Gasket Material Type - Galvanic compatablity and Voltage differential

Transcription

1 Galvanic relationship of Metals Gasket Material Type - Galvanic compatablity and Voltage differential Group Metallurgical Category EMF (Volt) Graphite Silver Glass Silver Aluminium Nickel Monel Stainless Steel Copper beryllium Tinned CuBe Tinplate copper clad steel Aluminium 1 Gold, Platinum Cathodic 2 Graphite or most 3 Silver Noble 4 Nickel, Monel Copper, Low Brass, Bronze Stainless steel Naval Brass % Chromium Steel Direction 8 Chromium, 12% Chromium Steels of corrosion 9 Tin-Plate, Tin-Lead Solders Lead Duralumin Alloys Iron, Low Alloy steels Aluminium alloys Cadmium Galvanized Steel Anodic 16 Zinc or least 17 Magnesium Noble Suitable for marine environments or harsh conditions Suitable for wet non-saline conditions Suitable for dry, indoor conditions Galvanic mis-match where corrosion will occur

2 Galvanic Compatibility Galvanic reactions are one of the most important aspects to consider when choosing or designing EMC gasketing for your application. It is said up to 5% of an industrialized nation s income is spent on corrosion prevention, or maintenance and replacement of parts due to corrosion. In EMC terms, an incompatible match between gasket and surface can mean inconsistent shielding and in extreme cases lead to the mechanical failure of the component. When present in benign electrical devices such as home computers and Televisions, this is little more than an inconvenience often manifesting itself as poorer audio/visual quality or reduced processing efficiency. But when galvanic reactions occur in life critical applications such as medical devices or a Vehicles computer controlled ABS, the effects can be devastating and costly both financially and potentially in human lives. Reports of actual deaths associated with EMC and galvanic reactions are mercifully small, and tend to come from military sources, incidents involving critical devices failing through interference, caused by their EMI gasketing being degraded by galvanics are however more common. Galvanic reactions Galvanic corrosion is a very specific type of reaction, by definition, it is the accelerated electrochemical corrosion produced when one metal is in electrical contact with another, more Nobel metal. The effect of this is to create a basic voltaic cell, where one metal will act as an anode, the other the cathode, ions from one material are transferred across the cell, altering the electrical charge of the metallic surface. This leaves the altered surface susceptible to chemical reaction with atmospheric molecules, mostly oxygen. Thus an oxide layer is formed. The alteration of the surface and subsequent chemical reaction is usually detrimental to the materials conductive ability. In some cases, the galvanic corrosion can be so extreme as to cause serious mechanical and structural damage to the materials involved. The effect is increased in the presence of an electrolyte, such as water or a salt (typically marine environments) which act as a transfer medium for the electron flow and as a reactant donor. As the oxides build up on the surface of the unit, or gasket, several events occur, each one potentially damaging. Initially the thin oxide layer tends to reduce electrical conduction through the joint, as good electrical conduction is essential to the performance of any EMC gasket, this will clearly degrade its ability to perform correctly. As the reaction increases the sheer bulk of oxide generated can physically displace the gasket or deform the panel it touches, as the gasket and panel move apart, conduction decreases again, and where gaps form, Electromagnetic radiation escapes. Finally the material fails completely, eaten away by chemical reactions the panel, literally falls apart, splintering particles into electronics, or causing actual structural damage to the unit. There is another lesser known effect of galvanics particular to iron based metals, when Iron oxidises it becomes increasingly ferretic in its properties, thus as a ferrite, it exhibits semi-conductor characteristics, in essence a diode. The diode acts to multiply any incoming signals (intermodulation). This can exacerbate the problem dramatically. Obviously, It is essential that careful consideration is given to the choice of materials used for the base component

3 and gasketing. Minimising the effects of Galvanic reaction The above table (table 1) gives some indication of compatible materials, and when choosing a shielding gasket, ideally the material should be in the same group as your base application. The list is not exhaustive, and a much more useful guide is table 2, here a much wider range of metals and potential gaskets can be cross referred. As a guide, for indoor environments, where dry conditions prevail, the difference in EMF values should not exceed 0.5V (See Table values). For wet conditions, the differential voltage should not exceed 0.3V. For harsh environments where salt water / mist may be encountered the differential value should not exceed 0.1V. Choosing the correct product is the first and arguably the most important step in design, but to ensure long term stability several other factors must be considered. The actual design and style of the gasket is important. Galvanic reactions can be surprising quick, reactions occurring within minutes of contact, but most reactions are very slow, often only appearing months after the unit has been completed. There are obvious dangers here, as products tested and certificated to comply with current EMC regulations at one point, may corrode to a point where they are no longer effective within a year. To minimise the rate of reaction the environmental conditions under which the gasket is used needs to be controlled, the most obvious method of control is to avoid contact with moisture, which as discussed can act as an electrolyte. In an indoor environment, where conditions are not damp or humid, this can be overlooked. But in external applications, the shielding section of the gasket needs to be protected. Wire mesh gaskets, which have thin strands of metal knitted around a soft sponge, are ideal for indoor use, but externally the loops of the wires can act as water traps, holding moisture close to the joint. To protect these wires, closed cell sponge can be bonded to the shield, this service should be provided by gasket supplier. The recent insurgence of conductive fabric materials, such as Warths Fabshield, are exceptional EMC gaskets, but even here, the loops between the weave can act as traps, so special fabric gaskets which incorporate closed cell sponges have been designed to protect the delicate shields, an example is Warths Fabtrack materials. Very common gaskets are those where the shield is embedded

