I would like to make vias under ground pins of HMC204MS8G. I will use RO5880 subtrate which dielectric constant is 2.2. What diameter is appropriate for via design in this substrate?
We used a total of nine (9), 10mil vias in 10mil Rogers 4350B underneath the HMC204MS8GE. Rogers 4350B has a thermal conductivity of 0.69 W/m/°K @ 80°C. Half ounce copper was used on our evaluation boards and this was bonded to an additional 50mils of FR4 with similar copper. By contrast, Rogers 5880 Duroid has a thermal conductivity that is much lower (only 0.2 W/m/°K @ 80°C) and therefore isn't a good choice for this application in my opinion. With a nominal conversion loss of +17dB and the need for high drive levels (+15dBm) a lot of power (read heat) will need to be dissipated by this device, the substrate and heat sink if used. My recommendation would be to use Rogers 4350B or something with similar thermal conductivity.
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