Green Environmentally Friendly Technology For Tantalum And Niobium Oxide Capacitors

Transcription

1 CARTS Europe 2004: 18 th Annual Passive Components Conference, October 18-21, 2004 Green Environmentally Friendly Technology For Tantalum And Niobium Oxide Capacitors F.Priban, T.Zednicek, S.Zednicek, J.Tomasko AVX Czech Republic s.r.o., Dvorakova 328, Lanskroun, tel , fax , Abstract The requirements for modern technology today are not only to enhance product parameters or reduce cost but more and more importance is paid to use environmentally friendly materials and evaluate the whole product life cycle. In addition, legislation has been created that bans use of specific hazardous materials such as lead, mercury, cadmium etc. The impact of the green technology to the selection of material and processes can be quite significant. The paper discusses modifications of technology and materials needed on tantalum and niobium oxide capacitors including elimination of lead from terminations and alternatives to halogenated flameretardants in plastic packages for tantalum capacitors while continuing to provide excellent reliability and cost efficiency. electronic components and packaging comply with the recent environmental requirements. The requirements define elements and components, which are banned, restricted or under control. However the true Environmental Friendly products involving the whole production period, starting on preparation of basic compounds and materials, continuing onto processing, waste recycling and energy saving. All these requirements need to be built-into a development of new materials, processes and equipments. This phase is the most efficient point to consider all the critical environmental aspects. This paper discusses some GREEN opportunities for the introduction and practice on Tantalum and Niobium Oxide capacitors production. Legislation Introduction Electric and electronic industries are increasingly important in everyday life. The modern electronic devices are becoming consumer type electronic equipments with relatively short lifetime. Cellular phones are a good example where the average lifetime has dropped during the last five years from 3 to 1 year. Electronics represents only about 5% of solid waste, however 90% is land filled, and only around 2% of waste is today re-used or recycled. This situation is not possible to maintain from a longterm perspective. The task today is to find ways for us to live in harmony with the environment, actively working on replacement of toxic components (PCC, PBC, CFC, heavy metals ), reduction of waste, energy saving procedures and materials etc. This all is necessary to keep life sustainable in the very long term and be good global citizens. EU, Japan and some manufacturer directives specify that materials and processes used for The basic legislation requirements are RoHS (Restrictions on Hazardous Substances), WEEE (Waste from Electrical and Electronic Equipments), ELV (End of Life Vehicles), and other directives, Short summary of RoHS and WEEE banned substances in new electronic equipment in the European Union from 1 July 2006: heavy metals - lead, cadmium, mercury and hexavalent chromium brominated flame retardants PBB (polybrominated biphenyls) and PBDE (polybrominated diphenylethers) These directives apply to products manufactured and imported into European Community member states. Restrictions are for categories: - Large and small household appliances - IT & Telecommunication equipment - Electrical and electronic tools - Consumer equipment - Lighting equipment - Medical equipment systems 91

