A CPU shim is a shim used between the CPU

Central processing unit

The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...

 and the heat sink

Heat sink

A heat sink is a term for a component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems and the radiator in a car...

 in a computer. Shims make it easier and less risky to mount a heatsink on the processor because it stabilizes the heatsink, preventing accidental damaging of the fragile CPU packaging. They help distribute weight evenly over the surface.

CPU shims are usually made of thin and very flat aluminium

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 or copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

. Copper has good heat

Heat

In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

 dissipation capacity but is electrically conductive. CPU shims should be non-conductive to prevent any accidental short circuit

Short circuit

A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....

ing. Aluminium shims are often anodized, which makes them non-conductive and improves their appearance (see case modding

Case modding

Case modification is the modification of a computer chassis , or a video game console chassis. Modifying a computer case in any non-standard way is considered a case mod...

). It is also very important that the shim is the proper thickness. If it is too thick then the heatsink will not make contact with the CPU, resulting in poor cooling and possibly overheating.

Most shims are CNC manufactured, often using laser cutting

Laser cutting

Laser cutting is a technology that uses a laser to cut materials, and is typically used for industrial manufacturing applications, but is also starting to be used by schools, small businesses and hobbyists. Laser cutting works by directing the output of a high-power laser, by computer, at the...

. Cheaper ones may be pressed or stamped which could make them less accurate.

They can usually be bought for less than USD

United States dollar

The United States dollar , also referred to as the American dollar, is the official currency of the United States of America. It is divided into 100 smaller units called cents or pennies....

$10.

Usage

CPU shims are not common at all in OEM

Original Equipment Manufacturer

An original equipment manufacturer, or OEM, manufactures products or components that are purchased by a company and retailed under that purchasing company's brand name. OEM refers to the company that originally manufactured the product. When referring to automotive parts, OEM designates a...

 computers, but are used by some computer hardware enthusiasts who may use heavier heatsinks because they wish to have a cooler or less noisy system or perhaps to overclock

Overclocking

Overclocking is the process of operating a computer component at a higher clock rate than it was designed for or was specified by the manufacturer, but some manufacturers purposely underclock their components to improve battery life. Many people just overclock or 'rightclock' their hardware to...

 the CPU for better performance. A heavy heatsink puts more pressure on the CPU and motherboard. Shims are very useful for people who often change CPU, heatsink and/or cooling solutions

Computer cooling

Computer cooling is required to remove the waste heat produced by computer components, to keep components within their safe operating temperature limits.Various cooling methods help to improve processor performance or reduce the noise of cooling fans....

, or use a heatsink that is heavier than the CPU manufacturer's recommended weight.

CPU shims are nowadays largely obsolete because most modern processors designed for home users since the introduction of the Athlon 64

Athlon 64

The Athlon 64 is an eighth-generation, AMD64-architecture microprocessor produced by AMD, released on September 23, 2003. It is the third processor to bear the name Athlon, and the immediate successor to the Athlon XP...

 and Pentium 4

Pentium 4

Pentium 4 was a line of single-core desktop and laptop central processing units , introduced by Intel on November 20, 2000 and shipped through August 8, 2008. They had a 7th-generation x86 microarchitecture, called NetBurst, which was the company's first all-new design since the introduction of the...

have Integrated Heat Spreaders (IHS) which prevents the CPU core from getting accidentally broken.