Steve Berry and Sandra Winkler Contributing Editors

Wafer-level packages (WLPs), created on an uncut wafer, offer the efficiencies of packaging an entire wafer en masse. These efficiencies can also be applied to the application of underfill on WLPs. Why would we want underfill on all devices?

Today's WLPs are often placed in handheld products, which are typically items that do not have an easy life. Besides being left in hot automobiles and occasionally sat on, they are often dropped.

To combat these stresses, packages are often underfilled in hand-held products, whether the package design requires it or not.

The position on the PWB generally determines whether underfill will be applied to a package or not, as different areas of the PWB flex more than others.

Many Benefits

There are many benefits to wafer-applied underfills. Gang application of the underfill is cost effective, of course. Since the underfill is pre-applied, these parts are picked-and-placed down on the board, like any other part.

There is no need to go through the additional time-consuming placement of capillary underfill, which can leave voids behind the solder balls while it flows beneath the package.

The underfill itself protects the die during dicing, which improves yields. This is particularly important with WLPs, as they often are not tested after dicing, a process that can chip the devices.

Wafer-Applied Underfill Developers

Who is offering or developing wafer-applied underfills? Current developers or users of wafer-applied underfills include, Aguila Technologies, Ablestik Laboratories, Emerson & Cuming Specialty Polymers, Henkel Technologies, National Semiconductor and National Starch & Chemical Co.

National Semiconductor was the first to market with a wafer-applied underfill; the underfill is applied after the bumps are placed.

Underfill material is positioned so that the underfill does not completely cover the solder balls, leaving them exposed for interconnection to the PWB.

The underfill does shrink due to the heat from the reflow oven, and the solder balls also collapse. The end result, because the solder balls and the underfill material are the same height, is that both touch the PWB and the bottom of the WLP at the end of reflow.

Aguila's technology applies the underfill to an unbumped wafer, then uses a laser to drill micro-vias into the underfill, exposing the bond pads. Conductive material is filled in the holes, and bumps are formed at the ends for interconnection.

Consumer products that use semiconductors, such as this Nokia N-Gage, are typically in for rough handling. (Nokia)

Ablestik Laboratories offers a reworkable underfill suitable for B-staging on silicon bumped wafers that will become flip-chip devices or WLPs.

This thermoplastic structure can be reworked when heated. A separate wafer-applied underfill made by Ablestik and National Starch is geared for lead-free applications and is still under development.

Henkel Technologies is developing a product that stencil prints the underfill over the solder balls, covering 50 to 70 percent of the balls, which fills the gap to the PWB, once the solder balls collapse during reflow.

Emerson & Cuming's underfill can be applied both at the wafer level or on individual CSPs. It is still under development, but is being designed to be applied at a partial stand-off height. It expands during reflow to fill the gap between the silicon and the PWB.

Expansion occurs through softening and activation of a blowing agent that causes it to become a syntactic foam structure, filling the standoff gap.

Strong Growth

WLPs are showing strong growth now and for years to come. Wafer-applied underfills will also show solid growth, and innovations in this area will continue to expand with this growing market.