A clear signal - preventing interference in electronic devices
Preventing interference in electronic devices has become a high priority for governments as well as manufacturers. Dr Tony Hart, Managing Director of Hart Materials Limited, discusses how nickel is contributing to this increasingly important area.
The electronics industry has grown remarkably in the past 35 years, but this expansion has not been without its difficulties. One critical problem associated with electronic devices is that they emit and receive extraneous interference signals that can seriously affect their operating efficiency – a phenomenon that is exacerbated by the huge number of devices now in common use. Consequently, in the past 30 years, a completely new branch of technology has arisen to combat the detrimental effects of interference. The importance of this can be judged by the fact that electronics shielding has become enshrined in the legislation of many countries, including the USA and some EU member states.
In response to the issue of electronic interference, one metal with a unique combination of suitable properties has come to the fore – nickel. Its electrical conductivity is inherently good and it is corrosion resistant, so products retain their beneficial properties even in aggressive environments. Although the surface oxidises, this does not prevent good particle-to-particle contact. It is ferro-magnetic, which extends its shielding capability, and is readily available in small particle form in large quantities at a competitive price. Nickel is the only one of the 92 elements available in the normal periodic table that gives the combination of properties that prove so useful in the electronics shielding industry.
There are many requirements of shielding technology, but one of the most common is to include, within the structure of the physical enclosure of the electronics device, features that reduce interference to tolerable levels. One effective means of achieving this objective is to coat the interior of the enclosure with an electrically conductive layer, such as a special paint, typically containing nickel flake. However, this leaves a weak spot in the defences – those areas where component parts of the enclosure come together. As a result, it has been necessary to devise gasket materials that not only eliminate contamination and moisture from the interior of the device but are also electrically conductive. In both sectors of this technology, small particles of nickel and nickel-related materials play an extremely important part.
In the case of paint, the nickel particles must
be capable of being incorporated into coatings that are no more than 50µm thick. Consequently, large, three-dimensional particles are not effective – instead, filamentary and flake particles are preferred.
One effective material is a filamentary nickel powder, Vale Type 255. The morphology of the particle increases the number of points of contact between individual particles, which increases the conductivity of the paint film. Another particulate nickel that has proved extremely effective is the flake format. Since its development in the late 1970s, Novamet’s conductive nickel flake grade HCA-1 has been one of the most popular electrically conductive pigments. These flakes are in the region of 1µm thick, meaning multiple layers can be used to build up a 50µm paint film that provides enhanced conductivity.
One of the most important techniques for refining nickel to a very high level of purity is the carbonyl vapour phase process invented by Dr Ludwig Mond in 1889. This depends upon the reaction of nickel with carbon monoxide gas to form volatile nickel tetra-carbonyl – Ni + 4CO = Ni(CO)4.
This reaction is reversible, meaning that changing the process conditions causes the nickel tetra-carbonyl gas to decompose, producing pure nickel metal plus carbon monoxide gas that can be recycled through the process stream. Because only three elements (iron, nickel and cobalt) undergo this type of reaction, the carbonyl process is an extremely effective method of refining nickel to a high level of purity.
Another vital advantage of this gaseous phase reaction is that it can be used to coat non-nickel particles with pure nickel. In electronic shielding and conductive gasket manufacture, nickel-coated graphite particles have become extremely important. Although graphite particles coated with 25–85% nickel have been used in the metal spraying industry for many years, in the electronics shielding market there are two grades that have predominated, 60% nickel and 75% nickel. They are mostly incorporated into silicone-based elastomeric resins, although they have also been used in polyolefins.
The shielding performance of gaskets manufactured from composite materials incorporating nickel-coated graphite has been found to be superior to those containing other types of filler – with the exception of those employing silver, the use of which is restricted by cost. This enhanced performance is attributed to the irregular shape of the particles, which provides a multiplicity of contact points between adjacent particles as well as the long-term stability of nickel in corrosive environments. Nickel-coated graphite materials are used to manufacture composite moulded and printed products, extruded forms, and form-in-place gaskets.
A magnetic personality
The magnetic properties of nickel and its alloys have been known for many years. However, until recently, no data has been available on their properties when incorporated into organic media in the form of small particles. The results on page 42 are taken from a recent study of the magnetic properties of nickel-based flake products when incorporated into thin coatings. The materials covered include pure nickel and stainless steel flakes as well as two special nickel alloys.
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 de-magnetised). All of the flake products studied are soft magnetic materials. The variation of three critical parameters in respect of the quantity
of nickel flake incorporated into organic films is shown in Table 1.
Nickel-related particles have occupied a prominent place in the electronics and related industries for more than 30 years as a result of their electrical conductivity, but it is the recent proliferation of electronic devices and the subsequent problems of interference that have made this material invaluable.