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

By David Francis and Linda Jardine, Contributing Editors Patent Number: 5,976,912 Assignee: Hitachi Chemical Co. Inventors: Fukutomi et al. Title: Fabrication Process of Semiconductor Package and Semiconductor Package

The patent in this discussion is not strictly a CSP patent. However, the method of fabrication is of interest and the resulting package size is only slightly larger than a pure CSP.

The process described in this patent is designed to use sheets, strips or a continuous tape. The patent is 39 pages in length with several variations of the basic process.

Figure 1 shows the basic process used in fabricating the package interconnect. A sheet or strip of copper is used as the base layer, and a nickel layer is plated on top of the copper to act as an etch stop when the bottom layer is removed. Figure 1. Basic lead fabrication process.

Photoresist Applied

A dry-film photoresist is applied over the nickel and patterned to create the package leads. Layers of copper, nickel and gold are then plated up to become the leads of the final package. The gold is used for wire bonding. (The vertical dimensions are exaggerated in the figures to show the various layers.)

The leaded package is assembled as shown in Figure 2. The die is attached using a silver-epoxy adhesive and then wire bonded. The package is transfer- molded to complete the assembly. Figure 2. IC attach and encapsulation.

After sealing, the base copper layer is removed using a suitable alkaline etch. The nickel layer acts as an etch stop. The nickel is then removed using a nickel stripper that has low copper-dissolving power.

In one version, the base copper layer can be replaced by a stainless steel foil, but mechanical means must be used to remove this layer.

Solder Mask

A solder mask is applied to the bottom side of the molded package to cover the leads, except for open areas that will be used to attach solder balls. The completed package assembly is shown in Figure 3. Figure 3. Base layer removal and bumping.

This same approach can be used if the die is bumped. In the example given, the die has gold bumps and is joined to the leadframe using thermocompression bonding.

Process Variation

Figure 4 illustrates one variation of the basic process. Instead of plating nickel over the entire surface of the base copper layer, the dry film resist is applied directly to the copper.

The first nickel layer is then selectively plated followed by the normal copper, nickel and gold layers described previously. This eliminates the need to deposit a nickel strip once the base layer copper is removed. Figure 4. First Ni layer is placed in lead area only.

In another variation, the plating sequence is reversed. In effect, the package is built in the reverse order with the last plated surface being where the solder balls are attached. The dry film resist is applied to the copper base and then patterned. The plating sequence is gold, nickel and copper. After plating, the resist is removed.

Mask Materials

Next, a solder mask is applied. Various masking materials can be employed, ranging from a liquid sealing resin to a polyimide film.

One material can be applied by screen printing, the other by lamination. With film, it is necessary to laser- form the holes for the solder balls.

In one variation, the polyimide film is pre-punched with holes corresponding to the solder ball location. This film is then laminated to the top surface of the leads (the bottom of the package).

The copper base layer is removed by etching, and the interconnect structureis inverted for assembly. The die is attached as described above.

Continuous Film

This approach lends itself to making the parts using a continuous polyimide film.

Other variations involve the use of a two-layer copper polyimide film to provide a continuous strip process.

Due to the small package size, it is possible to mold the entire assembly as a single block. After the copper foil is etched away, the block can be separated into individual packages using a diamond cutter.