Surface Defects Laboratory

Faculty of Electrical Engineering and Informatics\ Department of Electronics Technology
Surface Defects in Electronic Systems

Introduction of the research area

Due to a directive of the European Union, lead-free soldering has become a new production trend, where a number of different materials are used for surface finishes, solder alloys and unconventional technology settings. Due to these new technologies new kinds of reliability problems have been arisen. Our research concentrates on two of them: mitigation of tin whisker formation and the phenomenon of electrochemical migration. Both of these issues can result in failures (shorts), which in certain systems – like automotive systems – can even have a catastrophic outcome.

Brief introduction of the research place

The main focus of the research center – according to the Department's mission – is the material and technological aspects of physical connections between microcircuits and larger systems, where mainly knowledge on manufacturing, quality and reliability are used. As a consequence of demands on the part of electronics industry, including the miniaturization, the increase of integration, the increasing density of conductive layers and decreasing distances between the leads of components, smaller and smaller areas has to be investigated, while using interdisciplinary knowledge. For this reason, the research center is equipped with special machinery capable of investigating solid phase conductive, insulating and semiconductor structures and, in some cases, even liquid-phase measurements can be done, all between the scale of the human visibility to nano sizes.

 

History and context of the research

The main mission of the research center, beyond maintaining and developing the above mentioned competencies, is to provide experts with a solid knowledge for the domestic electronics industry. The rapidly changing technological skills are incorporated into the curriculum up to date and the research center also participates in the technological development. We consider an important task to strengthen and extend our industrial, corporate relations.

The classical model of electrochemical migration was first reported in the 1950s in the case of silver. The model describes the anodic dissolution in the presence of moisture and applied voltage, secondly the ion migration toward to the cathode side and finally, the metal ion deposition and conductive filament formation (dendrite).  The dendrites when forming short circuits, could even cause dangerous reliability and quality failures. Later it was proved that the classical model is also valid for several other metals, like tin, lead, copper, etc. This failure mechanism could significantly change in the presence of various contaminants. Additionally, recent appearance of new material systems in electronics required deeper investigations, which is well indicated by the increasing number of the novel publications.          

The phenomenon of tin whisker formation was first reported in the 1950s as well. Having clarified the whisker formation of classical tin-lead alloys, there had been relatively few papers published on whiskers up until the late 1990s. But this has changed on approaching the deadline (2006 July 1.) set by the European Union environmental regulations (RoHS) demanding the lead to be removed from electroplated tin films. There have been several reports on failures caused by whiskers, four satellites suffered complete failures due to tin whiskers, pacemaker models had to be recalled, and failed diodes in a nuclear power plant forced it to shut down. These events have also directed attention on the problem. Nowadays, more and more companies believe that tin whiskers might be the cause of failures and try making steps towards prevention.

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The research goal, open questions

Research on tin whisker and electrochemical migration failures has become more prominent because of the varied materials of contact surfaces and technological changes accompanying the introduction of lead-free soldering (RoHS). In both cases there is a danger that shorting occurs, thereby greatly reducing the lifetime of fine resolution circuit boards. The above mentioned processes typically do not occur in the nanometer size range, but the associated transport phenomena can be treated with the tools of nano physics. Examination of surfaces from the point of view of materials sciences will also help understand whisker growth – a phenomenon not yet satisfactorily explained. The aim of whisker research is to explore the factors that cause and affect this phenomenon, and possibly reduce its occurrence. This research focuses on exploring the environmental effects (temperature, humidity, oxidation, etc.) causing tin whisker growth, structural analysis of the use of barrier layers (nickel, silver, etc) and alloying metals to observe the grain structure of the tin and the intermetallic layers that is also partially responsible for whisker growth.

When investigating electrochemical migration on conducting-insulator-conducting structures in the presence of moisture, applied voltage causes the ionization of metals and the growth of tree-like filaments (dendrites), due to ion migration and deposition processes. The aim of the research is to examine the electrochemical properties of different surface finishes, as well as solder alloys on various substrate materials. It also aims to improve the existing test methods and to create novel monitoring and/or measuring methods.

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Methodology

To investigate the above mentioned two failure mechanisms, primarily reliability testing (climatic and environmental testing methods) are applied, however, in the case of electrochemical migration the so called water drop test is also widely used, nowadays. In case of the environmental tests, samples are stored in a climatic chamber where the temperature is higher than room temperature and/or relative humidity is also high.  This "aging" process is a common procedure in electronics for testing factors, which, separately or combined, influence the development of reliability issues. This test, however, is less suitable for qualitative observations since the process is not as fast as the water drop test. To quantitatively compare samples, they have to be tested under identical conditions (relative humidity, temperature, voltage) and the changes in electrical parameters (e.g. insulation resistance or the leakage current) monitored.

