Printed circuit boards are the substrate of choice for contemporary electronics. (Picture © Dan Thomas Brostrom.)
Printed circuit boards are the substrate of choice for contemporary electronics. (Picture © Dan Thomas Brostrom.)

Ask the proverbial man in the street which materials he would consider to be most important in modern electronics and he might respond with silicon, say, and copper. Few people would include fibreglass on their guess list. Yet, in a real sense, fibreglass underpins electronics.

At one time electronic assemblages, with their thermionic valves, crystals, condensers, potentiometers etc, were connected by wires and secured to a metal chassis. As components became smaller and valves were succeeded by transistors, chassis and wires gave way to insulating base boards, each carrying a layer of interconnecting copper track to which electronic components – discrete (separate) in the early days, but increasingly integrated today – are attached. Printed circuit boards (PCBs), also known as printed wiring boards, are the substrate of choice for contemporary electronics.

PCBs are found in a vast range of products, from digital watches to washing machines, from radios, TVs and DVD players to road, rail, sea and air vehicles. Accordingly, they account for a large and mainly unsung market for reinforced plastic substrate. The US alone spends nearly $5 billion per year on PCBs, a significant proportion of this being accounted for by substrates, many of which are ever more highly engineered. Most PCB manufacturing has now migrated to Asia, notably China of late, though some speciality boards are still produced in Europe and the USA.

General electronics

Early boards were based on various resin-paper combinations and these affordable materials are still widely used by hobbyists, in low-end consumer goods, and for prototyping (breadboard, stripboard etc). Synthetic resin-bonded paper (SRBP) is a popular generic and related products go by various trade names. Resins used are flame-resistant, a necessary quality for electronics, and are designated FR-2. The substrates are easy to produce and are readily drilled and machined, causing less tool wear than glass reinforced composites. They can, however, be subject to moisture absorption over time and this can adversely affect their dimensional stability and electrical properties.

Reinforced plastic board overcomes this drawback and an FR-4 epoxy/E-glass material has virtually become the industry standard for general electronics below the high frequency range. As suggested by the designation, a flame resistant epoxy is used. The composite offers a good combination of mechanical and electrical properties. For instance, it bonds strongly to a thin (30 microns is a typical thickness) conductive layer, usually of copper, adhesively bonded or electro-deposited to the substrate. This layer is then selectively etch-removed to form the conductive tracks that will connect the various electronic components. Due to the glass reinforcement, the flexural strength and resistance to cracking of composite boards are about five times those of paper-phenolic materials. Tensile and impact strengths are high, elongation is low, and the material has low mass.

Thermal tolerance is adequate to withstand the high temperatures involved in the flow and wave soldering processes that are part of volume PCB manufacture. The material is a good insulator and other electrical parameters, primarily dielectric strength (relative permittivity) and dissipation factor, are favourable. Mechanical, thermal and electrical characteristics are stable over time.

FR-4 material is easy to machine by drilling, shearing and cold punching, although one drawback is that the abrasive nature of glass fibre means that machining during volume PCB manufacture requires tools that are tipped in tungsten carbide, a durable but expensive material.

Alternative substrate materials include aramid, acrylic, quartz and carbon fibres, and polyester, vinyl ester or cyanate ester resins. Polyimide and bismaleimide triazine (BT) resins are used for demanding, high-temperature applications. FR-4 epoxy/E-glass remains, however, the dominant composite for most consumer and industrial electronics. Not all circuit substrates are rigid. Flexible printed circuits are based on a polymer film, typically polyimide.

Most rigid composite boards are compression moulded from pre-impregnated material. This is produced by impregnating woven E-glass cloth with resin which is left in its beta (partially cured) state. The resulting prepreg is pressurised and heat cured in a laminating press to form the rigid substrate board, which is then cut to the forms and sizes required. Copper layers are added to the substrate on one or both sides, after which they are subjected to selective etch removal to form electrically interconnecting tracks. Boards are drilled so that connecting ‘vias’ and components can be inserted for soldering. PCBs range from simple single-sided products through to double-sided and complex stacked multi-layer boards.

