How to Avoide Conductive Anodic Filaments (CAF) Your Delegate Webinar Control Panel
How to Avoide Conductive Anodic Filaments (CAF) Ling Zou & Chris Hunt 22 January 2013 1 Your Delegate Webinar Control Panel Open and close your panel Full screen view Raise hand for Q&A at the end Submit text questions during or at the end 2 NPL Management Ltd - Commercial 1 NPL EI Free Technology Webinars Benefits of Using Nitrogen During Soldering and determining PPM Levels Thursday 14 February Attraction of Sn Tin Whiskers Wednesday 6 M arch BGA Reliability - The Effects of Solder Joint Voiding and Uneven Stand-Off Height Tuesday 30 April Reuse of Electronic Products Wednesday 15 M ay Through Hole Reliability for High Aspect via Holes Tuesday 11 June Functionalisiation and Applications of Nanomaterials for Electronic Applications M onday 18 July Environmental Robustness: Performance Coatings & High Temperature Interconnects Tuesday 3 September www.npl.co.uk/ei “IPC/NPL Cleaning & Contamination Testing Center Live” stand 1549 30ft AV & Seminars SIR Testing 20ft Ionic Tester Microscope AOI for Flux & Coatings Aqueous Cleaner Contamination Flux Test Kits Ion Chromatography Ionic Tester Bobs Office Table http://www.ipcapexexpo.org/html/main/cleaning.htm 2 How to Avoide Conductive Anodic Filaments (CAF) Ling Zou & Chris Hunt 22 January 2013 5 Three main drive for CAF concern in past ten years CAF failure mechanism was first reported in the 1970;s by Bell laboratory. Three main drives: The drive to increase circuit density with smaller printed wiring board geometries. The rapid increase of the use of electronics in harsh environments and for high reliability and safety critical application. Lead-free soldering process affect laminate stability and increase CAF material choices. 6 NPL Management Ltd - Commercial 3 Conductive Anode Filament (CAF) CAF formation inside the PCB is an important failure mode for electronic circuit. It is a electrochemical process, and initially caused by anodic Cu corrosion. CAF is Cu corrosion products grow along the glass/resin interface from anode to cathode R - + NPL Management Ltd - Commercial Mechanisms Ionic species Low pH Anode corrosion Moisture Bias Susceptibility • • • reflow Electrochemical cell Mobile corrosion products Migration down pathway pathway CAF • • pH increase Deposition of salt 8 NPL Management Ltd - Commercial 4 Electrochemical process for CAF v V Cu HOH+ M +n M C6 H9 OH2 HO H+ + M +n Cu + Corrosion of metal Cathode Anode Cu+n + ne Cu H+ + 2e H2 O2 + H2 O + 4e 4OH - Cu Cu+n + ne 2H2 O O2 + H+ + 4e Cu (OH)2 Cu O + H2O Cu+2 + 2OH - Cu (OH)2 CuO: Black - CuCl2: Yellow brown CuSO4 5H2O : Blue The Cu salt or Cu (OH)2 can be deposited at high pH NPL Management Ltd - Commercial 9 Conditions for CAF formation Electrical charge carriers must be present to form Electrochemical cell Ionic species inside PCB, H+ and OH- from water Water must be present to dissolve the ionic material and sustain them in their mobile ionic state Moisture, humidity Acid environment around conductors is needed to initiate Cu corrosion at anode. Acid contaminations from glass fibre surface, fluxes from assembly process or/and acid residues from plating process Pathway is needed for Ions to move Delamination between glass fibre and resin due to reflow Bias acts as driving force for ion transport Circuits need to be powered up in service NPL Management Ltd - Commercial 5 Traditional CAF measurement Test PCBs at high temperature and humidity (85°C/85% RH) condition with bias (50V) for 1000 hours. Monitor Insulation Resistance (IR) continually. Take long time for the evaluation. 