4 in silicone or sponge, these gaskets are generally designed to hold to stringent environmental standards, but their use is limited to smaller devices, such as external telecom base stations etc. Often these gaskets take the form of conductive elastomers, where particles of a conductive material, usually Silver- Aluminium or Nickel are suspended in an elastomer core. One of these materials, the Silvered-Aluminium type, has very particular characteristics, which broadens its range of uses significantly. The metal particles are, as the name suggests, flakes of Aluminium, silver-plated. These particles enable this gasket to be used on pure aluminium surfaces. This is initially a surprise, as Silver and Aluminium are separated by some 0.75V (EMF) well over the suggested limit of 0.3V. There are numerous reasons for this, not least the particles, locked inside the silicone are fully protected from any moisture which may act as an electrolyte. The silver plating is also not perfect, so areas of pure aluminium will remain exposed to the aluminium of the base unit. Obviously Aluminium Aluminium seals are perfectly stable and will not react with each other. Perhaps the most surprising aspect of the conductive elastomers performance is their conductivity. Although they have some of the highest shielding figures available, they are on average 1000 times less conductive than a perfect metal to metal seal. This lower conduction slows the galvanic process further, thus making this a very stable solution to Aluminium EMC issues. A new approach is the use of low modulus silicone gels that surround the gasket material and once placed down for an effective water tight seal that all but stops corrosion by blocking the ingress of an electrolyte. These gel materials, known to Warth as Emiseal, are usually made up of a knitted monel wire cord, with the gel impregnated around the perimeter. These gels are fully cured, so, while they are very soft and conformable, they stay held to the metal. As suggested, not all gaskets need environmental protection, but do suffer because of the surface conditions they are mounted onto or pressed against. rough surfaces should be avoided as the sheer surface area of the material gives a large scope for reactions to occur. Many gaskets use a sweeping action to maintain good conduction, displacing any oxide layers that form, these gaskets tend to be stamped metal forms, often using Copper beryllium for its performance, or Stainless steel for its cost. The use of conductive metal tapes can also be advantageous, Aluminium based foil tapes can be applied to a plain aluminium box. The tape effectively stabilises the surface, giving good electrical contact, and raises the galvanic group, thus allowing a greater range of gasketing materials to be considered. Copper, and Tin-plated copper clad steel tapes can have the same effect on other materials. The choice of gaskets is clearly essential for the success and longevity of many electronic devices, and as the number of devices used within our daily lives increases the importance of compatibility grows with them.

5 SHIELDING Warth Shielding ZSF Shielding Foils ZSF Frequency Field db Resistance through tape per 2.5cm/sq Temperature Range C Properties 1 MHz Electric > W -20 to +155 Description ZSF is a Copper, Aluminium, or Tin plated copper, foil coated on one side by an electrically conductive pressure sensitive adhesive. The conductive adhesive is acrylic based with special filler, which provides a low contact resistance through to the foil. The foil offers excellent surface contact and low resistance. These characteristics allow the conductive foil to be used in a wide range of electromagnetic shielding applications, where good adhesion and surface conductivity are needed. ZSF materials are available in standard rolls or die cut to customer s requirements. Typical applications include providing a Bus-bar connection on Zemrex shielded windows, making electrical contact with connector backshells and cable shielding and shielding seams in shielded rooms. Shielding Effectiveness db C = Copper, A = Aluminium, TPC = Tin plated copper Frequency Field C A TPC Typical Properties C A TPC 10kHz Magnetic kHz Magnetic MHz Magnetic kHz Electric 88 >100 >100 Resistance through tape, per 2.5cm/sq (Ω) Foil thickness (µm) (excluding adhesive) kHz Electric >130 >130 >130 Adhesive thickness (µm) MHz Electric >120 >120 >120 Adhesive Synthetic resin 10MHz Electric 112 >130 >130 Temperature range C -20 to MHz Electric Tensile strength N/cm MHz Electric Peel strength N/cm GHZ Electric GHz Electric Release material Siliconised paper Ordering Procedure Width Length Copper Aluminium Tin plated copper 12.5mm ± 1.0mm 20m ZSF-C-12 ZSF-A-12 ZSF-TPC-12 25mm ± 1.0mm 20m ZSF-C-25 ZSF-A-25 ZSF-TPC-25 50mm ± 1.0mm 20m ZSF-C-50 ZSF-A-50 ZSF-TPC ± 1.0mm 20m ZSF-C-100 ZSF-A-100 ZSF-TPC-100 Page 1 of 1 : Issue 08/01