Fibre volume fraction in substrate composites ranges from some 30% for commodity boards to up to 70% for top quality boards required for specialist applications. E-glass suppliers such as PPG Industries Inc, Saint Gobain Vetrotex and AGY produce fabrics that are tightly specified according to particular client and PCB needs. For example, a trend in modern electronics to require ever thinner boards has stimulated the production of yarns that have reduced twist and are therefore flatter. Yarns have to survive the weaving process with minimal broken filament and other damage. High quality yarns needed for certain niche applications require filaments as fine as five microns, compared with the typical nine micron commodity reinforcement. A particular requirement of PCB makers is that yarns must have very few of the hollow air spaces, known as ‘seeds’, that are created during the fibre drawing process and can create electrical anomalies.

Similar care is needed over the production of epoxy resin, especially since this element has great scope for variability during manufacture and hence variation in finished quality. Resins used in data processing equipment, for example, must have well defined, reproducible properties that can only be ensured by close monitoring of the levels of catalyst, solvent and process parameters during production, along with tight on-going inspection of chemical and physical properties. Dow Chemical, Huntsman Advanced Materials and Hexion Specialty Chemicals are among the primary suppliers of substrate resins.

PCB materials are tested according to standards and test methods developed or adopted by the Institute of Printed Circuits (IPC), formerly known as the Institute of Interconnecting and Packaging Electronic Circuits.

Highly specified

More specialised electronics, particularly those operating at frequencies above about 500 kHz and up into the microwave band, require more highly specified PCB substrates. Likewise, electronics for use in adverse environments have onerous requirements. Meanwhile, a transition from use of leaded solder to lead-free assembly – driven by Europe's Restriction of Hazardous Substances (RoHS) Directive – has added its own demands, raising the bar in terms of thermal tolerance. Other legislation, including the European End of Life Vehicles (ELV) and Waste from Electrical and Electronic Directives (WEE) are further influences. As a consequence of these and other factors, the need for speciality engineered substrates continues to grow.

PCBs for high power radio frequency (RF) work use plastics with low dielectric constant (permittivity) and dissipation factor, such as Rogers 4000, Rogers Duroid®, DuPont Teflon®, polyimide, polystyrene and cross-linked polystyrene. These plastics typically have poorer mechanical properties, but this drawback can be compensated by mechanical reinforcement and is accepted for the sake of superior electrical performance. Polyimide, for instance, is highly suitable for digital circuits above 3 GHz, as well as analogue circuits up to 2 GHz. Rogers Corp's Duroid materials combine polytetrafluoroethylene (PTFE) with random microfibre glass or ceramic material. Rogers also produces a number of thermoset plastic/ceramic and thermoset plastic/ceramic/woven glass composites. The company says that ensuring lot-to-lot consistency of electrical properties by selecting appropriate raw materials and implementing a controlled manufacturing process differentiates its RF/microwave materials from high-volume PCB materials like FR-4 and BT/epoxy.

Arlon Inc produces Teflon-based materials, while another of its products utilises polyphenylene oxide (PPO) thermoplastic to improve on FR-4 properties in both electrical and mechanical respects. Hotpcb, manufacturer of a range of PCB types, utilises Duroid microwave laminates and Teflon materials from both Rogers and Arlon, among other manufacturers.

Lead-free soldering requires higher solder process temperatures than the increasingly outlawed leaded product. A noted supplier of advanced substrate materials, Park Electrochemical Corp, has developed a number of epoxy formulations that result in low coefficients of thermal expansion (CTE) in the finished board, so reducing the risk of copper track disbonding because of CTE differences between the composite substrate and the copper overlay. CTE is also a major consideration where silicon memory chips are surface mounted on boards, since board CTE up to eight or ten times that of the silicon chip will cause differential expansion and contraction with high temperature swings occurring during PCB processing and shipment. This can damage the chip and crack soldered joints. Park Electrochemical's low CTE multifunction epoxy, launched in response to these factors and RoHS, is just one of a range of different materials the company now offers to the high-performance PCB industry.

