iNEMI End-of-Project Webinar:

Conformal Coating Evaluation Test Development

Webinar recording https://inemi-my.sharepoint.com/:v:/g/personal/haley_fu_inemi_org/EWCCh7- 7kgdJpf7c4W4zgW0BTQDPpS9FymFNOh2KmAUoJw?e=D5Oqh8

Project Leaders: iNEMI Staff: Prabjit Singh (IBM) Haley Fu Chen Xu (Nokia)

5 Jan 2021 Team

Prabjit Singh, Larry Palmer Mei Ming Khaw, Kok-Lieh Tan IBM Corporation, Poughkeepsie, Keysight, Penang, NY, USA Mike Huang, Ruby Chen Xu Wistron Corporation, Hsinchu, Nokia, Murray Hill, NJ, USA

Jason Keeping Haley Fu Celestica, Toronto, Canada iNEMI, , China

Marko Pudas Picosun, Masala, Finland Agenda

§ Project background and objectives § Conventional conformal coating test method § Thin-film based test method § Test conditions § Examples of coating characterization § Results § Conclusion

3 § Next phase Project Background

§ Conventional test determines the times to failure of coated components. § Conventional test has disadvantages: – Done in dry sulfur environment at 105 oC. – No moisture and no gas besides sulfur. – Takes many months.

4 Project Objectives & Impact

§ Test should be able to determine conformal coating performance – in less than a week – under environmental conditions like the application conditions.

5 Introduction: Conventional method

Results of corrosion of 0603 resistors in FoS chamber at 105 oC

HumiSeal® 1B59LU Control

EMCAST 1916

Courtesy: M. Gaynes and 2017-2018 project team

©2018 Universal Instruments PAGE: 6 Introduction: Thin film method

§ Cu and Ag thin films are conformally coated and exposed to a corrosive environment. § The corrosion rates of the coated thin films are electrically 7 measured and used to quantitatively characterize the conformal coatings. § The corrosion rates of coated and uncoated films are compared. Introduction: Thin film resistance measurement

Serpentine thin films and metal foils for measuring corrosion rates

§ The above structure enables accurate measurement of film resistance using the 4-point method. § A potentiostat is used to

8 a) pump known constant current through the film and b) measure the voltage across the film. The ratio of the voltage and current is the film resistance at the film temperature.

Film thickness measurement

Film thickness from resistance measurement

s. l ü Thickness, th

Rt( ==0, T0 ) ï wth.0( t = ) ï Width, w ï s. l ï RtT( ,00) = ý at temperature = T wth. ( t) ï ï But if we make resistance Rt( = 0, T) measurements at some other th( t) ==0 th( t 0)ï RtT, temperature T, we need to correct ( 0 ) þï for the effect of temperature on the resistance values.

Film resistance = f (temperature, thickness)

We need to eliminate the temperature variable

RtT(, )=+ RtT (,00 )1[ a ( T- T )] RtT(, ) Thickness, th RtT(,0 )= 1(+a TT-0 ) Width, w From previous page Rt( = 0, T) th( t) ==0 th( t 0) RtT( , 0 ) Compensated (corrected) resistance Rt( = 0, T) th t==0 th t 0 ( ) RtT(, ) ( )

1(+a TT-0 ) How do we measure a, the temp coeff of resistivity? Determining a to eliminate the temperature variable

Rt21()=+ Rt ()1[ aD T 1]

For each temperature step such

as DT1 , a can be determined. a values can be averaged.

Value of a for silver thin films was 2.7x10-3/oC with a standard deviation of 0.5x10-3/oC .

Value of a for copper thin films was 3.3x10-3/oC with a standard -3 o deviation of 0.4x10 / C . 11 Using the bottom thin film

Serpentine thin films and metal foils for measuring corrosion rates

§ The bottom film could be used for two purposes: ―To heat the top film ―To compensate for temperature so that thickness is the only variable determining film resistance. This approach did not work because of unequal temperature of the top and bottom films. éù êúRt(0,)= T Thickness of film corroded away at time ttht = (= 0)êú 1-Top 0 êúRtTTop (, ) RT(0, ) êúRtT(, ) Bot 0 The bottom film was not used in this work. ëûBot Test setup

The relative humidity in the chamber was kept constant using a saturated salt solution.

