Polymer science is a broad field that includes many types of materials which incorporate long chain structures with many repeated units. One useful way of categorising polymers for the requirements of electronic assembly is by functional behaviour. In the strictest sense these categories are not fixed, or even particularly precise, and you should be aware that some materials can fit into more than one category:
The stress-strain relationships for these groupings are substantially different, as may be seen in Figure 1.
Adapted from: Odian 1970
|Initial modulus of elasticity||0.1–1 MPa||10–100 MPa||1–10 GPa|
|Upper limit of extensibility (%)||100–1000||20–100||<10|
|Nature of stress deformation||Almost completely and instantaneously elastic||Partly reversible elasticity; little delayed elasticity;some permanent set||Some instantaneously reversible elasticity; some delayed elasticity; some permanent set|
|Effect of temperature upon mechanical properties||Elastic modulus increases with temperature (within a limited range)||Marked temperature dependency (over a wide range)||Little temperature dependency from –50°C to +150°C|
|Crystallisation tendency||Low (when unstressed)||Moderate to high||Very high|
|Molecular cohesion, cal/mol||1000–2000||2000–5000||5000–10,000|
ASTM D-156611 defines an elastomer as a ‘macromolecular material that returns rapidly to approximately the initial dimensions and shape after substantial deformation by a weak stress and release of the stress.’ Such elongations typically exceed 100%.
The earliest-used elastomer was natural rubber, obtained from the sap of the rubber tree, which contains around 95% of a polymer whose repeating unit is isoprene. As polymer chemists evolved more and more polymers resembling natural rubber in properties, the term elastomer has grown to represent these materials, rubber being reserved for its original use.
Elastomers have three main functions in electronic assemblies:
Although used for many centuries in its raw form, a significant step forward was made when Charles Goodyear succeeded in ‘vulcanising’ natural rubber by heating it with sulphur to induce what is now understood to be cross-linking. The significance of the great performance improvement resulting from this treatment has led to the term ‘vulcanisation’ often being loosely used to describe the cross-linking of any elastomer.
Elastomers consist of long chain-like molecules, linked together to form a three dimensional network. Typically, an average of about 1 in 100 molecules are cross-linked: when this number rises to about 1 in 30, the material becomes more rigid and brittle. Most elastomers are thermoset materials, and cannot be remoulded, an exception being the class of materials known as ‘thermoplastic elastomers’.
The most common elastomers used in electronics are silicones (Section 0), which are supplied either as thick pastes which can be dispensed, or as fully cured preforms. Silicones may be chosen because of their chemical inertness or, more commonly, because of their thermal performance. Silicone elastomers may also ‘double’ as adhesives.
Moulded cured elastomers can also be supplied with conductive filler materials, such as silver, copper, nickel and graphite. These gasket materials are intended to combine an environmental seal with shielding against electromagnetic interference (EMI).
‘Plastic’ is a term which can cover a wide range of polymer materials, all of which can be moulded, for example to produce the body of a QFP component, the casing for a computer keyboard, the hand set of a mobile telephone or the encapsulant cover for a PLCC.
There are two main groups of plastic polymers, thermoplastics and thermosets:
Thermoplastics are supplied fully polymerised and remain permanently fusible, melting when exposed to sufficient heat, and potentially they can be recycled and reused.
Although some thermoplastics can have a crystalline microstructure, the essential feature of their structure is that there are relatively weak forces of attraction between the chains. These are overcome when an external force is applied (resulting in the plastic deforming) or when the material is heated, so that it becomes first soft and flexible and eventually a viscous melt. For each thermoplastic there is a specific temperature at which the material will start to distort, which is known as the ‘heat distortion point’. However, when the material is allowed to cool it solidifies again. This cycle of softening by heat and solidifying by cooling can be repeated more or less indefinitely and is the basis of most processing methods for these materials.
Thermoplastics are usually supplied in the form of granular feedstock, which is heated, melted and moulded, and removed from the mould only when it has cooled below its ‘heat distortion temperature’.
Examples of thermoplastics are polyethylene, poly(vinyl chloride), polystyrene, nylon, cellulose acetate, acetal, polycarbonate, poly(methyl methacrylate), and polypropylene.
A thermoset material is produced by a chemical reaction which has two stages. The first results in the formation of long chain-like molecules similar to those present in thermoplastics, but still capable of further reaction. This second stage of inter-linking the long molecules takes place at the point of use and often under the application of heat and pressure.
