On Conductive Anodic Filament (CAF) Failures of Electronic Circuits

Effect of Bias (up to 1000 V DC) on Conductive Anodic Filament (CAF) Failures of Electronic Circuits

Ling Zou 16 July 2019

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Effect of Bias (up to 1000 V DC) on Conductive Anodic Filament (CAF) Failures of Electronic Circuits

Ling Zou 16 July 2019

NPL Management Ltd - Commercial

You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars About NPL …

The UK’s national standards laboratory 36,000 m2 national • Founded in 1900 laboratory • World leading National Measurement Institute • 600+ specialists in Measurement Science • State-of-the-art standards facilities • 360+ laboratories

• The heart of the UK’s National Measurement World leading measurement System to support business and society science building

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NPL/EMM/Electronic Interconnects Research Areas

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars How we work: Bespoke Research and NMS

. Bespoke 3rd party research and measurement • Mission extension consultancy and testing • Sn whisker behaviour • Replacement alloy testing • SIR & CAF testing • Condensation performance of components . National Measurement System – to develop capability – in collaboration with industry • Metrology for high temperature electronics • Coatings for harsh environments • High voltage SIR and CAF testing • Reliability of embedded components • Power cycling in low vacuum

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How we work: Collaborative Research

. Collaborative research • Multi-partner projects • Sn whisker mitigation • Condensation testing • UKRI (Innovate UK) • Recyclable electronic assemblies • Substituting Ag in electronic inks • High temperature interconnect materials • Protective coatings to operate at elevated temperatures • Cost effective high temperature substrates

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars How we work: Conditioning, Ageing, Stressing

Conditioning, Ageing, Monitoring Analysis Stressing (before, during & after)

. Thermal shock – air to air capability from -70 to . Mechanical shock testing - acceleration rate and 300oC, with programmable dwells and rates of shock pulse width accurately controlled and change of temperature of around 40oC per monitored using accelerometer and oscilloscope minute (typically used for fatigue life testing of . Hot mechanical shock testing – up to 250oC assemblies) . Shear testing of component attachments - . Temperature cycling – from -65 to 200oC, with degree of crack propagation and damage to programmable dwells and ramp rates of around solder joints or die attach, strength of the joint, 10oC per minute (typically used for fatigue life comparison of alloys, post thermal fatigue testing of assemblies) (typically used for testing joint formation with . Humidity bias testing – from 5 to 98% relative new platings, fluxes or finishes) humidity, -20 to 100oC with electrical bias to . Hot shear testing – up to 300oC 1000 V (typically used for electrochemical . Pull testing – up to 300oC reliability testing) . Solder joint reliability testing - low cycle fatigue, . Condensation testing – condensation using driven by the mismatch of the coefficients of custom platen to chill samples below dew point thermal expansion, solder joint cracking, of humidity chamber adhesive loss of glob tops and underfills, . Thermal ageing – up to 300oC delamination in multilayer boards and modules . Power cycling – multi-channel switching and . Reflow soldering - for sample build and monitoring of power devices using cooling simulated manufacturing conditioning platens to increase cycle time - Low Vacuum Capability

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How we work: Monitoring

Conditioning, Ageing, Monitoring Analysis Stressing (before, during & after)

. Constant continuity monitoring – in-chamber monitoring of electrical continuity across up to 900 channels allowing simultaneous measurement for interrupts of >100 micro-seconds . Electrical monitoring – in-chamber monitoring using programmable multichannel switching systems for resistance and capacitance . Surface insulation resistance (SIR) – in-chamber leakage current detection, approaching pA, and resistance values, monitored continuously over test periods up to 1500+ hours . Conductive anodic filament (CAF) testing – in-chamber monitoring of printed circuit boards for leakage current due to conductive salts forming . Solvent extract conductivity - using a mixture of isopropanol and water to remove soluble contaminants providing a measure of the ionic contamination . Solderability testing - performed on solder pads or component terminations . Tin whisker propensity – electrical monitoring of tin whisker growth, measuring wetting force and time . Adhesion testing – coating adhesion measurement using pneumatic or customised pull testing

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars How we work: Analysis

Conditioning, Ageing, Monitoring Analysis Stressing (before, during & after)