The sulfur gas concentration was kept constant at 0.15 ppm by keeping the chamber temp at 40 oC; and at 0.30 ppm at 50 oC.

The chamber air was not stirred. Metal films and conformal coatings tested

§ TheIntroduction corrosion rates of conformally coated thin films of silver and copper were measured in sulfur-bearing corrosive environment at four different temperature- humidity conditions. § The coatings tested were: – Atomic layer deposited (ALD) metal oxide coating – Silicone coating 14 – Acrylic coating Type Thickness Acrylic 39-45 µm Silicone 100 µm ALD 0.1 µm Atomic layer deposition (ALD)

§ Atomic layer deposition (ALD) is an advanced thin film coating method used to fabricate nanometer thick, highly uniform, dense, pinhole-free conformal coatings. § ALD process is performed in a vacuum reactor at relatively low temperature, typically 80-300 oC, depending on the material deposited.

15 Chamber temperature and humidity conditions

Temperature and humidity test conditions § Chamber temperature and humidity conditions were: psychrometric– 40 oC, 15%RH chart – 50 oC, 15%RH Chamber – 50 oC, 31%RH environment – 50 oC, 75%RH

Electrical and temperature test conditions for the FoS chamber at 40 °C and 15%RH

Duration, Current, Nominal film temp days mA and %RH, °C 0-1.85 100 42 °C, 13.5% 1.85-3 200 52 °C, 8.2% 3-4.16 300 64 °C, 4.6% 4.16-5.32 100 42 °C, 13.5% Coulometric reduction corrosion rates of Cu and Ag foils

§ The composition of a gas is near impossible to measure and interpret. § A more direct way of characterizing corrosive gas mixtures is to measure the corrosion rates of Cu and Ag metal foils. § ANSI-ISA 71.04-2013 standard states that for data center environment to be acceptable, the corrosion rates of Cu and Ag should be less than 10 and 6.7 Å/day.

§ In this work, L-shaped Cu and Ag foils were inserted into the chamber front door and exposed to the chamber atmosphere typically for one day, and the corrosion product thickness measured using coulometric reduction.

17 Coulometric reduction corrosion rates of Cu foils

These corrosion rates (rate of Total corrosion product thickness per day growth of corrosion products) of Red are for Cu; Black for silver Solid circles are for 40C, 15%RH Cu foils and are 2X faster than Open circles are for 50C, 31%RH the corrosion rates (rate of loss Triangle solid are for 50C, 15%RH of metal thickness) measured by Square open are for 50C, 75%RH resistance change method. 1000

800 50oC, 15%RH

50oC, 31%RH 600 Rate of growth of 400 Cu 50oC, 75%RH corrosion 200

Corrosion rate, Å/day rate, Corrosion product 40oC, 15%RH

0 0 1 2 3 4 5 6 Day 18 Coulometric reduction corrosion rates of Ag foils

Total corrosion product thickness per day Red are for Cu; Black for silver Solid circles are for 40C, 15%RH These corrosion rates Open circles are for 50C, 31%RH (rate of growth of Triangle solid are for 50C, 15%RH corrosion products) of Square open are for 50C, 75%RH Ag foils are1.7X faster 1000 than the corrosion rates (rate of loss of metal 800 thickness) measured by resistance change method. 600

50oC,15%RH 400 Rate of 50oC, 31%RH growth of 200 Ag Corrosion rate, Å/day rate, Corrosion 50oC, 75%RH corrosion 40oC, 15%RH product 0 0 1 2 3 4 5 6 Day 19 40oC, 15% RH chamber with sulfur

Silicone over Cu ALD over Cu Bare Cu 65 70 70

60 60 C 60 o C o C o 55 50 50 50 40 Temperature, Temperature, Temperature, Temperature, 40

Temperature, Temperature, 45 30

40 30 20 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 Days Days Days

Silicone over Cu ALD over Cu Bare Cu

5.8 6.5 5.6 W W 6.4 W 5.4 6.0 5.2 5.5 5.0 6.2 4.8 5.0 4.6 4.5 4.4 6.0 4.2 4.0 Uncorrected resistance, resistance, Uncorrected Uncorrected resistance, resistance, Uncorrected

4.0 resistance, Uncorrected

3.8 5.8 3.5 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 Days Days Days 20 40oC, 15% RH chamber with sulfur

