The weird and wonderful materials that make up your PC

22nd Jul 2012 | 09:00

The weird and wonderful materials that make up your PC

We find out what's hidden inside your computer

Silicon still the king

Call us sceptical, but we bet you didn't know 7,7,8,8-tetracyanoquinodimethane is contained within your PC. It's not nearly as well known as silicon, but today's motherboards wouldn't be nearly as reliable without it.

This is just the tip of the iceberg - PCs are built from a range of obscure materials, many of which are extremely expensive and difficult to mine or extract. Chances are you wouldn't be able to describe the properties of neodymium, ruthenium or gallium, but they all have a vital part to play in keeping your PC running smoothly.

Here we take a whistle stop tour of the PC, component by component, delving into the amazing substances that are used in its manufacture. We'll see something of their unique properties and why they're used, but we'll also investigate the materials themselves to see what they look like and where they come from.

We'll ponder what'll happen when supplies of the rarer elements run dry, investigate whether these chemicals pose a health risk, and reveal little-known and fascinating facts about these most mysterious of materials.

Of course, having these facts at your fingertips won't make you any more effective at improving your PC's performance or choosing your next upgrade, but you'll be off to a flying start when you're next asked, in a pub quiz, what links tantalum and niobium.

Silicon chips

CPU

So, silicon chips - and here we could include the processor, memory, GPU and the Southbridge chip - are made of silicon, right?

Well silicon is vitally important, but no chip made entirely from silicon would stand a chance of working. To look at a piece of silicon you'd think it was a metal, but if you were to drop it the chances are it would shatter, which isn't what you'd expect from a metal. With properties intermediate between metals and non-metals, silicon is part of a small group of chemical elements known as metalloids or semi-metals.

Semi-metallic is also how you'd describe silicon's electrical properties - it will conduct electricity, but not very well. However, by doping silicon - the technical term for adding a small amount of another element - its conductivity can be vastly improved.

This is the key to the operation of the transistor, the basic building block of all electronic circuits. To cut a long story short, doping with boron or arsenic turns the silicon into something called a p-type semiconductor, while doping with phosphorous or gallium creates an n-type semiconductor.

The reasons for this are too complex to explain here, but by combining n-type and p-type semiconductor materials you end up with a transistor, and by connecting enough of them together you can create a processor.

The transistors are connected using thin copper strips. A complicated chip will need several layers of copper tracks, so layers of insulating material must be used in between to prevent them shorting out.

The insulator is silicon dioxide - the same compound that makes up pure white sand. This can be produced easily by oxidising the surface of the silicon wafer as the chips are being manufactured.

The exotic mix of chemicals in a processor doesn't end here. Intel and IBM made the headlines a few years ago when they began using an element called hafnium to improve the performance of their chips. If you've not heard of this obscure element before, then you're in good company.

Suffice to say, it's a metal, and its close neighbours in the periodic table are the equally rare elements lutetium, tantalum, and the radioactive material rutherfordium. If you hanker after little-known facts, it's named after Hafnia, which is the Latin name for the Danish capital, Copenhagen, where it was discovered in 1923.

Worryingly, because of its use in the control rods of nuclear reactors, some experts have suggested that it will run out within 10 years at current rates of consumption. That's a long time in the world of semiconductors though, and we feel confident that an alternative - perhaps zirconium - will step up to the mark.

While the apparent scarcity of hafnium might suggest that it would be expensive, there's another element found in many processors that costs about 50 times more, gram for gram.

Gold is only used in very small quantities, but it's used as the plating on the pins or pads of most high-performance processors. The property that makes it such a good material for jewellery also makes it an excellent metal for plating components: as a fairly unreactive element, it doesn't tarnish due to reaction with the air.

This doesn't just affect its appearance - it means that the good electrical contact between the processor and its socket won't diminish over time.

If you consider yourself safety-conscious, you may have raised your eyebrows at the mention of one particular element used in semiconductor manufacturing. A hundred and fifty years ago, when the Cornish tin mining industry was in full swing, that element was a by-product of tin production, and was sold to America as a pesticide in the fight against the boll weevil beetle that was devastating the country's cotton crops.

The element in question is arsenic, a minor constituent of the tin ore cassiterite, and which was driven off by heat treatment in a calciner and condensed in a chamber called a labyrinth. Given the fact that the young boys who were employed to remove the arsenic from the labyrinth rarely lived beyond their early 20s, you'd be excused for questioning how sensible it is to put it in PCs.