In the case of water drop test a well-defined (in terms of volume, and concentration) liquid drop is placed onto a conductive-dielectric-conductive structure, and a few volts (DC) is applied. Meanwhile, the formation of dendrites can be visually observed, and the time dependence of leakage current can measured as well. This method is highly qualitative, as it does not model the real circumstances. One definite advantage of applying it is cost-effectivity, as the electrochemical migration susceptibility of metals can be easily and quickly established in this way.

Additionally to environmental tests and water drop tests, application of various surface analysis methods is also required. The surface of the samples is primarily observed with scanning electron microscope (SEM). Then the aim is topographical mapping of the surface, where up to 100,000 x magnifications can be reached. The composition of surface can be determined by mapping elements with EDX (Energy-dispersive X-ray spectroscopy) method. EDX is a method capable of determining the composition of materials in a defined point or area, and can present either atomic or mass ratio. The cross-sectional exploration of the layers is carried out by FIB (Focused Ion Beam) technique, where the sample is being etched with a focused ion beam. This gives more information about the cross-sectional structure, and in this way the grain structure of layers can be examined as well. Orientation of the grains and their element composition is analyzed by transmission electron microscope (TEM), which presents the structure in the nanometer range.

Results

The issue of electrochemical migration is now widely known in scientific circles, as it causes short-circuiting of electronic assemblies during operation under the presence of moisture. This issue is in the focus of research because it still raises many questions. Basically, two methods have been widely used for the monitoring of this phenomenon; one applied at room temperature and the other one under extreme conditions (high temperature and humidity), in climate chambers. Our basic goal was to obtain more information about the electrochemical migration processes. Therefore, we have developed an innovative monitoring system, which allows the "in-situ" and "real-time" observation of the process of water condensation and the following shorting under extreme conditions (high temp and humidity) in different surface conductor-insulator-conductor structures. The in-situ and real-time observation is possible with the following limits: -20 to +150 degrees Celsius, 100% relative humidity used. To verify the measuring system, electric signal changes were measured and video recordings have been made to gather information on water condensation and the subsequent formation of dendrites. It was observed that the condensation was more intense on metals, where the initial small water droplets had swollen and suddenly transformed from small water "islands" into a continuous moisture layer, which sometimes led across the surface of the conductor-insulator-conductor structure. After this, a dendrite formation which led to shorting could be observed. This observation is important, because it clearly shows that processes leading to shorts in climatic chambers can better model failure mechanisms than water drop method. The reason is that this latter method ignores the condensation time, which is presumably varied on different conductor-insulator-conductor structures.

The aim of our whisker research is to analyze the growth mechanism, investigate mitigation strategies and study their effect on whisker growth. Using underlayer materials between tin and copper helps mitigate whisker phenomenon. These metallic underlayers prevent the formation of Cu6Sn5 intermetallics which can otherwise cause high mechanical stresses at the border of the copper and tin coating. We had aged samples with nickel and silver underlayers in different aging conditions and it showed various results depending on the underlayer materials and aging methods applied. Our other aim was to analyze the whiskering effect of oxides on the tin layer. Interestingly, when ageing in 105°C/100%RH environment, the surface of both nickel and silver underlayered samples has changed. This environment generates whiskers in high density on silver underplated surfaces, but the whiskers were short because oxidizing process initiated by humidity prevented further growth of whiskers. We have also studied the impact of the so-called interdiffusional effects on the underlayer materials, where forces initiated by mechanical stress significantly influence the process of whisker growth.

Expected impact and further research

The underlayer materials between tin and copper help mitigating whisker phenomenon. The interdiffusional effects of the underlayer materials are being studied in respect of stress that initiate mechanisms leading to whisker growth. Sn-Ag and Sn-Ni interdiffusion is a field barely researched in literature. Our previous test results showed that these existing results are incorrect. Our plan is to build a more accurate and correct model based on our observations. Also, doping tin platings with additional elements like copper changes the properties of platings. Investigating the effect of these elements cause changes in whiskering and corrosion resistance. With the help of material analyzing methods, measurements can be made to evaluate its effect on whisker growth. Another point of focus is to examine the electrochemical properties of these alloys and pure tin. Since the research fields of both tin whiskers and tin oxides still feature a lot of open questions, we expect these measurements to bring valuable results to both electronics engineering and materials sciences.

With the introduction of lead-free soldering, electrochemical migration has become again a focus area in the reliability of electronic circuits, units and assemblies. New materials are applied on different substrates with different technologies, and are operated under various environmental conditions. This alone is a huge challenge that will surely fill our research capacity in the coming years. Susceptibility to electrochemical migration depends on (not limited to) many factors, including: substrate materials, surface finishes, conductive path distance, operating voltage, moisture appearance mode, moisture quantity and type of contaminants and their concentration, etc. As a first step, we are going to investigate the electrochemical migration behavior of lead free solder alloys using different soldering technologies. We expect that the results will be useful for electrical industry in their choice of material and in setting optimal technological parameters during manufacturing.

