The use of nickel based particles in the electronics industry. by Dr. Tony Hart Chairman and Managing Director, Hart Materials Limited, Wombourne, UK

Transcription

1 The use of nickel based particles in the electronics industry by Dr. Tony Hart Chairman and Managing Director, Hart Materials Limited, Wombourne, UK Key properties of nickel Nickel is an extremely versatile element with a vast number of commercially important applications in which it is used either as a pure metal or as an alloying element. The reason that nickel is so important is that it has or conveys not just one useful property but a number of such properties that are frequently required in combination in order to achieve the desired performance of the final product. These include:- Improved corrosion resistance Better toughness Better high temperature strength Better low temperature strength Good electrical conductivity Ferro-magnetic properties Availability as electro-deposited or chemically-deposited thin films Availability as small particles There is in fact no other element in the periodic table that can offer such a range of beneficial properties. Important uses of nickel By far the most important of these applications, which accounts for about 60% of the world's nickel consumption, is the manufacture of austenitic of stainless steels; these contain typically 8% to 10% nickel. The second most important use of nickel is in the form of thin surface coatings - most generally applied by electrodeposition. Alloys with a high nickel content have an important range of uses, frequently generally in very exacting conditions. For example, the invention of nickel-based alloys with exceptional high temperature properties led to the successful development of

2 the gas turbine engine. There are also many corrosion resistant high-nickel alloys that are used in aggressive sections of chemical production plant. Other significant uses of nickel include powder metallurgy, the manufacture of catalysts and production of rechargeable battery systems including nickel-metal hydride. Nickel-based particles in the electronics industry There are already many applications of nickel-based materials in the electronics industry. However, in recent years there have been a growing number of new uses that require small particles of either pure nickel of nickel composite products such as silvercoated nickel and nickel-coated graphite. In these systems the ability to obtain nickel and related products as small particles becomes a key advantage. What is even more important is that nickel-related particles are available in a number of morphological forms which adds to their versatility. The shape of small metallic particles is determined by the process by which they are manufactured. One of the commonest methods of manufacturing metal powders is by water or gas atomisation which generally results in the formation of irregular, approximately spherical, particles. Water atomisation produces particles of a smaller size than those made by air atomisation. However, in the case of nickel there is another process for producing particulate material which cannot be used for the majority of metals. This is the nickel carbonyl process, invented in 1889, by Dr. Ludwig Mond. The principles of the process depends upon the ability of nickel to react with carbon monoxide gas to form volatile nickel tetracarbonyl. Ni + 4CO Ni(CO)4

3 This reaction is reversible so by changing the process conditions nickel tetra-carbonyl gas can be made to decompose producing pure nickel metal and releasing carbon monoxide gas which can be recycled through the process stream. The carbonyl process has two very significant advantages. Firstly, because only three elements (iron, nickel and cobalt) undergo this type of chemical reaction it is an extremely effective method of refining nickel to a very high level of purity. Secondly, the reaction conditions under which the decomposition of the nickel tetra-carbonyl gas is carried out can be varied in a controlled way to produce a variety of particle shapes and sizes. Some of the commercially important type of particle that can manufactured by the nickel carbonyl process are described below. One of the most basic shapes is a spherical particle of the type shown below comprising a solid pellet 8 14 mm diameter that is used in both the electroplating and melting industries. The nickel carbonyl process is, however, capable of producing much smaller particles, effectively nickel powders, with a very regular spherical shape - as demonstrated below by one of the basic standard nickel powders, Vale Type 123. This very regular spherical morphology is vital is vital in many of its applications.

4 Page 4) Type 123 nickel powder, has a d50 in the region of 10 microns, and a closely defined particle size distribution as shown below. Volume (%) Particle Size Distribution /104M PRE, 14 April :38:22 Particle Size (µm) Another basic particle shape which can be produced using the carbonyl process is a filamentary nickel powder as shown by the illustration of Vale Type 255 shown below.

5 Page 5) This product is widely used for the production of nickelcontaining batteries where the active mass sits between the filaments of nickel which effectively conduct the electron flow. It is also however, particularly useful for the manufacture of conductive coatings. The structure reduces the quantity of metal that needs to be incorporated into the non-conductive resin base to achieve the particle-toparticle contact required for conductivity. The high purity of nickel manufactured by the carbonyl process gives rise to a further advantage in that the particles are very ductile. They can therefore be deformed mechanically which enables them to be processed into the form of flakes. This is carried out by ball milling under closely controlled conditions to give a morphology which maximises electrical conductivity when the flakes are incorporated into organic media. Page 7) Conductive Grade Nickel Flake Grade HCA-1, as illustrated, was specifically developed by the U.S.A. based NOVAMET Specialty Products Corporation to provide optimum particle morphology for the production of conductive paints and inks. The thickness of the flake is about 1 micron and the flakes typically have an aspect ratio between 20:1 and 30:1.