You'll be relieved to hear that only small portions of a silicon chip are doped and, even then, concentrations measured in a few parts per million are all that's needed to provide those all important semiconductor properties. The bottom line is that integrated circuits contain no more than trace amounts of arsenic.

Hard disk platters

hard drives

We all know that hard disks store data magnetically, but there are few similarities between them and the old DAT format used in the 1980s. Both use a head to read and write data to magnetic media, but there the similarity ends.

Magnetic disks use a much more efficient form of recording than is possible with a flexible tape, although that efficiency comes at a price. While magnetic tape is a simple, cheap strip of plastic film impregnated with iron oxide (rust), or more recently chromium dioxide, the platter of a hard disk drive is manufactured in a much more expensive multi-stage process that demands an extremely high degree of accuracy, and uses some interesting and esoteric materials.

The starting point is a plain disk, machined to high tolerances from a non-magnetic material. This way the magnetic properties can be accurately fine-tuned by adding thin layers of other materials.

The disk is typically made from either glass or metal. Aluminium is a good choice since it's light and non-magnetic, but hard disks need to be robust and aluminium isn't hard enough by itself. Instead, the material used is an alloy of aluminium and magnesium, plus small amounts of other elements like silicon, copper and zinc.

There are therefore five or more substances in the blank platter, although none of them are exactly rare or unusual - the more esoteric materials are used later.

The platter will spin at up to 7,200rpm, and the read/write head will hover millionths of a millimetre above it, so any unevenness would be catastrophic - the head would crash immediately. It isn't possible to polish aluminium alloy to the necessary smoothness, so the blank must be coated in a layer of a substance called NiP, which can take the high polish.

This material is an alloy of nickel and phosphorous. An alloy is usually thought of as a mixture of substances that shares their properties, but NiP couldn't be more different from its constituent parts.

Phosphorous is a non-metallic element, and a very reactive one at that. In its white elemental form, it will ignite spontaneously in air and burn fiercely. It's also highly toxic and glows in the dark. However, like nickel, NiP has all the properties of a metal and is relatively inert.

Strangely though, whereas nickel is magnetic, NiP is not. Most of the other materials on the platter are responsible for its magnetic properties. Iron is the most familiar magnetic material, but those you'll find in a hard disk are far more interesting. There are many magnetic (or, to be more accurate, ferromagnetic) metals, but those used in a hard drive are chosen for the ways in which they interact.

Since a more detailed explanation would take us into the intricacies of physics, we'll stay well clear of the 'why' and concentrate squarely on the 'what'. First there's something called the soft magnetic underlayer, which is made from an alloy of cobalt, nickel and iron.

In the highest performance disks, the soft magnetic underlayer is divided into two by a thin layer of the element ruthenium. Only a very small amount it is needed, which is just as well - as the 74th most abundant element on Earth (and there only are 90 naturally occurring elements), ruthenium is rarer than both gold and platinum.

Accounting for one part per billion of the Earth's crust, just 12 tonnes are produced annually, which is only enough to make a one-metre cube. Its name comes from Ruthenia, the 13th century Latin word for the ancient land of Rus, which comprised parts of current day Russia, Belarus, Ukraine, Slovakia and Poland.

The true recording layer is where we find the really expensive materials though, as we're now looking at an alloy of cobalt, chromium and platinum. Although platinum is more abundant than ruthenium, because it has so many more uses - mostly as a catalyst in the chemical industry and catalytic converters in cars - it's much more expensive. Currently it sells for $1,500 per Troy ounce, which works out as over $48,000 per kilogram.

Hard disk heads

Rare and expensive elements aren't only found on the platter, as we'll see when we look at that other important part of a hard disk drive - the read/write head. The head is attached to an arm assembly, which can move to access any of the concentric tracks of data on the platter.

An integral part of the arm is a wire coil that moves in a magnetic field whenever an electrical current is applied. That magnetic field is provided by a very powerful magnet, of which a major constituent is neodymium.

Neodymium looks much the same as any other metal, but it's unique in being the most magnetic of all the elements. In its raw form that property isn't particularly useful, since neodymium has such a low curie point - the temperature above which magnetism is lost - that anything containing a neodymium magnet would have to be refrigerated.

This is where those other metals come in. By mixing two parts of neodymium with 14 parts of iron and one part of boron, a combination of strong magnetism and a high curie point is achieved. And we really are talking about strong magnets - a neodymium-iron-boron magnet can lift over a thousand times it own weight.