11 NPL Management Ltd - Commercial Time to failure 12 Log Log SIR IR () (ohm 11 217 hours 10 217 hours 217 hours 9 8 7 6 5 0 50 100 150 200 Time (hour) 250 300 12 NPL Management Ltd - Commercial 6 A 800µm Traditional CAFGmeasurements C K I In-line Horizontal 300µm 550µm In-line In-line Vertical Vertical E Anti-Pad Anti-Pad Inner Pads No Pads In-line Ground Anode Ground Anode J F D 400µm 800µm In-line In-line Vertical Vertical 400µm 400µm Horizontal B 400µm H 400µm 400µm 400µm Anti-Pad Anti-Pad Inner Pads No Pads Via Anode Via Anode Staggered 400µm geometry L No via 13 NPL Management Ltd - Commercial In-Line /staggered combs m 300 400 550 800 _ + Via to via + + - - Fibre Weave Fibre Weave In-Line 14 Staggered NPL Management Ltd - Commercial 7 Anti-pad combs Anode Via Comb I - + Comb J + - Comb K - + Comb L + - Inner Layer Inner Layer Pads Cathode Via 15 NPL Management Ltd - Commercial Parameters tested Factors affect CAF formation: Via wall gap Bias Pattern orientation Pattern arrangement Via polarity Presence of Inner-layer pads Surface finish Drill speed Reflow conditions PCB materials Board house 16 NPL Management Ltd - Commercial 8 Effect of tested variables on CAF formation-failure time 17 NPL Management Ltd - Commercial Conclusions- how to avoide CAF • Via to via gap • Fail faster with smaller gap • Bias • High bias significantly decrease the time to fail • Laminate Wicking, & inter yarn voiding Images courtesy of Isola • CAF resistance material reduce risk of CAF formation • Identically specified laminates from different suppliers have different CAF performance • Manufacture • The in house process of laminate at board house resulting in different CAF performance • Via and ground planes • Anodic via fail faster than cathodic via • Alignment to reinforcement • Warp and weft direction • Via staggered at 45° has greater CAF resistance • Reflow condition • High peak temperature in reflow is harmful to CAF performance of the PCB • Board finish, drill speed, inner pads, Tg of laminate have a less significant effect on CAF performance 18 NPL Management Ltd - Commercial 9 Publication Lead-Free and Other Process Effects on Conductive Anodic Filamentation Resistance of Glass-Reinforced Epoxy Laminate” Ling Zou, Alan Brewin & Christopher Hunt, The ELFENT Book on Failure Mechanisms, Testing Method, and Quality Issues of Lead-Free Solder Interconnects, 2011 p255-271 Susceptibility of Glass- Reinforced Epoxy Laminates to Conductive Anodic Filamentation, Alan Brewin, Ling Zou & Christopher Hunt, MATC(A)155, January 2004 19 NPL Management Ltd - Commercial Identifying CAF Typically CAF occurs in buried layers inside PCBs It is difficult to find and inspect, and hence it is difficult to study Work at NPL has investigated a method for controlling the growth of CAF, and then subsequently studying the growth. NPL has developed a sample structure where glass fibres are introduced, encapsulated, and then provide the correct electrochemical conditions to grow the CAF in relatively shot time. 20 NPL Management Ltd - Commercial 10 Simulated Test Vehicle (STV) Simulated Test Vehicle (STV) was developed, which provides a controlled way to grow CAF. This enables us to investigate different variables separately. CAF formation: Different resin systems and glass fibres: STV & STV with drilled hole Reflow process: STV Desmear process: STV with drilling hole Glass bundle size: STV + - CAF can be easily seen using microscope with backlight - + 21 NPL Management Ltd - Commercial STV1 sample - + + Polyimide substrate with 2 oz Cu Resin powder dissolved in acetone Resin cured at 150°C for 60 minute 22 NPL Management Ltd - Commercial 11 Test sample preparation NPL Management Ltd - Commercial Different variables 0.025ul 0.3mm 5mm Acid condition at anode (plating solution CuSO4+H2SO4 ) Ionic contamination inside PCB (NaCl coated