Density of PCBs

1) SMD component  on top side except raspberry pi which is in Through Hole Technology. 

    4 to 6 layers of routing. Having all SMD components on one side will accelerate the assembly process as all components are soldered in on go.

2) Modestly miniaturised with SMD components on both sides ( half of the surface of point 1) as components are on top and bottom of the PCB.

     6 to 10 layers. Assembly time is longer as two passages in the vapor phase machine are needed.

3) HDI (High Density Interconnect) miniaturisation  with Microvia technology. Use of 0201 resistors and capacitors wherever possible

    8 to 12 layers. This would imply that we eliminate all big components such as all Headers and Test points and we would always have to use the smallest components available. (DFN QFN ) technology instead of SOIC and PLCC.  Long and expensive PCB manufacturing and very high assembly cost for prototype yet cheap in mass production. Very small like cell phones.

The time needed to do the work will triple to quintuple with each point ( for point 3 up to 25 times more time needed)