These types of materials would allow for a largely additive process, with minimal subtractive processing. The reason that these materials would exhibit straighter side walls, with much the same cross-sectional width from top to bottom, is that the etch process would be short enough, due to the very thin copper base, so that the etching solution would spend very little time attaching to the side walls of the plated features (FIGURE 2).
[FIGURE 2 OMITTED]
Using these materials and a process such as this, the sidewalls of these features would be approximately the same size as the image that was projected, with minimal etch factor. Such materials could possibly be utilized by current PCB manufacturers to produce geometries at the levels required without the need to spend many millions of dollars on additional--and very expensive--equipment.
With such materials, traditional lamination techniques could be utilized, along with traditional and laser drilling equipment. Imaging techniques would need to be modified to allow for the resolution of very fine lines and spaces, such as 10 microns. However, equipment manufactured today that can produce these geometries could be modified for just such a use, and purchased at a cost far less than that of current production line equipment for additive processes. Plating techniques would only need to be modified slightly, and etching costs would be minimal where small amounts of base copper would need to be removed.
In such a scenario, the circuit features produced could much more closely approach the size of the imaged feature, as the process would be largely additive. These features would exhibit little of the side wall etch associated with today's subtractive processes, due to the minimal amount of starting foil that would need to be etched off. This would allow for much better control of line widths, allowing for better predictions of product performance during design and modeling.
An additional benefit of these materials is a resistance to cathodic anodic filament (CAF) growth. This typically occurs where holes are placed very close to each other, usually 300 microns or less, and where copper migrates along the glass reinforcement fibers between these two holes, causing a short. This is typically not an issue with additive materials in that there is no glass reinforcement, and hence no pathway down which the copper can migrate. These new materials would therefore not need to possess glass reinforcement down which the copper could migrate in order to decrease geometries even further.
These materials should also possess the ability to control dielectric thickness tolerances very tightly. This would be necessary to allow designers to have confidence in the ability of manufacturers to produce product that would closely resemble the modeling done during the design phase, with minimal variance. This, again, coupled with the CAF considerations expressed above, suggests materials that have no glass reinforcement, as this introduces more tolerance into the dielectric thickness equation.
Should such materials be developed, one would assume that the manufacturers of these materials would be able to vary the dielectric thicknesses so that designers would be able to develop substrates using these different dielectric thicknesses to tune for their specific product need. If this is correct, it follows that the materials manufacturers would be able to develop these same materials in thicknesses that could address the needs of the PCB industry as well. This would mean that the PCB industry would have materials, and processes, available to them to produce cost-effective feature pitches well below those that are currently available, allowing further miniaturization of the semiconductor package.
The side benefit to this scenario would be that the semiconductor package and the PCB would be made of materials that would possess the same electrical and physical attributes, making for a very robust union between the two through the assembly process, and on into finished product life.
