An Independent Journal Dedicated to the Advancement of Chip - Scale Electronics January - February 2000 Email the editor

"A variety of materials will find their way onto the scene."

The market for array packages-BGAs, CSPs and newer versions of PGAs-is growing rapidly. A distinguishing feature of array packages is the use of a substrate rather than a leadframe for the internal interconnection.

To meet the cost and performance requirements of the wide variety of semiconductor devices, package substrates are made from a number of materials.

Essentially all of the early array packages used a ceramic substrate. In recent years, organic materials have been improved and are now the material of choice for most array packages.

By Steve Berry and Sandra Winkler Contributing Editors

Organic substrates can be either rigid or flexible in nature. The rigid substrates are generally made from BT (bismaleimide-triazine) resin, but a small percentage are made from FR-4.

The market for BT resin substrates was originally dominated by a few suppliers, resulting in complaints that the material was high priced and of questionable reliability. Today the BT resin market has many competitors, and the price has been lowered substantially. Additionally, quality has improved, and the material is now a commodity for the two- and four-layer substrate market.

Flexible organic substrates are generally referred to as "flex tape" and offer superior thermal and electrical performance over thicker substrates. This tape is generally, but not necessarily, made using polyimide as the dielectric material.

Flex tape permits finer traces than BT resin. In addition, the smallest ball pitch possible with BT resin is about 0.8 mm, while flex tape allows for a pitch as small as 0.5 mm.

For wafer-level packages (WLPs), the "substrate" typically takes the form of material layers deposited directly onto the wafer, using a variety of methods. Polyimide is typically employed for the dielectric layers, with copper and gold commonly selected to provide the interconnection between the pads on the die and the solder balls on the package.

In addition to the substrates mentioned, there is a variety of other materials that will find their way onto the scene in the coming years.

These include a substrate that is a BT resin-flex combination, materials that are similar to, but competing against BT resin, upgraded FR-4 materials such as FR-5, liquid crystal polymer and proprietary polymer systems. The latter two are associated with Siemen's PSGA, which is assembled by CS2 in Belgium.

Over the past five years, PGAs have largely moved from ceramic substrates to BT resin. This trend will continue in the coming years, but most of the movement has already occurred. Substrates for full-sized BGAs are overwhelmingly made from BT resin. Flex tape and FR-4 will only make small inroads into this market in the future.

Substrate Forecast

The table summarizes the substrate unit forecast, showing that the overall market for array substrates is dominated by BT resin. The demand for flex CSPs, however, will enable unit demand for flex tape to grow rapidly.