What is an Air Cavity Ceramic Package?

An Air Cavity Ceramic Package is a packaging technology that usually used for Si, GaAs, and GaN RF power transistors. It is ideal for high-power RF applications.

A typical air cavity ceramic (ACC) package structure consists of a gold-plated lead frame, flange (or heatsink), a ceramic lid, a ceramic window frame, and sealing epoxy between the lid and the window frame. The construction of the air cavity ceramic package can be seen in the figure below.

Air cavity ceramic package construction

In an ACC package, the die (containing functional circuits) is typically attached to an Au (gold) plated flange or heat spreader made from a Cu (Copper) based laminate or similar material using high conductivity die attach. The lowest resistance die attach used in the semiconductor industry is Au-Si eutectic die attach.

The die and wire bonds are surrounded by a low dielectric constant material such as air and protected from the environment by a ceramic lid (hence, the name air cavity ceramic package). The leads and flange (heatsink) are separated by a window frame that forms the cavity for the die and wire bonds. The leads and the flange are typically gold-plated to protect the underlying solderable surface.

Key Benefits of Air Cavity Ceramic Packages:

• The use of ceramic material (typically, Al2O3) for sidewalls and lids of the packages makes enable it to withstand high-temperatures without cracking, melting, flowing, and decomposing. 
• Flange materials (CPC, CuW, CuMo, etc.) provide low thermal resistance that allows the package to provide better heat. Better heat dissipation makes ACC packages ideal for use in high-power applications.
• Provides excellent mechanical reliability
• Dry air inside the package has a considerably lower dielectric constant than molding compounds which reduces losses at high frequencies, hence providing improved electrical performance in high-frequency semiconductor circuits.

Drawbacks of Air Cavity Ceramic Packages:

• Ceramics are expensive, hence the manufacturing costs for ceramic air cavity packages are relatively higher and not ideal for high volume production. 
• Brazing (is a joining process) of the ring-frame with the flange creates stress and distortions.
• Each package variation (such as additional leads or shorter leads) demands the creation of a unique header (The flange is brazed with the ring-frame at high temperature, and the resulting component is known as a header).

The drawbacks of the air cavity ceramic package (ACC) can be overcome by the air cavity plastic package (ACP), which is another packaging technology. However, ACP reduces the ability of the component to withstand high temperatures.