. Micro-sectioning - High quality polished micrographs . Fourier-transform infrared spectroscopy with for various investigations, including crack detection and microscope capability - used to identify organic and grain structure analysis polymeric materials, using infrared light to scan test . Optical microscopy and image analysis – a variety of samples and observe chemical properties high resolution optical imaging and measurements . Electrochemical impedance spectroscopy - used for the systems characterisation of electrochemical systems and to . Scanning electron microscopy - for analysis of surfaces investigate mechanisms in electro-deposition, electro- with complex topography with a magnification of over x dissolution, corrosion studies and the study of 100,000, used to locate tin whiskers, examine biosensors intermetallic layers, investigate crack failure modes and . Surface Profiling – 3D micro coordinate and surface general failure analysis roughness using contact and non-contact methods . FIB (Focused Ion Beam) . Surface energy – drop shape analyser for measuring . Scanning acoustic microscopy - uses ultrasonic waves surface free energy reflecting or transmitting at material interfaces to study . Electrokinetic analyser - automated surface zeta buried solid interfaces of dissimilar materials and non- potential analysis of solids destructive detection of features such as bonded . Thermal analysis interfaces, delaminated interface, voids and cracks • Differential scanning calorimetry (DSC) - rapid . X-ray florescence - used to determine the atomic technique that measures the heat flow associated with content of material, screening for ROHS compliance, material transitions as a function of temp. and time thickness measurements and validation of solder • Dynamic mechanical analysis (DMA) - a versatile content technique used for characterising time, temp. and . Energy dispersive X-ray spectra – electron frequency dependent mechanical behaviour microscopes for image and elemental analysis • Thermo mechanical analysis (TMA) - used for . X-ray radiography – nano-focus X-ray inspection measuring dimensional changes in a material as a function of time, temperature and the applied force

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Introduction

. Why electrochemical reliability is important? . What are the main causes for electrochemical failures? . How to use the Insulation Resistance (IR) technique to monitor CAF failures at higher test voltages. . Test design and results of CAF failures under different biases up to 1000 Vdc will be presented. . Conclusions on the effect of bias on CAF formation. . Potential new collaborative research project for CAF propensity up to 1000Vdc.

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars Electrochemical Reliability

. Achieving high reliability in service is the key issue in today’s high-density electronic circuits. . Electrochemical reliability becomes more critical, as electronic circuits move to increased packaging densities, and also operated at higher voltages and in harsher environments (hotter/damper/ condensing). . Electrochemical reliability failures are mainly caused by contaminants and bias under damper conditions, facilitating the electrochemical corrosion process that results in a loss of continuity/short circuits.

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Electrochemical Reliability

. For powered circuit failures, voltage bias is one of the main driving forces . The corrosion process is initiated and accelerated by the voltage bias. . Electrochemical failures are very sensitive to the bias, all other aspects being equal, the electronics used in the aerospace and automotive applications will suffer more failures due to the high service voltages. . As operating voltages increase towards 1000V, the effect of higher voltages on CAF failures needs to be investigated.

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars CAF Formation

. For metal corrosion (Solder, PCB Cu track, via and component termination) to occur the following must be present • Contaminants (from production processes: PCB chemicals) • Bias • Moisture (Humidity environments) R . CAF formation is initially caused by anodic Cu corrosion. - + . CAF is Cu corrosion products grow along the disrupted glass/resin interface from anode to cathode to bridge two opposite charged Cu conductors.

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Conditions for CAF formation

. Electrical charge carriers must be present to form the electrochemical cell . Ionic species inside PCB, H+ and OH- from water . Moisture must be present to dissolve the ionic contaminants and sustain them in their mobile state . An acid environment around conductors is needed to initiate Cu corrosion at anode. . Pathway is needed for ions to move . Weakened interface between glass fibre and resin due to drilling and reflow . Bias acts as driving force for Cu corrosion and ion transport.

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars Electrochemical Reactions for CAF Formation

v IR

- - + + HO H+ M+n H - + Cu M - +n Cu Corrosion HO- C6H9O M H2 of metal Cathode Anode +n Cu + ne  Cu Cu  Cu+n + ne + H + 2e  H2 + - 2H2O  O2 + H + 4e O2 + H2O + 4e  4OH

. Corroded Cu ion form insoluble Cu compounds (CuCl2, CuSO4 5H2O) deposited at high PH to form CAF along the interface of glass fibre and resin. . Cu corrosion is driven and accelerated by the applied bias.

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NPL New Facility: CAF Measurement up to 1000V

. CAF failures can be identified and evaluated using Insulation Resistance (IR) technique, . This technique is based on the measurement of a leakage current (IR) associated with the formation of the electrochemical corrosion products. 18 NPL Management Ltd - Commercial

You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars IR measurement at high test voltage up to 1000V

. Most IR measurement is undertaken below 250V. . Requirements from automotive industry are for significantly increased operating voltages (up to 1000V). . Existing CAF testing methodologies and standards need to be developed and updated to meet these requirements. . NPL has multichannel IR testing capability from 5V to 1000V DC.

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NPL 1000V CAF Test Vehicle - TB163

• 16 IR patterns • Each pattern is a chain of 450 vias with 420 CAF opportunities. • All vias are 800m diameter with different via wall to wall separations: • 3 x 550m wall to wall • 3 x 800m wall to wall • 3 x 1100m wall to wall • 3 x 1500m wall to wall • 3 x 2100m wall to wall • 1 x 550m SIR control

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars NPL TB163

. 7628 glass fibre has been used for both prepregs and cores. . 7628 has a high glass/resin ratio and a high propensity for CAF failure. 21 NPL Management Ltd - Commercial

Six layers PCB – AuNi finish

Ø 800m H-H distance (L) (m) 550 L 800 1100 1500 2100

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars H-H distance

550m 800m 1100m 800m

via via via via via

H-H distance (L) 1500m 2100m (m) 550 800 via via 1100 1500 2100

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IR test patterns

H-H distance Via diameter IR pattern (m) (m) code 550 800 3 13 16 800 800 5 6 15 1100 800 4 8 9 1500 800 1 10 12 2100 800 2 11 14 550 800 7 (SIR, control)

There is no surface failures on control patterns

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars Test matrix

PCB code Test voltage (V) 1-4 100 5-8 250 9-12 500 13-16 1000

. 16 PCBs were tested at different voltages. . Test condition 85C/85%RH.