Since the cross-linking of the molecules is by strong chemical bonds, thermoset materials are characteristically quite rigid and their mechanical properties are not heat sensitive. Once cured, thermosets cannot again be softened by applying heat: if excess heat is applied to these materials they will char and degrade – as with eggs, once hard-boiled, they cannot be softened! Examples of thermosets are phenol formaldehyde, melamine formaldehyde, urea formaldehyde, epoxies, and some polyesters.
Thermoset raw materials are supplied in an uncured or partially cured state and fully cured during fabrication. The various stages of cure of a catalysed thermoset resin are known as ‘A-stage’ (uncured), ‘B-stage’ (partially cured), and ‘C-stage’ (fully cured). Many moulding compounds and laminating fabrics can be processed whilst in the B-stage, and some must be kept refrigerated until ready to use.
Compared with thermoplastics, thermosets are much less soluble in organic solvents, and have harder surfaces. Inherently somewhat brittle, thermosets can be combined with reinforcements such as fibre-glass to form very strong composites.
Many thermoset mixes produce an ‘exothermic’ reaction, that is they give off heat during curing. The amount of heat generated will depend on the material and the amount of catalyst used. This effect needs to be taken into account when processing thermosets, particularly when producing large castings.
Explain the essential differences between thermoplastic and thermoset polymers.
Adhesives can be classified by the method used for curing, and a number of different mechanisms have been developed to suit different applications:
The lack of a detailed understanding of the adhesion process has not hindered progress in developing very strong adhesives for most materials. The only problem is that the wide range of chemical structures makes it impossible to produce an adhesive which is compatible with all polymers. It is always prudent to check recommendations on suitable adhesives and surface preparation with the material manufacturers.
There are two main classes of adhesive for polymeric materials:
The most versatile range of organic adhesives is that based on epoxy resins, and these are particularly widespread in electronics, although they are relatively expensive. The major advantages of epoxy adhesives are that:
There are however, the disadvantages that:
Epoxy adhesives are sold either as two-part adhesives, where the epoxy resin is mixed with a catalyst just before use, or single-part materials, where the catalyst is incorporated during manufacture. Single-part adhesives are generally less reactive, needing to be heat-cured, and often require refrigerated storage to increase storage life.
Note that refrigerated materials generally need to be brought to room temperature before use, and it is unwise to try and accelerate this process. Remember to read the manufacturer’s recommendations on storage life both before and after thawing!
Other adhesives you may encounter are:
The range of polymer materials available is enormous, as slight changes in the chemical make-up of the monomers or the conditions of polymerisation can result in dramatic changes in the material characteristics of the end of processed polymer.
Polyethylene is an example of a polymer which can be used in a wide variety of applications because it can be produced with different forms and structures. The first to be commercially exploited was called low density polyethylene (LDPE), which is characterised by a high degree of branching, which forces the molecules to be packed rather loosely. The resulting low density material is soft and pliable and has applications ranging from plastic bags and textiles to electrical insulation.
By contrast, high density (HDPE) or linear polyethylene demonstrates little or no branching, so that the molecules are tightly packed and the plastic can be used in applications where rigidity is important, such as plastic tubing and bottle caps. Other forms of this material include high and ultra-high molecular weight polyethylenes (HMW; UHMW), which are used in applications where extremely tough and resilient materials are needed.
New materials can also be tailored by combining monomers with desirable properties. In some cases, these combinations are just physically mixed polymers, but more typically new ‘co-polymers’ are produced. Some types have a random structure of the constituent monomers, others may have a regular, repeating structure of the different materials:
Nylon is an example of a common ‘alternating copolymer’ with two different monomers alternating along the chain. One useful material, which is in fact a ‘terpolymer’, is ABS. This is a combination of three monomers: acrylonitrile, butadiene and styrene, in varying proportions depending on the application (Figure 2). A rigid but tough material, it is used for water pipes, refrigerators and Lego bricks!
after Crawford 1985
To obtain the desired properties, the chemist takes into account three key factors: the chemical composition of the building blocks; the possible shape of the polymer chains they can produce; and the alignment of these chains within the final product. A wide range of fillers and additives can then be used to modify the properties of the material. In fact, the plastics industry depends on additives to convert polymers into materials which have useful properties and improved ease of processing – one commentator even makes the point that ‘polymer plus additive equals plastic’! Different combinations can produce synthetic products as unlike as nylon tights and vinyl flooring.