusion Waste Cleaning Device

Areas of Expertise

Copper-Clad Aluminum (CCA) Technology Copper-Clad Steel (CCS) Manufacturing Bimetallic Composite Materials PV Ribbon for Solar Cells Battery Tab Materials for EV Applications Continuous Extrusion Technology

Selected Publications

• Research and Application of Rolling Method for Manufacturing Metal Laminated Composites, Aluminum Processing Journal, 2008
• Annealing Process Research of Copper-Aluminum Composite Strip
• Research on Preparation Process of Copper/Aluminum Composite Strip for Cables
• Interface Microstructure Evolution of Rolled Copper/Aluminum Composite Strip During Annealing

Mr. Xu Gaolei is a distinguished expert in non-ferrous metal processing with over 15 years of experience. He is recognized as a Young Talent under the Zhejiang Provincial High-level Talent Special Support Program. He leads R&D initiatives in bimetallic composite technologies and has contributed significantly to the standardization of copper and bimetallic materials in China.

Click standard/patent codes to view official documents

1. Introduction

1.1 Diffusionin Conductor role

1.2 as Diffusionimportant

2. DiffusionFundamentals

2.1

2.2 temperature

2.3 Diffusion

3. Diffusion Mechanisms

3.1 Mechanism

3.2 Diffusion

3.3 InterfaceDiffusion

4. Intermetallic Compounds

4.1 Mechanics

4.2 Mechanics

4.3

5. Kirkendall

5.1

5.2

5.3 Strategy

6. SpecificDiffusion

6.1 - (CCA)

6.2 - (CCS)

6.3 - (NCC)

6.4 - (SCC)

7.

7.1 Processtemperature Control

7.2 management

7.3 Diffusion

7.4 Strategy

8.

8.1 Interface

8.2 Thermal Cycling

8.3 Prediction

9. Conclusion

9.1 Key

9.2 Design

9.3 future directions

Figures

Tables

References

Authoritative References

Gaolei Xu

Credentials & Honors

National Standards (Lead Author) View Official

Patents (Inventor) Search Patents

Areas of Expertise

Selected Publications

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