2 (with the exception of all implanted and infected products) - Automatic dispensers - Toys - Monitoring and control instruments The exemptions: - Lead in high melting temperature type solders - Lead in glass in electronic components - Lead in piezoelectric devices - Lead in servers, storage and storage array systems (exempt until 2010) -Lead in solders for network infrastructure, equipment for switching, signaling, transmission as well as network management for telecommunication WEEE [1] The main requirements of the Directive are the collection, treatment, recovery, financing and information regarding waste from electrical and electronic equipment. The purpose of this Directive is, as a first priority, the prevention of waste electrical and electronic equipment, and in addition, the reuse, recycling and other forms of recovery of such wastes so as to reduce the disposal of waste. It also seeks to improve the environmental performance of all economic operators involved in the life cycle of electrical and electronic equipment and in particular operators directly involved in the treatment of waste electrical and electronic equipment. RoHS[2] Member States shall ensure that new electrical and electronic equipment marketed after 1 January 2006 does not contain lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB) or polybrominated diphenylether (PBDE). Sony technical standard SS00259 [3] Targets are the parts, materials, and other articles that are produced by the Sony group or by third parties to which the Sony group subcontracts the production of products. Restrictions are for categories - Semi-finished products - Parts - Screws - Accessories - The materials employed in subsidiary parts and materials used for products - Instruction manuals - Repair parts - The packaging materials that parts suppliers use for delivery and protection - Substances that are in regulation - Heavy metals and components Cadmium, Lead, Mercury, Hexavalent chromium - Polychlorinated compounds - Polybrominated compounds (exemptions is TBBP-A) - Organic tin compounds - Asbestos - Formaldehyde - Azo compounds Adherence by suppliers to these Sony requirements is recognized by a suppliers certificate Sony Green Partner. AVX is one of the few companies outside Japan to have received this recognition. Green Tantalum and NbO Capacitors The first step to achieve GREEN PRODUCT is to determine non-green processes and materials, which are used for production and product. Production processes for tantalum and niobium oxide capacitors is composed from many steps and materials, and is not limited to the finished product but part processses (i.e. washing medium, solvents) Analysis defined next base critical parts: encapsulant plating Capacitor termination leadframe plating: Lead Encapsulant material= mould resin: antimony trioxide and TBBP-A Fig.1. Green Concerns on Tantalum Capacitors Termination Plating Capacitor terminations are plated on their surface in order to improve solder wetting during board assembly process. The traditional plating material for a long time was tin-lead alloy 90/10 (melting point 215 C). Advantage of this material is in cost, solder stability, resistance, easy processing and excellent solderability. 92

3 The main task for green requirements is to find the surface plating with adequate parameters, but lead-free (heavy metals free) that is suitable to be used with the current processes with little or minimal modification. It can not be also assumed that the whole industry will change to lead-free overnight, hence such material needs to be compatible not only with the new lead-free assembly but also with the current lead based process. Comparing potential substitute materials, all positives and negatives, we can find that there is practically no existing equivalent material to SnPb alloy. However, some very similar materials can be found with some limitations. The requirements on lead free material: Low cost (Indium is good, but price Non hazardous Mechanical reliable (contact strength) Thermal fatigue resistant Compatible with a variety lead free solder Good wetting Good thermal and electrical characteristics Compatible with current equipment and chemistries Plating comes from electrolysis process. Electrolysis is an ideal process for pure elements, however preparation of alloy can be complicated, with regards to a different electric potential of elements. Modern electrolysis methods are capable to maintain the required process with high level of accuracy and stability; hence the material plating is possible to be produced in good quality with some care. Al +3 / Al Zn +2 / Zn Fe +2 / Fe Cd +2 / Cd Reaction Potential (V) Al e - -- Al - 1,68 Zn e - -- Zn - 0,76 Fe e - -- Fe - 0,44 Cd e - -- Cd - 0,40 In +3 / In In e - -- In - 0,34 Ni +2 / Ni Ni e - -- Ni - 0,23 In +1 / In In +1 + e - -- In - 0,20 Notes Easy oxidation, whiskers, corrosivity Toxic, WEEE, RoHS Cost, easy oxidation, poorcorrosivity resistance Cost, easy oxidation, poorcorrosivity Sn +2 / Sn Pb +2 / Pb Fe +3 / Fe H + / H 2 (g) Sn e - -- Sn - 0,14 Pb e - -- Pb - 0,13 Fe e - -- Fe - 0,04 2H e - -- H 2 +0,00 Bi +3 / Bi Bi e - -- Bi +0,32 Cu +2 / Cu Cu +1 / Cu Ag +1 / Ag PdO / Pd Au +3 / Au Cu e - -- Cu+0,34 Cu +1 + e - -- Cu +0,52 Ag +1 + e - -- Ag +0,80 PdO + 2H e - -- Pd + H 2 O +0,92 Au e - -- Au+1,42 resistance Very sensitive on lead during assembly, intermetalics with Sn, intermetalics with Sn Cost, wetability Cost Cost, wetability Fig.2. Electopotentials for selected materials The closest material from SnPb 90/10 alloy is pure tin, the same metal already in the majority of the solder i.e. Sn Pb 90/10 to 100% Sn. Some components with an external termination finish of pure tin, such as ceramic capacitors or resistors, are already available and so there is wide practical experience with this kind of termination finish. Whiskers [6] are thin hair like crystals that can grow from the pure tin surface under certain conditions. Concerns remain that if allowed to grow, these may cause short circuits between metalized pcb tracks. (Fig.3. Tin whisker - starting growth, Fig.4. Tin whiskers) Fig.3. Tin whisker - starting growth 93