RtT(, ) RtT(,0 )= 1(+a TT-0 )

Silicone over Cu ALD over Cu Bare Cu 6.0 6.2 6.0 W W W 5.5 6.1 5.5

5.0 6.0 5.0

4.5 5.9 4.5 Corrected resistance, resistance, Corrected Corrected resistance, resistance, Corrected Corrected resistance, resistance, Corrected

4.0 5.8 4.0 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 Days Days Days

21 40oC, 15% RH chamber with sulfur

éùRt( = 0, T0 ) Corrosion = th( t= 0) - th( t) == th( t 0) êú 1- ëûRtT( , 0 )

Silicone over Cu ALD over Cu Bare Cu 2500 2500

40 2000 2000

20 1500 1500

0 1000 1000

-20 Copper corrosion, Å corrosion, Copper 500 Å corrosion, Copper Å corrosion, Copper 500

0 -40 0 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 Days Days Days

22 40oC, 15% RH chamber with sulfur

Silicone over Cu Bare Cu

6.4 7.0

6.3 6.8

6.6 6.2 6.4 6.1 6.2 6.0 6.0

5.9 5.8

5.8 5.6

5.4 5.7 Activation energy = 0.22 eV Activation energy = 0.6 eV 5.2 5.6 ln(Copper corrosion rate, Å/day) 0.00290 0.00295 0.00300 0.00305 0.00310 0.00315 0.00320 Å/day) rate, ln(Coppercorrosion 5.0 0.00290 0.00295 0.00300 0.00305 0.00310 0.00315 0.00320 1/T 1/T

Activation Silicone/ Silicone/ Bare Ag Bare Cu Acrylic/Ag Acrylic/Cu energy, eV Ag Cu 40 °C, 15%RH 0.6 0.6 1.0 0.8 1.7 0.6 50 °C, 15%RH 0.2 -- 0.6 0.5 0.2 -- 50 °C, 31%RH 0.3 0.1 0.5 0.9 0.2 0.2 50 °C, 75%RH 0.26 -- 0.4 0.26 0.2 --

23 Corrosion rates of coated and uncoated metal films

Cu - acrylic coating Ag - acrylic coating Cu - silicone coating Ag - silicone coating Cu - bare Ag - bare

Rate of loss of metal thickness

24 Bare silver and copper films corrosion rates

th( t = 0)

Rate of loss of metal thickness

25 Acrylic coating performance

Rate of loss of metal thickness

26 Silicone coating performance

Rate of loss of metal thickness

27 50 °C, 15 %RH chamber, film corrosion after 5.32 days

Silver thin films Copper thin films

Silicone protects ALD protects Ag Silicone protects ALD and acrylic Ag well at 40% well. Cu poorly. protects Cu well. RH but not at Acrylic protects 50% RH. Ag poorly. Conclusions

§ This study shows that conformal coatings can be characterized by the degree of corrosion protection provide by the coatings as measured by the rates of corrosion of the underlying copper and silver thin films. – The test duration is less than a week compared to a year long test if done by conventional means. § In this study: – ALD coating was found to protect the underlying Cu and Ag to near zero corrosion rates. – The acrylic coating protected Cu well, but protected Ag poorly. – The silicone coating protect Cu or Ag very poorly, except for protecting Ag well at 40% RH.

29 Considerations for the next phase

§ Refine test vehicle (add air stirring scheme) § Consider 3-D structures § Consider testing other coatings such as fluoropolymer coatings § MFG vs FoS § Coating thickness effect § Automotive applications

30 Q&A

Dr. Prabjit Singh (IBM) Dr. Chen Xu (Nokia Bell Labs) Job title: Senior Technical Staff Member Job title: Distinguished member of technical staff

Experience: Experience: 1. 41 years of metallurgical engineering and 1. BS Degree in Chemistry from Tongji University in failure analysis of computer power packing Shanghai, China; Ph.D. in Physical Chemistry from Ruhr-University Bochum, Germany and cooling 2. ~100 publications, 40 presentations, 3 book chapters, 2. 2 years of superconductor manufacturing and 15 approved and pending patents. characterization. 3. Member of ACS, AVS, AESF and has participated in 3. 7 years of thin film technology and electro- various AESF, iNEMI, HDPUG and IPC committee migration characterization. activities resolving industry-wide issues and 4. 81 patents issued. developing standards

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