Publications, references, links

Major publications of the last 5 years:

Bálint Medgyes, Balázs Illés, Richárd Berenyi, Gábor Harsányi. In situ optical inspection of electrochemical migration during THB tests. JOURNAL OF MATERIALS SCIENCE–MATERIALS IN ELECTRONICS 22:(6) pp. 694–700. (2011) IF: 1.020*, Full document, Full document at the publisher, DOI: 10.1007/s10854-010-0198-4

Bálint Medgyes, Balázs Illés, Gábor Harsányi. Electrochemical Migration Behaviour of Cu, Sn, Ag and Sn63/Pb37. JOURNAL OF MATERIALS SCIENCE–MATERIALS IN ELECTRONICS, in press , DOI: 10.1007/s10854-011-0435-5, IF: 1.020*

Balázs Illés, Barbara Horváth, Gábor Harsányi. Effect of strongly oxidizing environment on whisker growth form tin coating. SURFACE & COATINGS TECHNOLOGY 205: pp. 2262–2266. (2010) IF: 1.793*, WoS link, DOI: 10.1016/j.surfcoat.2010.09.012

Barbara Horváth, T Shinohara, Balázs Illés, Gábor Harsányi. Tin Whisker Growth at High Humidity Environments – Investigated on Ni and Ag Underlayered Leadframes. 57th Materials and Environment Debate. Okinawa, Japan, 20-22 October, 2010., 304–307. Paper C-301.

Barbara Horváth, Balázs Illés, Gábor Harsányi. Investigation of tin whisker growth: The effects of Ni and Ag underplates. In: 32th Spring Seminar on Electronics Technology. Brno, Czeh Republic, 13-17 May, 2009, (IEEE) Brno: pp. 1–5. Paper D07, DOI: 10.1109/ISSE.2009.5207031

Major publications prior the last 5 years:

Harsányi G, Inzelt Gy. Comparing Migratory Resistive Short Formation Abilities of Conductor Systems Applied in Advanced Interconnection Systems. MICROELECTRONICS AND RELIABILITY 41: pp. 229–237. (2001) IF: 0.548, WoS link

Harsányi Gábor. Irregular Effect of Chloride Impurities on Migration Failure Reliability: Contradictions or Understandable? MICROELECTRONICS AND RELIABILITY 39: pp. 1407-1411. (1999) IF: 0.374, WoS link

Harsányi Gábor. Copper May Destroy Chip-Level Reliability: Handle with Care – Mechanism and Conditions for Copper Migrated Resistive Short Formation. IEEE ELECTRON DEVICE LETTERS 20:(1) pp. 5-8. (1999) IF: 3.018, WoS link

Harsányi Gábor. Material Design Aspects of High Reliability high Density Interconnects. MATERIALS CHEMISTRY AND PHYSICS 45: pp. 120-123. (1996) IF: 0.583

Harsányi Gábor. Electrochemical Processes Resulting in Migrated Short Failures in Microcircuits. IEEE TRANSACTIONS ON COMPONENTS PACKAGING AND MANUFACTURING TECHNOLOGY PART A 18:(3) pp. 602–610. (1995) IF: 0.179, WoS link

Links:

http://mycite.omikk.bme.hu/search/slist.php?lang=0&AuthorID=10001333

http://mycite.omikk.bme.hu/search/slist.php?lang=0&AuthorID=10001162

http://mycite.omikk.bme.hu/search/slist.php?lang=0&AuthorID=10005055

http://mycite.omikk.bme.hu/search/slist.php?lang=0&AuthorID=10005593

Presentation of participants:

Prof. Dr. Gábor Harsányi

Doctor of the Hungarian Academy of Sciences, Head of department at the Department of Electronics Technology, Budapest University of Technology and Economics. Research areas: hybrid circuits, multichip modules, sensor technology.

dr. Balázs Illés

Electrical Engineer, Ph.D., currently an assistant professor at the Department of Electronics Technology, Budapest University of Technology and Economics. Research field: modeling and optimization of soldering mechanisms and investigating the mechanism of tin whisker formation.

Barbara Horváth

Electrical engineer, M.Sc., currently a third year Ph.D. student at the Department of Electronics Technology, Budapest University of Technology and Economics. Research field: growth mechanism of tin whiskers, printed wiring boards.

Bálint Medgyes

    Electrical engineer and economist, M.Sc., currently a research engineer at the Department of Electronics Technology, Budapest University of Technology and Economics. Research Area: quality management and reliability in electronics, electrochemical migration.