6 Page 6) The figure below demonstrates the relative behaviour of Conductive Nickel Flake Grade HCA-1 compared to spherical and filamentary nickel powders. With both the filamentary and flake materials the resistivity initially decreases sharply as film thickness increases but then reaches a limiting value at about 25 microns. The spherical powder provides a demonstrably lower level of conductivity than the other two forms. Although spherical nickel particles give a much lower level of conductivity than either of the other two this does not exclude them entirely from use in in conductive coatings; they can be used in conductive inks since their small particle size enables the inks to be produced that can be printed through fine screens. SPECIFIC APPLICATIONS As a result of the combination of beneficial properties shown by nickel- based particulate materials they have become adopted for use in a number of electronics industry applications that have grown up in recent years.

7 Page 7) Interference shielding It is more than obvious to state that the growth of the electronics industry since the end of World War 2 has been massive. This has resulted in society becoming increasingly dependent upon electronics devices in all areas of human activity. Failure of these devices can have catastrophic effects upon the systems into which they are incorporated. For example, an aircraft operating on a `fly-by-wire' control system requires totally fool-proof computer systems to avoid disastrous consequences. In other applications the result of equipment malfunction may be less dramatic but nevertheless highly inconvenient, resulting perhaps in the loss of vital data on a computer memory. Unfortunately, all electronics equipment can be affected by extraneous electrical and magnetic signals that interfere with their operation. These devices can, in fact, act both as a receiver and transmitter of interference signals occuring generally in the frequency range between 10 kilohertz and 100 gigahertz. It can be generated by natural causes, such as lightning, or by other electronic or electrical devices. As a result since the 1980s a large branch of the electronic industry has grown up specifically to deal with this interference problem. The technology developed to control these phenomena is known as Radiofrequency and Electromagnetic Interference (RFI/EMI) shielding or Electromagnetic Compatibility (EMC). The seriousness of the problem is demonstrated by the fact that effective shielding from interference has become incorporated into legislation in many parts of the world. The USA was the first country to properly appreciate the extent of RFI/EMI and began to enact legislation late in the 1970s making shielding of electronic devices compulsory. Legislation in Europe was enacted considerably later when on January 1 st 1992 the EU Electromagnetic Compatibility Directive (Directive 89/336/EEC) came into force. During the last two decades it has, therefore, become vital for manufacturers of the types of equipment covered by the Directive to find practical and economic means of providing shielding from interference.

8 Page 8) The problem has, unfortunately, been exacerbated by another technological development which has taken place simultaneously with the expansion of the electronics market. This is the replacement of metals by plastics as the preferred materials of construction for the housings that contain electronics equipment. When housings consisted primarily of metal boxes the enclosure provided a degree of shielding from interference. However, the plastics resins that have replaced metal are electrical insulators and as such provide no shielding from extraneous interference. Many attempts have been made to develop electrically conductive plastics but it has not so far proved possible to produce a fully satisfactory material. As well as showing the relatively high level of electrical conductivity necessary for shielding the material must also be capable of being moulded, extruded or rolled into a suitable form in which it exhibits the impact resistance, corrosion resistance, tensile strength and flexibility of regular plastics resins. It would also, of course, need to have a similar cost to other engineering plastics. Coating systems for shielding applications Since it has not proved possible to produce inherently conductive plastics meet all of these targets, it has been necessary to devise other ways by which the electronics components contained within plastics housings can be effectively be shielded. The use of electrically conductive coatings has assumed an important role in this respect. Four different systems have been utilised to a greater or lesser extent for this purpose during the last 25 to 30 years. conductive paints coatings thermal spraying of metallic coatings, vapour deposited coatings electroless copper and nickel coatings

9 Page 9) Conductive paint coatings show a number of distinct practical advantages and consequently they have become and remain widely accepted in this industry. Spray painting is, for example, an eminently suitable technique for coating complex shapes, such as the interior of computer housings. In addition the spraying process allows the coating to be used selectively only in the areas where it is absolutely necessary. A further benefit is that the equipment required is relatively uncomplicated and requires much less expenditure than alternative processes. Consequently a paint spraying station can be situated either at, or close to, the location where the plastics parts are formed so reducing transport costs and logistics problems. Furthermore, it is often necessary to paint the exteriors of plastics devices to obtain a satisfactory appearance. Under these circumstances the application of the shielding coating can be done simply as an additional stage in the finishing process. However, paints suffer the same disadvantage as plastics materials in that they are not inherently electrically conductive. It has therefore been necessary to modify paint formulations so that the coating produced is conductive and capable of providing protection from interference. This is normally achieved by incorporating conductive pigments into the paint formulation. Pigments that have been used in this type of application are:- Graphite Nickel Copper Silver In view of the alternative materials available it might not be obvious why nickel should be the preferred material for shielding coatings. For example, graphite is much less expensive than nickel and copper has a higher electrical conductivity which should, in theory, give rise to better shielding properties. There are, however, other properties of the conductive filler that are almost as important as inherent electrical conductivity in determining the shielding effectiveness of the pigment used.

