Motherboards

Computer Engineering; Information Technology

The motherboard is the main printed circuit board inside a computer. It has two main functions: to support other computer components, such as random access memory (RAM), video cards, sound cards, and other devices; and to allow these devices to communicate with other parts of the computer by using the circuits etched into the motherboard, which are linked to the slots holding the various components.

The motherboard of a computer is a multilayered printed circuit board (PCB) that supports all of the computer's other components, which are secondary to its functions. In other words, it is like the “mother” of other, lesser circuit boards. It is connected, either directly or indirectly, to every other part of the computer.

In the early days of computers, motherboards consisted of several PCBs connected either by wires or by being plugged into a backplane (a set of interconnected sockets mounted on a frame). Each necessary computer component, such as the central processing unit (CPU) and the system memory, required one or more PCBs to house its various parts. With the advent and refinement of microprocessors, computer components rapidly shrank in size. While a CPU in the late 1960s consisted of numerous integrated circuit (IC) chips attached to PCBs, by 1971 Intel had produced a CPU that fit on a single chip. Other essential and peripheral components could also be housed in a single chip each. As a result, the motherboard could support a greater number of components, even as it too was reduced in size. Sockets were added to support more peripheral functions, such as mouse, keyboard, and audio support.

In addition to being more cost effective, this consolidation of functions helped make computers run faster. Sending information from point to point on a computer takes time. It is much faster to send information directly from the motherboard to a peripheral device than it is to send it from the CPU PCB across the backplane to the memory PCB, and then from there to the device.

Designing a motherboard is quite challenging. The main issues arise from the presence of a large number of very small circuits in a relatively small area. One of the first considerations is how best to arrange components on the motherboard's various layers. A typical PCB consists of sheets of copper separated by sheets of fiberglass. The fiberglass insulates the copper layers from each other. Most motherboards consist of six to twelve layers, though more or fewer layers are also possible. Certain layers typically have specific functions. The outer layers are signal layers, while other layers carry voltage, ground returns, or carry memory, processor, and input/output data.

Lines etched in the fiberglass insulating each layer allow the familiar copper lines, or traces, to show through. These traces conduct the electrical signals. Most motherboard designers use computer simulations to determine the optimal length, width, and route of the individual traces. For example, a motherboard will have a target trace impedance value, often fifty or sixty ohms, which must be kept constant. Widening a trace will decrease impedance, while narrowing it will make it greater. Another issue is crosstalk resulting from the high level of circuit density. If this happens, traces must be either better insulated or moved farther apart so that interference will diminish.

Some sophisticated computer users may try tuning their systems by adding or removing motherboard components, adjusting power settings, or “overclocking” the CPU to make it run faster. Overclocking can be risky, as it typically requires increasing the core voltage. Most motherboards have a built-in voltage regulator to ensure that the core voltage does not exceed the recommended voltage for the CPU and other processors. However, some regulators allow users to adjust their settings. While there is usually a buffer zone between the recommended voltage and the maximum safe voltage, setting the voltage too high can still cause processors to overheat or even burn out.

Though somewhat standardized, motherboards still come in different sizes and shapes. The main distinction is between motherboards for laptops and those for desktops. Motherboards designed for one of these categories generally will not fit into the other category. Most large desktop computer cases have enough room inside for just about any model of desktop motherboard, though smaller motherboards leave more space for peripherals to be added later.

A motherboard will have some basic software embedded in a read-only memory (ROM) chip. This software is called the BIOS, which stands for “basic input/output system.” When the power button is pressed, the BIOS tells the computer what devices to activate in order to locate the operating system and begin running it. If a computer malfunctions, it may be necessary to use the BIOS to change how the motherboard behaves while the system starts up.

—Scott Zimmer, JD