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Reflow profile – Pb free

. PCBs were reflowed 3 times using a typical LF temperature profile

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars Example of IR results – 100V

550m H-H 800m H-H

H-H distance (m) 550 800 1100 1500 2100

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Example of IR results – 1000V

283 hours1158 hours 1809 hours H-H distance (m) 550 800 1100 1500 2100

. Time to Failure (TTF): Time at IR first drop below 108 . Subsequent data based on average TTF for 12 patterns with the same H-H distance (on 4 different PCBs).

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars Time to failure (TTF) with H-H distance

. TTF increased with H-H distance (L) increase. . There is a quadratic relationship between TTF and H-H distance 29 NPL Management Ltd - Commercial

Time to failure (TTF) with (H-H distance) 2

. There is a linear relationship between TTF and (H-H distance)2 30 NPL Management Ltd - Commercial

You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars TTF was normalised with voltage

. TTF not only depends on H-H distance, but also test voltage. . 100V is the test voltage in current IPC CAF standard. 31 NPL Management Ltd - Commercial

Time to failure (TTF) with voltage

. TTF decreased with voltage increase. . There is a reciprocal relationship between TTF and voltage 32 NPL Management Ltd - Commercial

You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars Time to failure (TTF) with 1/voltage

. There is a linear relationship between TTF and 1/voltage, but this relationship is only for voltage from 250V to 1000V. 33 NPL Management Ltd - Commercial

Time to failure (TTF) with 1/voltage

. The TTF obtained from 100V test voltage can’t be used to predict TTF at higher test voltages. . When test voltage was 100V, TTF was much longer than it should be from the linear relationship obtained at higher test voltages (250 to 1000V). . In the other words the linear relationship between TTF and 1/Voltage was different between higher test voltage range (250V to 1000V) and lower voltage range (below 250V).

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars The relationship between TTF and H-H distance (L) and bias (V)

TTF = A x f (H,T) x L2 / V . A : a constant associated with . Material CAF property . Manufacturer/chemistry/processing . T, H - Test temperature and humidity . L - H-H distance . V - Bias applied (test voltage)

. A was different at different voltage ranges.

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Conclusions

. The effect of test voltage has been successfully investigated using a CAF board design with different H-H distances. . Test voltages play a very important role in CAF failures . Increasing the test voltage dramatically decreases time to failure (TTF). . There is a inverse relationship between TTF and test voltage for higher voltages (>250V). . TTF at lower test voltage (100V) is longer than predicted from the inverse relationship revealed at higher test voltages.

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars Conclusions

. At higher test voltage CAF failure cannot predicted using the formula revived at low test voltage. CAF failure was more faster at high test voltage. . TTF not only depends on test voltage, but also H-H distance. There was a quadratic relationship between TTF and H-H distance. . For the same PCB material and PCB supplier, TTF = A x f (H,T) L2/V cannot be simply used across all voltage ranges. . At different voltage ranges, CAF constant A is different

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Previous work

H-H distance (L) (m) 300, Ø 250 400, Ø 500 550, Ø 800

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars Previous work

CAF constant A is affected by all the above parameters

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Further work

. Investigate SIR (surface) at 1000V for typical process contamination levels. . Current CAF results were only for one material from one PCB manufacture. The results need to be verified by different materials and PCB suppliers. . The same CAF board design, material and supplier with shorter H-H distance will be tested at lower test voltages (50V-250V). . Comparison the difference in CAF constant (A) at different test voltages.

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars Multi partner project for CAF failure up to 1000V

. Higher voltage CAF board design will be validated for different materials from different PCB suppliers in a wider range voltages (up to 1000V). The CAF constant A will be compared. . Finalise the design for industry to use to evaluate their PCB materials and processes up to 1000V. . An invitation will be sent out later this year. . If you are interested in participating using your PCB lay-up and resin system, please contact us

Contact details Ling Zou ([email protected]) Martin Wickham ([email protected]) Tel: 0208 943 6065 Tel: 0208 943 7027

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NPL High voltage IR testing

. High voltage IR test system available for reliability assessments. . NPL can work with you to set test parameters to suit your individual purpose. . Please contact us . For advise on IR technique. . IR measurements under high voltage. . Contact detail Ling Zou ([email protected]) Tel: 0208 943 6065

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars NPL Webinars – 2019

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You can book your next webinar at https://www.npl.co.uk/electronic-and-magnetic-materials/electronics-reliability-webinars