This has given rise to safety concerns - if you ever try dismantling an old hard drive, make sure your fingers are well out of the way if the pair of neodymium magnets find themselves drawn together.

That's not all - although neodymium, iron and boron are all malleable, the alloy used in magnets is brittle, so if you do allow a pair of neodymium magnets to snap together from any distance, watch out for flying shards of alloy.

What's on your motherboard?

motherboard

To the engineer, the motherboard might seem the poor relation to the processor and the hard disk, but while the complexity of its manufacture might not compare, the substances used are no less intriguing.

The printed circuit board (PCB) is a sheet of material with conducting copper tracks printed onto both sides and sandwiched in several intermediate layers. The tracks make the connections between the components, and the sheet onto which they're printed provides mechanical strength while insulating between the layers of connections.

Copper is far more common than the likes of ruthenium and neodymium. It's been used for millennia, is 10,000 times more abundant than platinum and thousands of times cheaper. It's one of the most important materials in a PC, and you'll also find it in the various cables that connect components like the hard disk and the CD/DVD drive to the motherboard.

The property of copper that makes it so important to the electronics industry is its electrical conductivity. Although it's not the most conductive material, if it was pensioned off in favour of silver - the only metal that's more conductive - the cost of electronics would soar.

Perhaps the most astonishing thing about copper is the vast scale of its production. From its discovery in the fourth century BC in tiny mines on the island of Cyprus (where copper gets its name), copper ore is now extracted from huge open cast pits. Bingham Canyon Mine near Salt Lake City in Utah, currently the world's biggest producer of copper, is 1.2km deep and 4km wide, covering an area of 1,900 acres. The annual production of 300,000 tonnes of copper is brought to the surface in trucks standing seven metres tall and weighing more than a jumbo jet.

The other element of a PCB, the insulating sheet, might seem simple, but appearances can be deceptive. The PCBs used in today's motherboards have to provide the ideal combination of electrical, thermal and mechanical properties. They also have to meet safety requirements so that if a component fails and overheats, the board won't catch fire and emit toxic fumes.

The most common insulating material today is FR-4, which comprises sheets of woven fibreglass bonded by flame-retardant epoxy resin. Many manufacturers use a similar material for simple car body repairs, but its function in PCBs is much more complex.

Here we get into the murky world of organic chemistry. The two compounds that form epoxy resin are chloromethyloxirane (otherwise known as epichlorohydrin), and 4,4'-(propane-2,2-diyl)diphenol (otherwise known as bisphenol-A). We could tell you that the former has the chemical formula C3H5ClO, and the latter is C15H16O2, but this isn't particularly helpful since in organic chemistry, two substances can share the same chemical formula but differ in their structure.

We're not going to get embroiled in molecular diagrams, though. All we need to know is that when these two liquids are mixed together, a polymerisation reaction takes place that results in a much larger molecule, which forms a solid. This in turn bonds the fibreglass sheets and fills the holes in the weave to produce a tough, insulating, flame retardant sheet to which copper foil can be applied.

Resistors

If you take a look at a motherboard you'll see lots of small components that look like tiny black blocks and metallic cylinders. Most of these will be either resistors or capacitors, which electronics engineers refer to as passive components.

There isn't room here to explain the functions of each one (see here for a guide to basic electrical components), but as with most parts of a PC, delving inside the passive components brings to light yet more unusual substances.

Resistors do what the word suggests - they resist the flow of an electrical current. They differ in their resistance (measured in Ohms, a unit of how much they impede the flow of electricity) and their power rating (how much power they can dissipate by resisting that current without burning out).

Early resistors were little more than a cylinder of carbon, but those days are long gone, and even in this most basic of all components we can now find quite a mix of elements.

The major part of a resistor is a ceramic block, typically made of aluminium oxide (alumina), which gives the finished component enough bulk to be handled and soldered. This material is used because it's a near-perfect insulator.

The electrical path is made by depositing a film of ruthenium oxide. Unlike most metal oxides, this material has limited electrical conductivity, which is exactly what's needed. To cap it all off, metal electrodes made from an alloy of platinum, palladium and silver are attached at each end.

Palladium, like ruthenium, rhodium, osmium, iridium and platinum itself, is referred to as a platinum group metal. These elements are grouped together in the periodic table, but they're also found together geographically - most notably in the Bushveld complex of South Africa - and they all command a high price.

Since they're often found together in the same ores, platinum group metals have to be separated before use. This is difficult due to their similar properties, and necessitates an expensive multi-stage chemical process, which you can read about here.

