How motherboards are made: a miracle of modern electronics

15th Aug 2010 | 07:00

How motherboards are made: a miracle of modern electronics

The process dissected

1-5: What goes into making a motherboard?

Perhaps you've fallen into the trap of thinking that a motherboard is just a slab of fibreglass for the all important processor to slot into. Well, it's time to rethink things: the motherboard is the nervous system of your PC.

It provides the essential communication pathways that enable the rest of your machine to do its job, handles the video circuitry and connections to external devices and even resists scrabbling hands trying to rip out graphics cards or rubbing all those essential components. Like all true workhorses, when it does its job, you barely notice it.

Manufacturing them remains a challenge. True, processors have features that are so small that they can't be seen with the naked eye, but the amount of technology at work when building a motherboard is no less impressive.

It's an intensive process – and one that you're about to learn in detail.

1. Raw materials

Like any other electronic item, tracing the motherboard back to its roots leaves us staring at a hole in the ground – or, to be more accurate, a couple of them.

The two dominant constituents of a printed circuit board are fibreglass – which provides insulation – and copper, which forms the conductive pathways, taking us back to their birthplaces in a sand quarry and open-cast copper mine respectively.

Turning sand into glass and copper ore into metal are processes that are hundreds of years old, but what we do with the materials next is anything but ancient.

2. Fabricating copper-clad laminate

Molten glass is extruded to produce glass fibres that are woven to create a sheet of fibreglass fabric. Next the sheet is impregnated with epoxy resin and heated to partially cure the resin; the resulting sheet is called 'prepreg'. Multiple sheets of prepreg are stacked to produce a laminated sheet of the required thickness.

Sheets of copper foil are applied to both sides of the laminate and the sandwich is placed in a heated press. This completes the curing of the resin, making the laminate rigid and causing the layers to bond together.

The result is an insulating sheet of fibreglass with copper foil on both sides: copper-clad laminate. The overall thickness of the printed circuit board (PCB) is typically 1.6mm. This means that, for a six-layer board, the fibreglass laminates will be about 0.35mm thick and the copper foil will be about 0.035mm thick.

The fibreglass is thick enough to provide adequate mechanical strength and rigidity, and the copper is sufficient for good electrical and thermal conductivity.

3. Etching away unwanted copper

A photosensitive material called photo-resist is applied to both sides of the copper-clad laminate, totally covering the copper layers. This is usually a dry film process, in which thin films of solid photo-resist are laminated onto both sides of the board using equipment that's fairly similar to an office laminator.

Step 3

Now a transparent artwork showing the pattern of the PCB's pads and tracks is placed over the photosensitive copper-clad laminate, and is then exposed to ultraviolet light. Ultraviolet is used rather than visible light so the board can be handled safely in daylight.

Where the photo-resist is exposed to ultraviolet, the chemicals polymerise, forming a plastic. Since the board has two copper layers, each of which has a photo-sensitive coating, this process is carried out twice using different artworks for each side.

Next, the board is immersed in a chemical solution to develop the latent image. The developer washes away the unexposed photo-resist, leaving only material that was polymerised and which corresponds to the pad and tracks. The areas of the copper film that aren't protected by the remaining polymerised portions of the photoresist are etched away.

In an oxidation reaction, metallic copper is transformed into a copper salt, which is water-soluble and therefore washes off during the etching. For quick etching, the board passes through a chamber in which the etchant is sprayed at a high pressure and at a temperature of about 50C.

After etching, the board is washed to remove surplus etchant and the remaining photo-resist is removed using an organic solvent. The insulating fibreglass board now has a pattern of copper tracks on each side that will form the circuit's interconnections. This assembly is called a core.

However, motherboards have a multilayer construction, which means they have more than two copper layers. This means that the above process has to be carried out several times. In the case of a six-layer motherboard, two of these cores will be needed to provide four of those layers. We'll see later how the other two layers are made.

4. Building up a stack

Double-sided cores are now sandwiched together to start the creation of a multilayer PCB. Two cores are used for a six-layer board (a common figure for motherboards), but they can't be stacked directly on top of each other because this would cause the copper tracks on the top of the bottom core to short with the tracks on the bottom of the top core.

To stop this from happening, a sheet of prepreg is placed between them. Sheets of prepreg are also applied to the top and bottom of the stack before it's subjected to pressure and a high temperature to complete the curing of the prepreg and bond everything together.

Step 4

For a six-layer board, the stack would comprise: prepreg / core / prepreg / core / prepreg. This means that the final result will be: fibreglass / copper / fibreglass / copper / fibreglass / copper / fibreglass / copper / fibreglass.

5. Drilling the holes

Holes are now drilled through the board. First come the mounting holes, which will be used for mechanical fixing (bolting the motherboard into the PC's case).

