A semiconductor package is a metal, plastic, glass, or ceramic casing containing one or more discrete semiconductor devices or integrated circuits. Individual components are fabricated on semiconductor wafers (commonly silicon) before being diced into die, tested, and packaged. The package provides a means for connecting it to the external environment, such as printed circuit board, via leads such as lands, balls, or pins; and protection against threats such as mechanical impact, chemical contamination, and light exposure. Additionally, it helps dissipate heat produced by the device, with or without the aid of a heat spreader. There are thousands of package types in use. Some are defined by international, national, or industry standards, while others are particular to an individual manufacturer.
A semiconductor package may have as few as two leads or contacts for devices such as diodes, or in the case of advanced microprocessors, a package may have hundreds of connections. Very small packages may be supported only by their wire leads. Larger devices, intended for high-power applications, are installed in carefully designed heat sinks so that they can dissipate hundred or thousands of watts of waste heat.
In addition to providing connections to the semiconductor and handling waste heat, the semiconductor package must protect the "chip" from the environment, particularly the ingress of moisture. Stray particles or corrosion products inside the package may degrade performance of the device or cause failure. A hermetic package allows essentially no gas exchange with the surroundings; such construction requires glass, ceramic or metal enclosures.
Manufacturers usually print—using ink or laser marking—the manufacturer's logo and the manufacturer's part number on the package,
to make it easier to distinguish the many different and incompatible devices packaged in relatively few kinds of packages.
The markings often include a 4 digit date code, often represented as YYWW where YY is replaced by the last two digits of the calendar year and WW is replaced by the two-digit week number,
typically the ISO week number.