Capacitors

Capacitors allow an AC current to pass while blocking a DC current. At their simplest, they're formed from two sheets of metal separated by an insulator called the dielectric. Of course, things are rarely that simple and a motherboard will contain lots of different types of capacitor, each suited to a particular application.

Those with the lowest capacitance (a property measured in Farads) tend to be ceramic capacitors, and while they look similar to resistors, their construction is different. The dielectric is made from ceramic, which is plated with silver to form the two metal sheets. This structure is stacked vertically to achieve the necessary capacitance without taking up too much space on the PCB.

If they were built this way, the large capacitors that are needed in a PC's power supply and audio circuitry would be huge, so various types of electrolytic capacitors are used instead.

The original type of electrolytic capacitor was formed by rolling up layers of aluminium foil, sandwiched with paper soaked in an electrolyte. That electrolyte was usually boric acid or sodium borate, with other materials added to prevent it from evaporating.

You'll still see this type of capacitor on some motherboards as little metallic cylinders but they have one drawback - if they overheat the electrolyte evaporates, causing the case to bulge or burst and gunge to leak out all over the motherboard. Needless to say, this doesn't have a particularly beneficial effect on the workings of the PC.

Liquid electrolyte capacitors are being phased out in favour of so-called polymer capacitors, which brings us to that 7,7,8,8-tetracyanoquinodimethane, which provides a replacement for the liquid electrolyte. Polymers based on this organic compound are plastics, but unlike most plastics they can conduct electricity.

One other important type of capacitor that can provide a particularly high capacitance for its volume is based on an element that we've not come across so far. We won't go into detail about the capacitor's internal construction, but the element it uses is tantalum.

Although it's a metal, tantalum has unusual properties that make it particularly suitable for manufacturing capacitors. But like hafnium, our supplies are running out. It's not that it's inherently rare - at one part per million in the Earth's crust, it's a thousand times more abundant than ruthenium. The problem is that, for a variety of political reasons (not least of which being that it's mined in the troubled Democratic Republic of Congo), experts have suggested that stockpiles will run dry sometime between 2015 and 2105.

If you know your way around the periodic table you might just know where to go if and when supplies of tantalum do run out. Tantalum has an atomic weight of 73, which puts it in the group of elements known as rare earth metals. Immediately above it is element number 41 – niobium.

While not strictly speaking a rare earth metal, niobium has remarkably similar properties to tantalum, and is much more abundant. It looks much the same, and has a similar density, melting point and boiling point to tantalum. Most importantly, scientists think it could form the basis of a new form of capacitor.

Solder

Unlike the big and expensive components like the processor and memory, which are fitted into sockets on the motherboard when the PC is built, the passive components are soldered in place by the motherboard manufacturer.

Solder, the low melting point alloy used to make an electrical and mechanical connection between the lead or pin of a component and the copper track on the PCB, was originally made from tin and lead. The type normally used in electronics was 63 per cent tin and 37 per cent lead, and had a melting point lower than that of pure tin, pure lead, or any other alloy of these two metals.

Once again we're seeing that although an alloy is essentially a mixture of two metals, its properties aren't always intermediate between those of its constituents. All that changed in 2006, when the European Union's Waste Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Substances (RoHS) directives came into effect.

Lead was banned in all consumer goods because of its toxicity, and the electronics industry had to find a new form of lead-free solder. An alloy of tin, silver and copper is most commonly used, and has a melting point a touch higher than old-fashioned lead solder. However, lots of other formulations have been used, with zinc or manganese being added to the mix to improve other properties.

An organic future?

While silicon is perhaps the most important material in an average PC, we've unearthed a wide range of elements and compounds, without which modern computing would be impossible. It's been estimated that well over 50 of the 90 naturally occurring elements are used to make the digital age a reality.

Many of these are in limited supply and for some there's no known alternative. Given the rate at which irreplaceable resources are being used up, we have to question how much longer we can expect business as usual.

There is a glimmer of hope though. While each element has unique properties that can be matched by no other, organic chemistry can provide a means of designing molecules with the desired properties.

What's more, the synthesis of organic compounds doesn't rely on esoteric and rare elements. Already scientists have produced conducting and semi-conducting polymers and built electronic circuits from nothing more than plastics - although, as yet, they can't match the performance of silicon.

Given the vast number of possible organic compounds, a PC of the future may contain no fewer amazing substances, but if the organic revolution takes off, the chances are they'll be radically different from the materials in today's PCs.

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