Is it required to connect two ground surfaces using via?

Strip line

Hi ,

Is it required to connect the two ground surfaces using via for striplines. Any criteria for via placement.

Regards
GV

Re: Strip line

Yes. You need to put via. The exact calculation of via diameter and spacing is possible, but I think you may use simple empirical rule: via spacing should be about 2-to-3 via diameter for MW frequencies. This approach works well for vast majority of practical cases, but sometimes you need to do optimization for via spacing. It may be the case for mass production like sell phones and so on. I developed such a method and started to write the paper, but it is not finished yet. I hope to publish it soon. Unfortunately, I cannot give you any reference because in technical literature it is practically impossible to find good information about via optimization.

Best regards,
RF-OM

Re: Strip line

Detailed guidance is rare.

Look at your requirements (frequency and bandwidth in particular)
Consider suggestions in EMI/EMC reference materials
Space vias 1/10 to 1/20 λ in the absence of other guidance
Model with a 3D field solver if the issue is critical
Build and test brassboard model

Re: Strip line

Usage of such a rule may lead to a problem. There is well known rules that distance between via should be approximately equal to the width of the stripline. Let’s say you have about 3 Ghz frequency signals and stripline on substrate with 3.7 dielectric constant, which may be a very popular for this frequency Nelco 4000-13 material. In this case actual wavelength will be c/((sqrEr)*F)=51.95mm, roughly 52mm. When you chose the smallest 1/20 via spacing it gives about 52/20=2.6mm via pitch. 50 Ohm stripline on Nelco 4000-13 59mils thick substrate (again, very popular thickness) will require about 29mils trace width. Let consider you use via 15 mils diameter (this is the less expensive in PCB production number). In such a case you may chose two via diameter spacing of 30 mils and it should be okay and it is practically match the striplane width. But if you chose to have the via spacing to be 1/20th of wavelength or 2.6 mm, which is more than 102 mils, you will end up with via spacing about 3.5 times more than your stripline width. This is the problem. For less thick substrate this problem will be even worse. Azulikeit is right when talking about EM solver, so if have such a program you may simulate your particular situation and compare the return currents with currents between via. If latter is more than 5-to-10 % of return currents you need to decrease via pitch. Moreover, often you have unwanted higher frequency currents though transmission line and you do not want they couple with other part of the circuit. In case you probably will need the second raw of the via, but it is the different story.
Best regards,
RF-OM