Park has developed its Nelco ® product line principally to meet the rigorous needs of telecommunications and internet infrastructure, plus high-end computing. Manufacturing its materials in North America, Europe and Asia, it supplies prepreg and laminate systems that provide superior mechanical, electrical and thermal performance in high layer count digital designs and RF/microwave applications. Epoxies are available with glass transition temperatures of 140–210°C, low or standard CTE and in halogenated or halogen-free formulations.

The Nelco range also includes polyimide, BT and cyanate ester-based substrates, and specialised substrates suitable for circuits operating at up to 77 GHz. Low dielectric materials are available for high-speed low-loss circuits. BT or polyimide resin, the latter being available in toughened and non-toughened formulations, can be selected for high temperature applications. Cyanate ester resin with quartz fabric, glass reinforced PTFE and woven glass/ceramic loaded PTFE are among other Nelco products. Two years ago, the company launched a new phenolic-cured FR-4 product, N4000-13EP. In recent years, it has increased manufacturing capacity for its materials in Asia, specifically at its Nelco Products Pte Ltd subsidiary in Singapore. A new mixing facility there can mix all solvent-based resins used in the Nelco product line, while a laminate press has been installed that can be used at up to 400°C.

Another composite that finds application in high-performance circuitry is nonwoven aramid fibre mat, impregnated with epoxy or polyimide. Aramid has a low dielectric constant (4, compared with 6.2 for glass) while a low CTE means that it is well matched thermally with silicon devices. DuPont's Thermount® is an example of this type of material, which can be combined with E-glass in multilayer boards.

Thin

A further trend in the advanced electronics industry, for which high circuit density is a totem, is that towards ever thinner boards. To meet the need for board which is thin while also being stiff and strong, certain manufacturers have introduced carbon fibre reinforcement.

A leader in this move has been US company ThermalWorks Inc with its Stablecor carbon fibre reinforced plastic (CFRP) laminate. Stablecor, made from continuous polyacrylonitrile (PAN) and/or pitch fibres in a high-temperature resin, comes in flat sheets of 4-mil to 10-mil thickness (greater if required) to which copper layers are added on each side. It is designed for use as a ‘plane’ layer (non-signal-carrying layer) in a ply stack with other dielectric layers to make a composite printed circuit board or device substrate.

According to the company, Stablecor is priced competitively with copper-Invar-copper and other common solutions for thermal conduction cooling. Another claimed advantage is that the material has a small CTE that can be tailored to be close to that of silicon. This means the composite is much better matched with attached electronic chips and components. Mounting a silicon die, for instance, that has a CTE of 3 ppm/°C on organic material having a similar expansion rate reduces or eliminates thermal strains at the solder joint and allows silicon to be attached without the need for underfill and adhesive. Direct silicon mounting onto PCBs permits signal paths to be shortened, leading to higher circuit performance.

Methods have been developed to isolate the carbon from conductive vias on multiplayer boards so that carbon composite layers can be included in such structures. High thermal conductivity ensures that heat is distributed from electronic hot spots throughout the laminate, contributing to effective thermal management. However, because carbon laminates are electrically conductive, they typically need insulating with a non-conductive layer before the copper conductive layer is applied.

Carbon is not the only candidate for board thinning however, and much can be accomplished with glass. Dielectric Solutions LLC uses its GlasFab® process to produce flatter, thinner glass fibres that offer mechanical and electrical properties said to be superior to those of other commercial glass products. This facilitates the production of compact, high-performance PCBs. In particular, a low-dielectric glass fabric is proving popular, says the company, in the high speed, high bandwidth electronic equipment market. A strategic alliance formed with American PCB specialist Compunetics Inc and Austria's Isovolta Group, known for its composites activities, is aimed at spreading the benefits of this technology.

As we have indicated, perforce briefly, while FR-4 epoxy/E-glass continues to be the composite workhorse of the general electronics industry, there are many material options available for more specialised circuitry. Choosing the right combination for a particular application can be a challenge. Here, a service from Underwriters Laboratories Inc (UL) may help. The Laboratory's UL IQ™ for Plastics is a powerful on-line database that allows users to search more than 50 000 grades of UL recognised plastics based on application requirements. UL also evaluates plastic materials for electronic device manufacturers and others.