fibres) Pathway between two conductors (Reflow process not necessary) NPL Management Ltd - Commercial 12 Test different resins and glass fibres - STV Anode contaminated with or without plating solution Resin Glass fibres 1080 finished Suppler DICY Cured* A 7628 finished 7628 finished Phenolic cured 7628 heat cleaned no finish B 7628 loom state no finish *DICY absent DICY cured resin 20m 25 NPL Management Ltd - Commercial Phenolic cured resin - 7628 heat clean fibre 1 2 3 4 5 6 7 8 Not reflowed 0 30 60 90 120 Time (hour) 150 180 13 12 11 10 9 8 7 6 5 Log R (ohm) Log R (ohm) 13 12 11 10 9 8 7 6 5 1 2 3 4 5 6 7 8 Reflowed 0 30 60 90 120 Time (hour) 150 180 Anode: no contamination Fibres: clean CAF formed on reflowed sample - + - + NPL Management Ltd - Commercial 13 Phenolic cured resin - 1080 finished fibre Log R (ohm) 1 2 3 4 5 6 7 8 0 30 60 90 120 Time (hour) 150 180 13 12 11 10 9 8 7 6 5 1 2 3 4 5 6 7 8 Log R (ohm) 13 12 11 10 9 8 7 6 5 0 30 60 90 120 Time (hour) 150 180 Anode: 100% plating solution Fibres: clean No CAF formed on both reflowed and no reflowed samples NPL Management Ltd - Commercial CAF formation with different resins and glass fibres Anod e c o n ta m in a tio n G la s s fib re s R e s in C A F fo rm a tio n N o t re flo w ed re flow e d x x 1 0 8 0 fin is h e d (A ) 7 6 2 8 fin is h e d (A ) 1 0 0 % p la ti n g s o lu tio n 7 6 2 8 fin is h e d (B ) 7 6 2 8 h e a t c le a n e d 7 6 2 8 lo o m s ta te D IC Y absent D IC Y c u re d 7 6 2 8 h e a t c le a n e d x 7 6 2 8 lo o m s ta te N one 1 0 0 % p la ti n g s o lu tio n 7 6 2 8 h e a t c le a n e d x 7 6 2 8 lo o m s ta te x x 1 0 8 0 fin is h e d (A ) 7 6 2 8 fin is h e d (A ) P h e n o l ic c u re d 7 6 2 8 fin is h e d (B ) - + - + NPL Management Ltd - Commercial 14 Different resins and glass fibres With anode contamination only: There is CAF formation for heat cleaned and loom state fibres with both resins (DICY cured and phenolic cured). There is no CAF formation for all finished fibres with both resins. NPL Management Ltd - Commercial Finished glass fibre - DICY resin Anode 100% plating solution 100% plating solution 20% plating solution 20% plating solution 100% plating solution 100% plating solution 20% plating solution 20% plating solution Fibre 3% NaCl 1% NaCl Reflow Yes No Yes No Yes No Yes No Anode contamination Fibre coated in NaCl Reflow & no reflow NPL Management Ltd - Commercial 15 Conclusion for finished fibres Anode 100% plating solution 100% plating solution 20% plating solution 20% plating solution 100% plating solution 100% plating solution 20% plating solution 20% plating solution Fibre 3% NaCl 1% NaCl Reflow Yes No Yes No Yes No Yes No CAF formation Yes No Yes No No No No No Three factors must be met for CAF formation Low pH at anode (Plating solution contamination) Ionic contamination inside PCB (NaCl coated fibres) Pathway between two conductors (Reflow process) NPL Management Ltd - Commercial CAF formation - different glass fibre bundle size Phenolic cured resin Anode: no contamination Fibres: Clean heat and loom state glass fibres Bundle size: small (~10) & large (30~50) Reflowed 32 NPL Management Ltd - Commercial 16 Small bundle Log R (ohm) 0 30 13 12 11 10 9 8 7 6 5 60 90 120 Time (hour) 1 2 3 4 5 6 7 8 150 Large bundle 0 30 60 90 120 Time (hour) 150 Small bundle 13 12 11 10 9 8 7 6 5 1 2 3 4 5 6 7 8 Log R (ohm) 13 12 11 10 9 8 7 6 5 loom state fibre 180 0 1 2 3 4 5 6 7 8 13 12 11 10 9 8 7 6 5 30 60 90 120 Time (hour) 150 180 Large bundle 9 10 11 12 13 14 15 16 Log R (ohm) Log R (ohm) Phenolic cured resin Heat cleaned fibre 180 0 30 33 60 90 120 Time (hour) 150 180 NPL Management Ltd - Commercial CAF formation - - + + - + - + - + 34 NPL Management Ltd - Commercial 17 STV with drilled hole - - + + Cu Polyimide Tap 100% 30% 10% STV is sitting on adhesive tape. Hole filled with 1µl different concentration plating solutions. Filling solution dried for 2 hours at room temperature before CAF testing. 