Whisker Mitigation Strategies for Pb-Free Electronic

Geoffrey Wilcox (1252749)

Mark Ashworth (1258119)

English

Loughborough University Institutional Repository

Whisker Mitigation Strategies for Pb-Free Electronics M.A. Ashworth and G.D. Wilcox SMART Group Advances in Electronics Assembly Technology Seminar 16th June 2015 Outline of presentation • Background on tin whiskers  • Introduction to WHISKERMIT • Tin deposits on Cu and brass • Electrochemical oxidation  • Conformal coatings • Summary 2 Background – tin whiskers • Crystalline growths from a metal surface (e.g. Sn, Zn and Cd) • Uncertain incubation period before growth • A few micrometres in diameter and up to several millimetres in length • Various growth morphologies possible • Whisker related problems increasing due to environmental legislation, device miniaturisation, lower voltages and harsher environments        10µm 1µm 10µm 20µm Filament Hillock/nodule Odd-shaped eruption Spiral Kinked 1µm 3 Documented electronic failures due to tin whiskers  Aerospace Military Medical Industrial NASA: “More than 50 Electronic Failures due to Whisker Growth from 1946-2006”  Galaxy satellite Military failures Heart pacemaker recall Nuclear power plant  Documented Incidents:  Only 10% of Failures known to NASA                   Images: www.google.co.uk [Accessed June 2014]. L. Panashchenko, “The Art of Metal Whisker Appreciation: A Practical Guide for Electronics Professionals”, NASA Goddard, 2012. 4 Tin whisker research at Loughborough University WHISKERMIT Process optimisation (commercial bright tin) Deposition current density  Deposit thickness Pulse plating Incorporation of particulates In-service mitigation strategies Conformal coating Electrochemical oxidation to mitigate whisker growth  Effect of process variables: Sn deposits on Cu Deposit thickness 10 mA cm-2 20 mA cm-2 30 mA cm-2 40 mA cm-2 #1 #2 #3HT #1 #2 #3HT #1 #2 #3HT #1 #2 #3HT 2 µm X X X X 5 µm X X 10 µm X X X 2µm 2µm 2µm 2µm 2µm 1µm 10µm 1µm 2µm 10mAcm-2 2µm #1 10mAcm-2, 2µm, #3 20mAcm-2 2µm #1 30mAcm-2 2µm #1 10mAcm-2 5µm #3 10mAcm-2 10µm #1 20mAcm-2 10µm #3 30mAcm-2 10µm #1 30mAcm-2 5µm #1 No whisker growth observed from ‘batch 2’ samples No increase in whisker growth for samples stored in oven No whisker growth from any sample electroplated at 40 mA cm-2 ~ 2 years after deposition Effect of elevated temperature on intermetallic formation 2µm 5 µm tin 20 mAcm-2 2 years room temperature storage 2µm 10 µm tin 20 mA cm-2 2 years storage includes 5000 hours at 55°C/85% RH 1µm Cu6Sn5 Cu3Sn 1µm Cu6Sn5 FIB analysis ~ 2 years after tin deposition • Cu3Sn layer develops during storage at 55°C • IMC layer becomes more planar • No acceleration in whisker growth observed Effect of process variables: Sn deposits on brass 29 months after deposition  Increased deposition current density Increased deposit thickness 2µm, 40mAcm-2 10µm, 40mAcm-2 10µm 2µm, 10mAcm-2 10µm, 10mAcm-2 1µm 1µm Deposit surface Deposit surface Effect of deposition current density on whisker growth 680 1775 020406080100120140160180200Filaments Eruptions Eruptions/nodules < 10 µmNumber of features /mm2 Whisker feature type 10mAcm-220mAcm-230mAcm-240mAcm-2Evaluation of whisker density for 2µm tin deposits on brass Analysis approx. 33 months after tin deposition Storage at room temperature Effect of deposition current density on whisker growth 232 01020304050Filaments Eruptions Eruptions/nodules < 10 µmNumber of features /mm2 Whisker feature type 10mAcm-220mAcm-230mAcm-240mAcm-2Evaluation of whisker density for 10µm tin deposits on brass Analysis approx. 