As the transmission frequency of PCB continues to move toward more than 100 GHz, copper interconnects are now reaching the performance threshold as the mainstream PCB interconnect technology. Ultimately, dielectric loss, copper layer roughness, and data transmission capacity can hinder its development. However, the factor that has the greatest impact on PCB interconnect performance is the volume of the conductor. On the other hand, the performance of the metal waveguide is better than that of the conventional transmission line, but it is bulky, costly, and not planar.
The Carrying Capacity Is limited
This is mainly due to the effect of the width of the wiring - usually the width of the wiring is between 3mil and 7mil. That is to say, the signal carrying circumference of the strip line is 6 mil~14 mil, and the signal carrying circumference of the microstrip transmission line is half of this value, and the side wall and current crowding are not included. Due to the skin effect, regardless of the thickness of the copper layer, current crowding reduces the effective current capacity by limiting the flow of current to the outer surface.
The Dielectric Loss of Substrate Material Is Large
Standard high-speed material loss is too large, and this problem can be solved with similar ultra-low loss media. Although at present, the cost is too high compared with the existing ordinary insulating materials, when the PCB manufacturers must accept them, the cost of the PCB production materials is likely to be lowered.
The Copper Surface Is Too Rough, Causing an Increase in Resistance Lose
At high frequencies, the current must cross the entire surface profile, adding an extra transmission distance, and the effective resistance of the copper will increase. This can be alleviated with smooth copper. However, the smooth copper foil needs to be roughened in the second stage to prevent delamination.
Signal Data Transmission Capacity is Limited by Diffusion Loss
When the clock frequency is higher than 1 GHz, practical effects (such as frequency-dependent losses) have an effect. They are associated with faster rise times and longer wiring lengths, such as multiple gigabit serial lines. This frequency correlation causes the rise time to decay and the bandwidth at the upper end of the signal to decrease, thereby reducing the channel through which data is transmitted. But substrate-integrated waveguides can be used to increase bandwidth, but switching from well-known microstrip transmission lines or CPWs to SIWs is a challenge.