10 Page 10) For example, the chemical properties of the surface (particularly its mode of oxidation) control the corrosion resistance of the material which has a critical influence upon the effective conductive properties. In addition the particle morphology is of critical importance, as demonstrated above in the figure comparing the behaviour of three different shapes of nickel particle. The other vital factor determining the suitability of a material for this application is, of course, cost. Precious metals, including silver, give excellent properties but are unfortunately much too expensive for the majority of uses. Silver is actually used in a number of applications where the cost is tolerable particularly in the aerospace or military markets. Silver coated particles, such as silver-nickel or silver-glass, are find a wider range of applications since they can offer a high level of performance at a lower cost than pure silver. Alone amongst the available metal pigments, nickel possesses the unique combination of the properties described above together with a cost which is acceptable for a broad range of applications within the normal commercial sector. Consequently it has become one of the preferred materials in this new technology. A key factors in determining the performance in shielding coatings is the chemical nature of the surface of the conductive filler. Discrete particles in either a paint or an ink film will only provide electrical conductivity when they make intimate contact with one another. If the surface condition of the particle prevents the creation of an effective conductive path between particles then it will render a potentially useful material worthless. Aluminium demonstrates this point well. This metal has an electrical conductivity about three times greater than that of nickel. However, aluminium readily forms a highly protective oxide film on its surface which is an excellent insulator. Consequently particle to particle electrical continuity cannot be achieved in an organic medium, regardless of filler loading so that aluminium finds no application in shielding technology.

11 Page 11) Nickel on the other hand forms an oxide film that allows electrical continuity between contacting particles whilst at the same time rendering the metal surface passive. This provides good corrosion resistance, another vital property of any pigment that is to be used in this application. The corrosion resistance of the particles is of great importance for two reasons. Firstly, the pigment can, if it is not resistant to chemical attack, react with the organic vehicle in which it is dispersed. This can destroy both the electrical conducting properties of the particle and possibly many of the basic properties of the organic medium. In addition, the coating will be required to show good durability, probably for many years, in a range of environments, some of them quite aggressive. If the corrosion resistance of the pigment is poor the electrical properties of the film will deteriorate with time and produce a marked decrease in shielding effectiveness. This is the prime disadvantage of copper as a pigment material for shielding since although it is basically more conductive than nickel it has much poorer corrosion resistance. Consequently it can react chemically with some organic vehicles both during manufacture and storage. Also, the excellent initial conductivity of a copper-containing paint film can deteriorate on exposure due to corrosion of the metal thus reducing shielding effectiveness. Another advantage of nickel is its inherent ferro-magnetism. This is important since it increases the shielding effectiveness of nickel-containing coatings in certain regions of the electromagnetic spectrum. Of the other ferromagnetic materials iron is totally unsuitable because of its extremely poor corrosion resistance. Cobalt, although showing some useful properties, is considerably more expensive than nickel in small particle form. Organic-based conductive coatings consist of large numbers of small conductive particles suspended in a non-conductive matrix. Since the conductivity depends on these particles making physical contact with each other the shape of the particles will have a marked influence on the level of conductivity achieved.

12 Page 12) Spherical nickel particles - as might be expected - give a much lower level of conductivity than either of the other two shapes. This might suggest that these materials have no use in conductive coatings. This is not, however, entirely the case since they can be used in conductive inks. In this type of product their small particle size and the close control which can be exercised over size distribution are advantageous in enabling the ink to be printed through finer screens. This diversity of morphology obtainable with small nickel particles allows them to be used as a conductive filler in a range organic-based products. The formulator of the system can choose the shape most suited to the particular product, bearing in mind all of the other properties required of the coating, such as applicability, appearance and durability. Despite the inherent advantages shown by nickel pigments there are situations in which their properties are not adequate to fulfil all of the requirements of the application. In these instances the properties of nickel pigments can be improved is by coating them with another material which has in better electrical conductivity, particularly silver. Silver is used as a conductive filler on its own but is very expensive and therefore only used for the most exacting applications Coating, typically 15%, silver onto spherical nickel particles produces a filler that gives enhanced conductivity, close to that of pure silver. The typical structure of commercially available silver-coated nickel particles is shown alongside.

