Feature Article - Substrate Materials Part 2 - By Christopher Henderson

There are other package substrate and dielectric isolation materials in use. One important material is polyimide. Manufacturers use this material extensively in redistribution layer and bump processes. It is easy to deposit, has a high glass transition temperature, a reasonable dielectric constant, and can be easily patterned. However, it does have a high coefficient of thermal expansion. Another important material is benzocyclobutene. It is also easy to deposit and has a low dielectric constant, making it good for high frequency applications. We discuss materials properties of these materials and others a little later in this presentation. However, it also has a high coefficient of thermal expansion. Another material in use is bismaleimide-triazine, or BT. It is also easy to process and has a low dielectric constant, and it has a relatively low CTE of 14 parts per million per degree centigrade.

BT epoxy resin is used commonly with laminates. Engineers can create build-up layers and plated through holes on the finished laminate. BT resin epoxy laminates make an excellent choice for BGA substrates since the BT epoxy allows for higher frequencies, and the higher glass transition temperature allows for higher temperature use. Furthermore, BT substrates can be processed in strip format for high volume manufacturing.

Polyimide can be used not only for isolation in redistribution layer technology, but also as a substrate material. DuPont makes polyimide materials that go by the trademarks Kapton and Pyralux that can be used for flexible substrates. Amkor also uses polyimide in one of their BGA packaging processes. fleXBGA is Amkor’s term for their fine pitch BGA substrate technology. They process this as either a 2-layer tape without adhesive and wet-etched holes, or a 3-layer tape with adhesive and punched holes for connections to the solder bumps.

BCB polymers are also used for microelectronic packaging and interconnect applications. During the 1990s, they gained commercial status in applications including the fabrication of gallium arsenide integrated circuits, bumping and redistributing GaAs chips, and for planarization and isolation in flat-panel displays. More recently, Dow Chemical and others have developed techniques to B-stage or partial cure the BCB material, and then spin deposit it onto a wafer or other substrate. Engineers then use the BCB as an isolation layer between dice in a multi-die package, as an isolation layer for interconnect in a redistribution layer, and other configurations. One can etch BCB with dry etch techniques, or one can formulate photosensitive BCB resins for lithography applications.