4 Fig.4. Tin whiskers Typically, users will assess the risk based on their own pcb processing and end application conditions. Suppliers will take necessary precautionary measures to prevent or retard such growth. At a component level, under reflow conditions, the solder paste into which the component is placed will wet most of the termination surface. This area will not be susceptible to whisker growth, as the additional elements alloyed in the solder paste will also alloy with the tin from the termination plating. Only a minor portion of the termination area will remain pure tin coated as in Fig 5: Negative Termination PCB Mould Body ANODE Pure Tin Surface after Board Mounting Positive Termination Solder Paste Most lead-free solder systems will require a peak reflow temperature from 240 C to 260 C. Component manufacturers will need to review all design parameters necessary to enable product to cope with the higher thermal stress resulting from these soldering processes. Replacement of some materials and development of low stress alternatives will be necessary to reduce the thermal stress inside the component. The reflow profile below is recommended and tested by AVX for eutectic SnAgCu and SnAgBiCu based lead-free pastes in combination with pure tin finish tantalum capacitors. - max peak gradient: 2.5 C /s - peak temp. recommend /- 5 C - peak temp. max 260 C - time at >230C: 40s max The capacitors are designed and treated to withstand up to 3times lead-free reflows with conditions above. More references on recommended reflow peak temperature and leadfree soldering see [5]. Attention must be paid not to exceed the maximum peak temperature during reflow because of accelerated oxidisation of termination surface. Significant reduction of solderability may result in such case. This effect can be minimized in the case of reflow with nitrogen atmosphere. Examples of tests by multiple reflow exceeding specification above - see Fig. 6 (without nitrogen) and Fig (with nitrogen). Fig.5. Tantalum Capacitor after Board Mounting Hence the whisker growth risk is related mainly to pure tin present on board such as pad plating rather than component termination finish. There are preventive methods that minimise occurrence of whiskers: - nickel underplating - matt tin with low organic content - reflow after plating for recrystalization - higher temperature plating bath to remove hydrogen - minimized chemical and mechanical stress All of the above whisker preventive items have been used and applied to the green tantalum and NbO capacitor technology. More references to whiskers and prevention methods can be found in reference [7, 8, 9, 10]. Higher reflow peak temperatures Fig.6. Soldering with multiple excessive peak reflow in air atmosphere Fig.7. Soldering with multiple excessive peak reflow with nitrogen atmosphere 94

