Author's Note: Reference slides from Dr. Werner Kuhr's presentation to CPMT/IEEE Chapter, Santa Clara California, September 9, 2009 ( ).
Two seemingly conflicting goals are impeding the progress of PCB technology as we approach 2010:
- Need for higher density, for greater connectivity to meet the demands of integrated circuits (Ics) with ever higher transistor counts. The denser ICs result in higher I-O counts that PCBs must interconnect--in the same, or less, area!
- Skin effect resistance becomes critically higher as circuit speeds move above 10 gHZ in response to equipment performance demands. Finer lines and spaces increase skin effect and total resulting resistance.
- Skin Effect, From Wikipedia, the free encyclopedia: The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor. The skin effect causes the effective resistance of the conductor to increase with the frequency of the current. Skin effect is due to eddy currents set up by the AC current.
The same printed circuit structural problem frustrates attainment of both goals and accounts for the conflict between the two. It is the process/material-related problem of surface roughness (see Karl Dietz comments below and the ZettaCore presentation.)
Karl Dietz, Development Manager at DuPont's Electronic Materials Laboratory, stated:
The situation is a little more complex in the semi-additive process where there is no copper foil on top of the dielectric film and the copper layer has to be built starting with an electroless copper seed layer of less than one micron. Traditionally, one has relied on the swell and etch chemistry to create a microroughness on the filled epoxy surface (Ra > 0.5 µm) to allow mechanical anchoring of the electroless copper and to yield reasonable copper adhesion. As very fine conductor widths on the buildup dielectric layers of flip-chip packages approach 10 µm, and are expected to become even smaller in the future, such microroughness is deemed too rough and smoother dielectric surfaces are demanded. This is not only desirable because of the previously mentioned signal integrity need at high frequencies but also to avoid high resistance shorts between fine lines due to residual palladium catalyst particles between copper tracks on the dielectric surface. It is more difficult to remove the palladium particles from recessed areas of a rough dielectric surface during a short flash etch than it is to remove them from a smoother surface.
The smooth surface of the electroless copper in contact with the dielectric film creates two adhesion problems: the issue of electroless copper adhesion to the dielectric, and the problem with dry-film adhesion to the smooth top side of the electroless copper. The latter is particularly troublesome because conventional copper roughening techniques cannot be used on the very thin seed layer. These surface preparation methods either create some roughness by microetching or by the conversion of surface copper into another chemical entity that forms a rougher surface as copper is converted. In any case, more metallic copper is lost in these processes than can be spared in the semi-additive process. So it is not surprising that suppliers of surface treatments are working on non-copper-consuming methods to yield some microroughness.
--"The Need for Smooth Surfaces in Electronics and the Troubles they Cause," Karl Dietz, CircuiTree, August 6, 2008.
ZettaCore has come up with a surface treatment for unclad PCBs that enables great progress in meeting both of the above-stated goals and completely resolves the conflict between them. It uses porphyrin, the core molecule of hemoglobin and chlorophyll. Porphyrin makes most animal and plant life possible.