Why do many vias?

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Chuck
 
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Why do many vias?

Post by Chuck »

Can someone explain why there are so many vias everywhere?

I'm asking because I can mill my own doublesided boards, but I have to solder a wire in place of a via. I don't have any through hole plating equipment. I'm just curious why there are so many on the same solid plane.

thanks

magician13134
 
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Re: Why do many vias?

Post by magician13134 »

When they're on the solid planes and don't seem to attach to anything, that's for more effective heat dissipation. Those are not necessarily required, especially if you're making it at home

cmolson
 
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Re: Why do many vias?

Post by cmolson »

I believe there are so many vias because of the RF nature of the board... I may be talking out of my a$$ but from a lot of boards that I have seen that deal with high frequencies make sure there is a very good ground plane., Also I saw it being used on the Battery board I believe for a heat sink for the regulator...

Bandit
 
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Re: Why do many vias?

Post by Bandit »

At microwave frequencies vias are uaually places all over the place and along ground edges at a quarter of a wavelength at the highest frequency of operation. This is to prevent parallel plate or substrate modes (resonances) from occuring. However, 900 Mhz and 1800MHz vias are still used, the main reason is to reduce the parasitic capacitances that could exist between groundplanes. As you go up in frequency these parasitics have a increasing tendency to interfere with circuit operation. This is especially important with amplifiers and oscillators.

Chuck
 
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Re: Why do many vias?

Post by Chuck »

Bandit,
Thanks thats totally makes sense.
Appreciate the response.

Bandit
 
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Re: Why do many vias?

Post by Bandit »

No problem, have fun! :twisted:

magician13134
 
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Re: Why do many vias?

Post by magician13134 »

Bandit wrote:At microwave frequencies vias are uaually places all over the place and along ground edges at a quarter of a wavelength at the highest frequency of operation. This is to prevent parallel plate or substrate modes (resonances) from occuring. However, 900 Mhz and 1800MHz vias are still used, the main reason is to reduce the parasitic capacitances that could exist between groundplanes. As you go up in frequency these parasitics have a increasing tendency to interfere with circuit operation. This is especially important with amplifiers and oscillators.
Wow, that's a lot more complicated than I was thinking, thanks for correcting me. How does this prevent resonance? Just by letting the waves pass through?

Bandit
 
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Re: Why do many vias?

Post by Bandit »

Often vias can be used to get rid of heat but the oscillators used here shouldnt be getting hot, or even warm (unless something is very wrong! :) ) Using vias to get rid of heat tends to be used when dealing with power amplifiers.

Resonant modes can be set up in a substrate only when the frequency of a signal is of the order of half a wavelength, similer to a parallel plate waveguide. By having the vias, they act as short circuits so that no electric field can exist in that space. It is not going to be a problem here we are only operating at 1.9 GHz.

For example, for 1.5mm thick FR4 with relative permitivity =4.5. If half a wavelength is 1.5mm in free space, in the substrate it is 1.5 x 4.5 = 6.75mm, so a full wavelength is 13.5mm. This corresponds to a frequency of around 22 GHz, It is just a rule of thumb. In any case you would not want to be using FR4 PCB material at much above 2 GHz anyway because it becomes too lossy.

Chuck
 
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Re: Why do many vias?

Post by Chuck »

Bandit wrote: In any case you would not want to be using FR4 PCB material at much above 2 GHz anyway because it becomes too lossy.
Well, while we're on the subject. What would you use?

Bandit
 
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Re: Why do many vias?

Post by Bandit »

For circuits above 2 GHz I'd use Rogers RO4003 or one of the BANNED TLX or TLY series. Theres also RT/Duroid. If you google those names you will find lots of info from the manufactures. There are lots of others but I'd recommend just looking at the datasheets. You can quite often get free samples if you ask nicely.

Chuck
 
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Re: Why do many vias?

Post by Chuck »

Googling away.
Thanks

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