5 Mould resin Mould resin (encapsulant) provides SMD components with a well-defined case, shape and dimension suitable for fast handling during SMD manufacturing process and precise placement. This material has many functions, and from this functionality comes inconsistent needs good flow and low viscosity during moulding, high mechanical resistivity, good humidity protection, high ignition energy etc. Commonly used epoxy based mould resin consists of the following key parts: epoxy resin filler hardener release agent coupling agent colour pigments flame retardants All the parts of the mould resin are environmental friendly compounds excluding the flame-retardants based on brominated components- TBBPA and antimony trioxide. There were two options for solution depending on capacitor type: 1. Remove flame retardants (new material is flame retardant free) group A 2. Replace flame retardants (new material contains alternative green flame retardants) group B The required level of flame redundancy depends on capacitor anode material (Ta vs Niobium Oxide-NbO) and cathode material (MnO 2 vs. CP-Conductive Polymer): Tantalum + Low ignition energy F.R. required MnO 2 Tantalum + CP High ignition energy F.R. free Niobium oxide High ignition energy F.R. free + MnO 2 Niobium oxide High ignition energy F.R. free + CP Fig.8. Table of technology vs flame retardant needs How difficult is to replace flame retardant? Before replying it is necessary to identify the burning and function of flame retardants on mould resin. - How does the material burn? All solid materials don t burn directly. It is necessary to be firstly decomposed (i.e by heat) to release flammable gases. Visible flames are burning these flammable gases with oxygen. These reactions are maintained by radical (O, OH ) breaking the molecules and prepare carbon, which can react with oxygen and generate more heat energy, which promote this reaction. - Why are flame retardants necessary? Flame retardants are chemicals inhibiting the combustion process. Their effect is to reduce the chance of a fire starting, this means increase resistance to ignition and reduce burning rate. The group of flame retardants is wide: halogenated components (TBBP-A, HBCD, DecaBDE, ) Phosphorous components (TCEP, TCPP,TDCP, ) Inorganic components (Al(OH) 3, MgOH, boron comp., silica, ) Nitrogen compounds For epoxy resin for tantalum and niobium oxide capacitors is most used flame retardant s material TBBP-A and Sb 2 O 3. - Why TBBP-A? Function of brominated components (TBBP-A) is effectively removing (capture) the radicals from gas. - Why antimony trioxide? [4] Antimony trioxide helps the breakdown of brominated components and prepares the bromine free radicals. It also reacts with bromine and produce brominated antimony compounds. They are effective in removing the free radicals (O, OH ) as well. Flammability of tantalum and niobium oxide capacitors was split in two groups see Fig.8. The biggest group includes capacitors with high ignition energy. Green mould material was from group A = flame retardant free. The second group is tantalum capacitors with MnO 2 - group B. For these components it was necessary to use a green flame retardant. The requirement is to develop green mould material with the same flame retardancy, (UL94, Oxygen index), free of phosphorus compounds. Green Flame Retardants (FR) 95

6 The new generation of mould materials has been developed in co-operation with external suppliers. It was not possible just to replacement FR in the mould resin as the FR type may have an effect to other process parameters that needs to be maintained. Thus new generation of mould resins had to be developed. One of the verification tests is a measurement and comparison of time, necessary for the snuff out of burned capacitors. Comparison for non-green and green flame retardant material see Fig.9. References [1] Directive 2002/96/EC of the European Parliament And of the Council of 27 January 2003 on waste electrical and electronic equipment (WEEE) [2] DIRECTIVE 2002/95/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 27 January 2003on the restriction of the use of certain hazardous substances in electrical and electronic equipment frequency Time to capacitor burn stoped Green FR material Non green FR material Fig.9. Flame retardancy comparison of original and green FR material Summary and Conclusion This paper summarises the current status of green technology and AVX s research to date. Sn (pure tin) was previously found to be the most serious candidates for replacement of the conventional tinlead termination finish. However, pure tin was found to be the only one available termination finish at the moment compatible with conventional tin-lead and new lead-free board mounting systems suitable for massive production of tantalum capacitors. The new green resin type has been proved to be comparable to the original brominated retardant resin in all electrical and mechanical parameters, including the flame retardancy function. The Tantalum and Niobium Oxide capacitors manufactured in accordance to the described processes were independently audited and received Sony Green Partner Certificate in A new generation of lead-free and halogenates-free Tantalum/NbO capacitors have been developed. This new generation is friendly to our environment and thus these are better components showing our responsibility for today s and the future life. [3] Management Regulations for the Environment/related Substances to be Controlled which are included in parts and materials, The 3 rd edition of SS (excerpts) [4] Volante, C.N.: A review of Green Flame retardants used in Epoxy Moulding Powders designed for Passive Components, CARTS 2004, SA, Texas [5] Zednicek, T. et col,: Lead-Free Soldering Effect to Tantalum Capacitors, CARTS Europe 2001, Proceeding [6] NASA Goddard Space Flight Center, Tin Whisker Homepage [7] Jay A. Brusse, Gary J. Ewell, and Jocelyn P. Siplon: TIN WHISKERS: ATTRIBUTES AND MITIGATION, CARTS US 2002 [8] Henry Livingston: GEB-0002: Reducing the Risk of Tin Whisker-Induced Failures in Electronic Equipment [9] Chen Xu, Yun Zhang*, C. Fan and J. Abys: Understanding Whisker Phenomenon: Driving Force for Whisker Formation, APEX 02 [10] Rob Schetty: Minimization of Tin Whisker Formation for Lead-Free Electronics Finishing, Shipley Co., LLC, Freeport, NY 96