13 Page 12) Nickel is, the ideal substrate for this purpose. The technology for depositing silver onto nickel is highly reliable and an excellent bond can be achieved between the two metals. Also nickel itself is a good conductor and it is advantageous to have a conductive, rather than a non-conducting, substrate for this type of particle. Nickel also retains its excellent mechanical properties after coating with silver. This makes the composite particle robust and able to withstand vigorous processing whilst being compounded into organic media. In addition, nickel is not prone to diffusion into silver at processing and post-processing treatment temperatures employed, for example in the production of siliconebased gaskets. As well as being used to manufacture macro sized pellets and fine nickel powders the nickel carbonyl process is also employed to coat small particulate materials with nickel metal. This provides an excellent opportunity to produce nickel-coated particulates. These exhibit the surface properties of nickel but also derive certain properties from the substrate material. By far the most suitable material onto which nickel can be coated is graphite. It is mechanically robust, relatively inert chemically and inherently electrically conductive. It is also relatively cheap and has the advantage of a low specific gravity. There are basically two grades of nickel-coated graphite available for this type of application, nickel grade. The 60% has a larger particle size than the 72%. Both types, however, have an irregular angular morphology as shown in the cross sectional illustration below. This morphology is considered to be one of the major factors responsible for the excellent performance of these particles in electronic shielding technology.

14 As a result of these favourable factors nickel-coated graphite has, during the last 25 years, become the preferred conductive filler for use in many radiation shielding applications. It is extensively used, for example in silicone-based conductive gaskets. The illustration shows some of the common shapes of gaskets that contain nickelcoated graphite particles. Primarily they are compounded into silicone based elastomers and are compatible with the formulation variations that are available to those who are expert in the use of this type of polymer. The can, however, also be sued in other elastomeric materials and polyolefins. Nickel-coated graphite containing composite materials are used to manufacture moulded products, extruded forms, printed gaskets and form-in place applications. However since nickelcoated graphite particles are three dimensional and in the region of 60 to 100 microns in size they are unfortunately not suitable for coating products. Magnetic properties of nickel-related flake materials The magnetic properties of nickel and many of its alloys have been known for many years. However, until recently no data was available on their behaviour when incorporated as small particles into organic media to be used in thin coatings. The following results are taken from a study of the magnetic properties of nickel-related flake products that exhibit ferromagnetism in this situation. Materials selected include pure nickel and stainless steel flakes as well as those from two special magnetic nickel alloys.

15 Magnetic materials can be divided into two categories, hard magnetic that retain their magnetism once the magnetising field has been removed and soft magnetic that readily become demagnetised. Nickel and stainless steel flakes are soft magnetic materials a typical hysteresis loop is shown below. The variation of three critical parameters in respect of the quantity of nickel flake incorporated into organic films are shown below. Pigment/ Binder Ratio 0.67:1 1.33:1 2.00:1 2.50:1 3.00:1 Saturation Magnetisation M S 10-3 emu Remanent Magnetisation M R 10-3 emu Coercivy H C Oe An unexpected result was found measurements carried out on the stainless steel flakes in that all of the five grades tested showed ferromagnetism of a similar order of magnitude to those exhibited by the nickel flakes. This is a significant finding, given that the UN material from which the flakes are manufactured would normally, as austenitic materials, be considered to be non-magnetic. This is probably attributable to the changes in metallurgical structure that takes place in the stainless steel as a result of the mechanical effect of the milling process used to convert the powder product into flake. Four grades of stainless steel tested produced similar properties.

16 Magnetic properties of Stainless Steel Flakes Flake Grade Saturation Magnetisation M S 10-3 emu Remanent Magnetisation M R 10-3 emu Coercivity H C Oe Fine Leafing Standard Leafing Fine Water Standard Water Leafing Grades were processed in mineral spirit plus stearic acid whereas Water Grades are processed in mineral spirit plus cetyl alcohol. Two commonly used nickel allow flakes also showed similar properties. Magnetic Properties of Nickel Alloy Flakes Alloy Type Nickel 3% Aluminium 82% Nickel Permalloy Saturation Magnetisation M S 10-3 emu Remanent Magnetisation M R 10-3 emu Coercivity H C Oe

17 Why nickel so important in these applications? This paper concentrates on one metal, nickel, and a number of nickel-containing alloys which begs the question why? The prime reason is the excellent track record of nickel and related products in the electronic shielding industry. Its widespread application has consistently increased over a more than 30 years. Put simply it is used because it works and it shows consistent and reliable properties - for the following reasons :- Its electrical conductivity is inherently good. Although the surface oxidises this does not prevent good particle to particle contact It is ferro-magnetic which extends its shielding capability. It is readily available in small particle form in large quantities at a competitive price. There a number of particle morphologies that can be selected for different applications. It is corrosion resistant so that electrically conductive composites retain their conductivity and their shielding characteristics even in quite aggressive environments. So the short answer to the leading question is that nickel is unique it is the only one of the 92 elements commonly available in the periodic table that gives this combination of properties that prove so useful in the electronics industry in general and in newer technologies such as electronics shielding in particular.