My PCB is two layers, Toplayer and bottomlayer(I use the Protel 99SE).When I try to put the copper of the Ground on both layers, I have to have Some Vias for the net, but,how many Vias do I have? Does it have many Vias a better?
Who can help me?
I’m so sorry, I come from china, My English is so bad.
:?
No worries about your English. I suspect it is much better than most of our Chinese!Soncon:
My PCB is two layers, Toplayer and bottomlayer(I use the Protel 99SE).When I try to put the copper of the Ground on both layers, I have to have Some Vias for the net, but,how many Vias do I have? Does it have many Vias a better?Who can help me?
I’m so sorry, I come from china, My English is so bad.
In general, more vias are better. More vias give you lower inductance and lower resistance. For general purpose ICs, I tend to have one via/pin. But unless the chip is doing something at very high frequency, or at very high current, it won’t matter much.
By the way, I am worried about how you are connecting some pins on your IC directly to large copper pours (in the four corners of your IC). Those 4 pins will be somewhat difficult to hand solder, as they have a fairly large thermal mass on them. If you are reflowing than it shouldn’t matter much at all. Just a thought!
NleahciM:
In general, more vias are better. More vias give you lower inductance and lower resistance. For general purpose ICs, I tend to have one via/pin. But unless the chip is doing something at very high frequency, or at very high current, it won’t matter much.
Thanks for your reply!
Do you mean that higher signal frequency,better for more vias? but, how should I define the higher frequency?
where is the boundary of the low frequency to the high frequency? 30MHz? More vias means multi-grounded by parallel, it also adds more area when the current flow back.It will radiate more noise when I do some EMC experiments on the PCB. The highest signal frequency on my board is 24MHz, consider the harmonic, there will be 48MHz, 96MHz,… and so on, and I have no ideal to dealt with it.
I'm not sure why he says "more vias are better"... unless more vias helps you improve the PCB layout in some important way. Of course, you do NOT want long traces from GND or PWR planes to the GND or PWR pin on any component, so a via right next to those pins is quite appropriate, and potentially important. The same goes for bypass capacitors - optimally the two pads of a bypass capacitor touch the GND and PWR pins of the component. Sometimes that's not practical, and at 30MHz you can generally afford to have several millimeters between a component GND or PWR and the via that connects to a GND or PWR plane (especially if those are nice fat/wide traces).Soncon:
NleahciM:
In general, more vias are better. More vias give you lower inductance and lower resistance. For general purpose ICs, I tend to have one via/pin. But unless the chip is doing something at very high frequency, or at very high current, it won’t matter much.Thanks for your reply!
Do you mean that higher signal frequency,better for more vias? but, how should I define the higher frequency?
where is the boundary of the low frequency to the high frequency? 30MHz? More vias means multi-grounded by parallel, it also adds more area when the current flow back.It will radiate more noise when I do some EMC experiments on the PCB. The highest signal frequency on my board is 24MHz, consider the harmonic, there will be 48MHz, 96MHz,… and so on, and I have no ideal to dealt with it.
You should begin to worry about high-frequency signals when speeds exceed ~250MHz, and especially when they exceed about 1GHz. 30MHz is not normally considered a “high speed signal”. However, even medium speed and slower signals can have “reflections” that can cause invalid operation, but that’s generally only a problem with very long traces (longer than you are likely to find on a small to medium-size PCB).
Again, more vias gives you lower resistance, lower inductance, and lower thermal resistance. This matters for things like DC/DC switchers, but not so much for most applications. For low power applications one via is almost always enough.maxreason:
I'm not sure why he says "more vias are better"... unless more vias helps you improve the PCB layout in some important way. Of course, you do NOT want long traces from GND or PWR planes to the GND or PWR pin on any component, so a via right next to those pins is quite appropriate, and potentially important. The same goes for bypass capacitors - optimally the two pads of a bypass capacitor touch the GND and PWR pins of the component. Sometimes that's not practical, and at 30MHz you can generally afford to have several millimeters between a component GND or PWR and the via that connects to a GND or PWR plane (especially if those are nice fat/wide traces).Soncon:
NleahciM:
In general, more vias are better. More vias give you lower inductance and lower resistance. For general purpose ICs, I tend to have one via/pin. But unless the chip is doing something at very high frequency, or at very high current, it won’t matter much.Thanks for your reply!
Do you mean that higher signal frequency,better for more vias? but, how should I define the higher frequency?
where is the boundary of the low frequency to the high frequency? 30MHz? More vias means multi-grounded by parallel, it also adds more area when the current flow back.It will radiate more noise when I do some EMC experiments on the PCB. The highest signal frequency on my board is 24MHz, consider the harmonic, there will be 48MHz, 96MHz,… and so on, and I have no ideal to dealt with it.
You should begin to worry about high-frequency signals when speeds exceed ~250MHz, and especially when they exceed about 1GHz. 30MHz is not normally considered a “high speed signal”. However, even medium speed and slower signals can have “reflections” that can cause invalid operation, but that’s generally only a problem with very long traces (longer than you are likely to find on a small to medium-size PCB).