35 NPL Management Ltd - Commercial CAF formation – desmear process No-desmear Desmeared 36 NPL Management Ltd - Commercial 18 CAF formation without desmear - Phenolic cured resin & 7628 finished glass fibre (B) 13 12 11 10 9 8 7 6 5 30 60 90 120 Time (hour) 150 Log R (ohm) Log R (ohm) 1 2 3 4 5 0 30% plating solution 13 12 11 10 9 8 7 6 5 100% plating solution 180 0 - + 30 60 90 120 Time (hour) 6 7 8 9 10 11 150 180 10% plating solution 13 12 11 10 9 8 7 6 5 Log R (ohm) 12 13 14 15 16 0 30 60 90 120 Time (hour) 150 More CAF formed with contamination increase 180 37 NPL Management Ltd - Commercial CAF formation with desmear -Phenolic cured resin & 7628 finished glass fibre Log R (ohm) 1 2 3 4 5 0 30 60 90 120 Time (hour) 150 30% plating solution 13 12 11 10 9 8 7 6 5 Log R (ohm) 100% plating solution 13 12 11 10 9 8 7 6 5 0 180 30 6 7 8 9 10 11 + 60 90 120 Time (hour) 150 180 10% plating solution 13 12 11 10 9 8 7 6 5 Log R (ohm) 12 13 14 15 16 0 30 60 90 120 Time (hour) 150 Desmear process increase CAF formation, but not significant. 180 38 NPL Management Ltd - Commercial 19 CAF formation on drilling sample + - - - + + - - + + 39 NPL Management Ltd - Commercial CAF formation - + 40 NPL Management Ltd - Commercial 20 STV1 and STV1 with drilled hole A STV has been successfully used to evaluate the effect of different resin systems, different glass fibres, desmear process, reflow process and glass fibre bundle size on CAF failure. Heat cleaned and loom state fibres form CAF more easily than finished fibres. Loom state fibre has the highest propensity to form CAF compared with others. Phenolic cured resin promote more CAF than DICY cured resin, this is probably because the DICY was removed when the resin was dissolved in acetone in our sample preparation. Desmear process can increase CAF formation, but not significant. Reflow process increase CAF formation significantly. Large bundle size increase CAF formation, but not significant. Further developments of a STV are underway 41 NPL Management Ltd - Commercial How to Avoide Conductive Anodic Filaments (CAF) Ling Zou & Chris Hunt 22 January 2013 42 21 Stand 1549 Presentations Tuesday 12.00noon “Cleaning and Contamination Failures – Causes and Cures” 2.00pm “Cleaning Under Low Stand Off Components” 3.00pm “Testing Printed Board Assemblies For Cleanliness” Wednesday 12.00noon “Evaluating a Cleaning Process Performance” 2.00pm “Reliability of Flux Residues Under Low Stand Off Devices” 3.00pm “Cleaning Package On Package Assemblies” Thursday 11.00am “Conductive Anodic Filament CAF Failures and Defect Analysis” 12.00 noon “Testing Adhesion of Conformal Coatings and Failures” 1.00pm “Testing Components for Cleaning Process Compatibility” Each presentation will be 15-20mins A copy of the presentation material can be obtained by visiting the stand http://www.ipcapexexpo.org/html/main/cleaning.htm NPL EI Free Technology Webinars Benefits of Using Nitrogen During Soldering and determining PPM Levels Thursday 14 February Attraction of Sn Tin Whiskers Wednesday 6 M arch BGA Reliability - The Effects of Solder Joint Voiding and Uneven Stand-Off Height Tuesday 30 April Reuse of Electronic Products Wednesday 15 M ay Through Hole Reliability for High Aspect via Holes Tuesday 11 June Functionalisiation and Applications of Nanomaterials for Electronic Applications M onday 18 July Environmental Robustness: Performance Coatings & High Temperature Interconnects Tuesday 3 September www.npl.co.uk/ei 22
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