33 months after tin deposition Storage at room temperature Sn deposits on brass: whisker mechanism 1µm 5 µm tin deposit on brass, electroplated  at 10 mAcm-2 FIB analysis ~29 months after tin deposition Blocky Cu6Sn5 intermetallic Zn oxide Intermetallic growth  in brass? Zn map of deposit surface For tin deposits on brass, whisker growth is primarily driven by zinc oxide formation at the deposit surface rather than intermetallic growth at the Sn-brass interface Tin deposit on brass: Zn present in tin whisker? Zn Lα x-ray map ‘Bulk’ analysis: SEM/EDX mapping Surface analysis: Auger electron spectroscopy 50 250 450 650 850 1050Kinetic energy (eV) 50 250 450 650 850 1050Kinetic energy (eV) C Sn Sn O O Zn Zn SEI Zn present on whisker surface Zn absent after oxide removed  No enriched Zn signal from whisker Prior to sputter After sputter 5µm Intermetallic growth  in Sn Zn oxide Intermetallic growth in brass? Sn deposit on brass: effect of elevated temperature 100µm 2µm Exposure of Sn deposits on brass generally results in a considerable increase in whisker growth Also samples where no whisker growth has occurred! Effect of electroplating parameters • For both Sn deposits on Cu and Sn deposits on brass, whisker growth is reduced at higher deposition current densities and increased deposit thicknesses • Current results only applicable for the specific Sn electroplating bath under investigation • Significant variation in whisker growth may be observed on nominally identical samples • Although whisker growth is strongly influenced by electroplating parameters, their control cannot be relied upon to successfully mitigate whisker growth  additional control measures are required  Whisker growth for tin deposition on silver • Ag has been considered as a potential underlayer material to mitigate whisker growth for tin deposits on copper • Sn deposits on silver sheet whisker much more rapidly than equivalent deposits on copper • Recent results have also demonstrated increased whisker growth for Sn deposits on Cu with an electroplated Ag barrier layer • According to Crandall 1: “Sn on Ag is a remarkable whisker producer!” and “holds our internal lab record for prodigious whisker growth” 50µm 1 E. R. Crandall, Factors Governing Tin Whisker Growth, PhD thesis, Auburn University, 2012 2µm 2 µm tin deposit on silver Deposition at 20 mA cm-2 ~ 2 years storage at room temperature The role of the surface oxide in whisker growth  In 1994, Tu proposed his “cracked oxide theory” 1  whisker growth occurs at certain weak spots on the surface where the oxide has been broken  In the absence of an oxide no whisker growth would occur   Later adding 2 …  “if the surface oxide is very thick, it will physically block the growth of any hillocks and whiskers”  Can we mitigate whisker growth by increasing the thickness of the surface oxide??  16 1 Physical Review B 49, 2030, 1994 2 Proceedings of the IEEE Electronic Components and Technology Conference, 2002 p1194–1200 SnCu deposits on Cu: Electrochemical oxidation at 1.2V 02004006008001000120014001600Native air formed oxide Borate buffer solution Potassium bicarbonate-carbonatesolutionWhisker density (whiskers/cm2 )  6 months 2 months2 µm Sn-Cu on Cu deposited at 10 mAcm-2 Greatly reduced whisker density for electrochemically oxidised samples Evaluation of whisker density Increase in whisker density between 2 and 6 months 17 Test coupons to investigate whisker growth  • To more fully investigate the effect of oxidation potential samples have been electrochemically oxidised in a potassium carbonate-bicarbonate bath at -0.66V, -0.5 V, -0.4V, 0 V and 1.2 V (all vs. SCE). Electrochemical oxidation at -0.5V Native air formed oxide 30 mC cm-2 passed 60 mC cm-2 passed • Regions  with both 30 and 60 mC cm-2 charge passed • Whisker growth evaluated using optical microscopy after ~ 2 months storage at room temperature Whisker density after ~ 2 months storage 1686 1230 1793 2477 3736 337 1161 3295 3031 2586 226 544 1015 1000 732 0500100015002000250030003500400045001.2 V 0 V -0.4 V -0.5 V -0.66 VWhisker density (cm-2) Oxidation potential (V vs. SCE) Native air-formed oxide30mC/cm^260mC/cm^2~87% decrease ~60% decrease ~43% decrease ~80% decrease ~56% decrease Effect of oxidation time: whisker growth 0500100015002000250030003500Native air formedoxide5 min at - 0.66 V 20 min at - 0.66 V 30 min at - 0.66 VWhiskers/cm2 1 µm Sn-Cu on Cu deposited at 10 mAcm-2 Electrochemical oxidation in potassium bicarbonate-carbonate whisker analysis 2 months after deposition 10 µm Native air formed oxide -0.66 V vs. SCE for 5 min -0.66 V vs. SCE for 20 min -0.66 V vs. SCE for 30 min 10 µm 2 µm 1 µm 20 FIB/XPS analysis of SnCu deposits on Cu: ~2 years storage SnCu on Cu Electrochemical oxidation at 1.2V borate buffer  SnCu on Cu Native air formed oxide Cu6Sn5 intermetallic Cu6Sn5 intermetallic 2µm 2µm 01020304050600 200 400 600 800Atomic % Sputter time (s) C1sO1sSn3d5 oxideSn3d5 metal01020304050600 200 400 600 800Atomic % Sputter time (s) C1sO1sSn3d5 oxideSn3d5 metalSn deposits on brass: whisker growth 20µm 20µm SEM/FIB analysis of 2 µm Sn deposits on brass Number of whiskers on electrochemically oxidised deposits after 2 years storage (Sample area = 4cm2) Native air formed oxide Oxidised at 1.2 V vs. SCE Oxidation potential (V vs SCE) Average number of whiskers per sample 1.2 V 6 ± 2 1.6 V 2.5 ± 2 2.0 V 3 ± 2 2µm 2µm Zn oxide 22 01020304050600 200 400 600Atomic % Sputter time (s) FIB/XPS analysis of Sn deposits on brass: ~30 months storage Sn on brass: Native air formed oxide 4µm Pronounced ZnO formation Large intermetallics Sn on brass: Electrochemical oxidation  1.2V borate buffer 4µm No ZnO formation Reduced intermetallic formation? Sn oxide O Zn Sn metal C 01020304050600 200 400 600Atomic % Sputter time (s) Sn oxide O Zn Sn metal C Grain boundary zinc oxide Whisker mitigation mechanism(s) • For Sn(Cu) deposits on  Cu, IMC formation appears unaffected by electrochemical oxidation and therefore whisker mitigation must simply derived from the increased thickness of the oxide layer • Although whisker density is greatly reduced long filament whiskers are still produced from electrochemically oxidised SnCu deposits • Electrochemical oxidation does not reduce the driving force for tin whisker growth (intermetallic formation) • For Sn deposits on brass, electrochemical oxidation reduces whisker growth by preventing the formation of Zn oxide at the deposit surface • Reduced IMC formation in electrochemically oxidised samples may also be an influence WHISKERMIT: Conformal coating trials • Evaluation of commercial urethane, acrylic, silicone and UV cure coatings • Samples cured both at room temperature and in the oven • Model coating materials (UV cure) prepared with tailored properties (flexible to rigid) • Evaluated using tin deposits on brass substrates  20µm 10µm 10µm 20µm Effect of physical properties on whisker growth  Original WHISKERMIT project demonstrated correlation between coating modulus and whisker growth/penetration  Develop conformal coatings that are specifically designed to prevent whisker growth 5µm No interaction between coating and penetrating whisker 5µm Low stiffness Intermediate stiffness 20µm High stiffness Tenting before whisker penetration No tenting or whisker penetration 2µm Penetrating whisker ‘flattened’ by interaction with stiff coating  The next step….. WHISKERMIT 2 Improving the whisker mitigation properties of polymeric conformal coatings  Summary • Whiskers do not always do what you expect! • Whisker morphology and density may be modified by control of deposition parameters • Greatly increased whisker growth for tin deposits on brass • Silver may not be an ideal barrier layer material • Electrochemical oxidation has the potential to mitigate whisker growth • Conformal coatings optimised to mitigate whisker growth are required   Any questions? 

Whisker Mitigation Strategies for Pb-Free Electronics

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Whisker Mitigation Strategies for Pb-Free Electronics

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