Interesting concept but I second the concerns on reliability and cost addition expressed by other commentators above. We can get highspeed controlled impedance boards today with the proper selection of core and prepeg materials, without the added process costs needed for airgaps.
What about thermal concerns? The air pockets can act as islands of heat and cause unintended thermal effects.
How about the thermal expansion behaviour in the Z-direction, typically higher than the XY CTE?
Even when used in chips, one has to carefully evaluate the effects of any TSV's in the vicinity of air gaps when used in a 3D stack.
The electric field is below the threshold voltage. Generally, it is lower than that for air-gaps on chips (inter-layer dielectric on-chip), which have been shown to be successful. Also, the breakdown voltage can be improved by adding a liner to the copper (should it be needed).
The dielectric loss on boards and substrates becomes a serious problem at higher frequency, and as power becomes a more important factor. The energy per bit is significanly lower as the permittivity and loss of the dielectric is lowered. The mechanical strength of the overcoat material is quite good. Physical processes that would rupture these structures would do significant damage to any board. They travel quite well and are easy to handle and work with.
A description can be found at:
Jayachandran, J.P.; Reed, H.A.; Zhen, H.; Bidstrup Allen, S.A.; Kohl, P.A. Air-Channel Fabrication for Microelectromechanical Systems Via Sacrificial Photosensitive Polycarbonates. Journal of Microelectromechanical Systems 2003, 12 (2), pp 147–159.
You can obtain background publications on this at: http://www.chbe.gatech.edu/kohl/Publications.html
The all-copper process bring little or no new chemcials or processing into a packaging or chip fab b/c it uses essentially the same chemicals as that for boards and chips.
The signal attenuation on substrate is greatly reduced.
This sounds like a somewhat messy process. Higher bandwidth PCB are designed with 50ish ohm characteristics. The difference between modern high performance dielectrics and air don't really provide a compelling reason to sacrifice mechanical integrity. This makes sense for chips where current lithography and fabrication technology supports this and the structures are very small. However, with PCB's, it's not clear to me why this would be of benefit. Using a low K dielectric that has some mechanical integrity seems to be a much more tractable solution.
Questions that pop up in my mind: How does the vapor escape without getting trapped anywhere? Any vapor trapped will surely lead to failure. Well, how would one go about failure analysis of such boards?