Second are the holes that are used to accommodate the leads of through-hole components when they're soldered to the board in a couple of steps' time.

Finally, there are the tiny holes that form vias (vertical interconnect access), which make electrical connections between the various copper layers – or will, when we get to routing, testing and QA.

Despite the use of a high-speed, numerically controlled drilling machine, drilling can be a very time-consuming process, especially if lots of different hole sizes are required. For this reason, it's common to stack boards together so that several are drilled at once, saving time and money.

6-10: Assembling the motherboard

6. Copper and tin plating

Electro-plating would be an obvious choice to make the vias conductive, except for one minor problem: only already-conducting surfaces can be electro-plated. To get around this, the board is immersed in various chemicals that coat its entire surface with a thin layer of copper. It's a slow method and very expensive, but it provides just enough conducting metal to electro-plate over the top.

Electro-plating the entire board would be wasteful because most of the copper would subsequently be etched away to produce the pads and tracks on the outer layers of the PCB. Instead a photo-resist is applied, exposed to UV light through an artwork and developed as when fabricating the copperclad laminate – but with one important alteration.

Step 6

Here, a different type of artwork is used so that the photo-resist remains in those areas that don't correspond to the pads and tracks of the finished board. Now the electro-plating will only increase the thickness of the copper on the areas without the insulating photo-resist.

The board is finally electroplated with tin, which, once again, only adheres to those areas of the board that will form the pads and tracks. The tin serves three purposes: it prevents the copper tarnishing; it provides a surface that can be soldered to more easily than copper; and it acts as a resist (after first removing the remaining photo-resist) in the next process – etching away the unwanted copper.

We now have a PCB with copper pads and tracks on the outer two surfaces, tracks on four internal layers, and vias making the necessary connections between the various layers.

To complete the bare PCB, a solder mask and component identification are applied via silkscreen printing. The solder mask covers all of the board where solder shouldn't adhere when the components are fixed in place. This prevents unwanted bridges between tracks that could occur during wave soldering in step 9.

The component identification provides a visible labelling of each of the components with their serial numbers. This is useful in manual inspection or board maintenance.

7. Routing, testing and QA

Steps 2 to 7 involved the processing of a panel – a sheet of material comprising several motherboard PCBs. Now the individual boards are separated using a numerically controlled router, which is also used to create any non-plated larger holes and slots that are needed.

The board is then given a going over by a 'bed-of-nails' tester, an automated process that probes both sides of the board to ensure that electrical pathways exist where they are supposed to and that there are no shorts.

Finally, before leaving the PCB fabrication facility, the motherboard is given a QA inspection to ensure it meets its specification in terms of the overall board size, mounting hole tolerances and so on.

8. Surface mounting

The first components to be soldered onto the bare PCB are the surface mountings. Solder paste – a mixture of solder powder and flux – is printed onto those pads on the top surface of the board where the contacts of the surface-mounting components (SMCs) will be soldered. The SMCs are placed on the board using a pick-and-place machine.

Step 8

The tackiness of the solder paste holds the components in place, but they're not fixed securely and there isn't a proper electrical connection.

The next stage is reflux soldering. The PCB is placed in a reflux oven and heated to over 200C. The solder in the paste melts and then solidifies when the board cools down again, providing good electrical connections and fixing the components securely.

9. Through-hole components

Next the larger through-hole components are fitted, often on a manual production line. Included are the processor socket, the memory and expansion card slots and the various connectors such as keyboard, mouse, audio and video sockets. The components are fitted to the top side of the board with their pins protruding through pads on the bottom side of the board.

Step 9

The board then enters a wave soldering machine. This contains a tank of molten solder that's pumped across a submerged edge, causing a raised wave of solder. As the board progresses through this apparatus, each part of the bottom side of the board comes into contact with the solder wave. The solder adheres to the board wherever it's free of solder resist, thereby making mechanical and electrical connections between the component leads and the pads.

10. Final testing and packaging

For final testing, processor and memory modules are plugged into their sockets. External PC components such as a hard disk, CD/DVD drive, monitor, keyboard and so on are also plugged into their appropriate connectors. With the motherboard now effectively built into a complete PC, a full functional test involving every socket is carried out.

Step 10

This is mostly an automated process, although humans do still have a part in the process for areas like audio circuitry. All this is followed up with a 'burn-in' test, which involves running diagnostic software on the motherboard for a protracted time while it's subjected to high temperatures and temperature cycles.

If the board passes this test, which is designed to cause any potentially faulty components to fail, the motherboard is complete. All that remains is for the finished board to be packaged in an antistatic bag and box, and it's ready to take pride of place in a new PC.

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First published in PC Plus Issue 297

Liked this? Then check out The ultimate guide to motherboards

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