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How does one determine the via size for "slow" signals?

gyuunyuu1989 , 04-11-2023, 04:32 PM
Via consists of a pad along with a hole in the center. The hole size subtracted from the pad size gives the anular ring. Vias have a whole lot of types and science behind them which matters for high speed signals so we come across terms like through hole via, buried via, blind via and then there are micro vias as well.

For signals that are "high speed" signals, the via size and type is a function of the distance from the ground and required characteristic impedance.

However, for other tracks that carry signals that do not fall into the "high speed" category, how does one determine via pad and hole size? How important is it to have one or more parallel via along signal via that are connected to GND?

Now, coming to the power supply. If a power rail needs to be connected to a different layer on the board, the only way to do that is to use one or more vias. How would one determine the via pad and holes size for this case? While signals could be categorized as high speed or low speed, I believe (please correct me if I am wrong) that when it comes to the power supply, we must always take steps to ensure power supply integrity. Therefore, the question of via size, count, placement of via and the GND vias becomes more important topic than siganl vias for not "high speed" nets.
Paul van Avesaath , 04-12-2023, 02:40 AM
in general, via sizes are plenty and mostly depended on the ratio drill dept / annular ring etc , but there are a few preffered sizes.. i suggest asking your supplier or looking into the IPC norm. ​

if you want to know how many you need to pass along a certain current from one plane to another. i suggest installing Saturn PCB toolkit.

is shows you all you want to know about the via.. just put in sizes you use and presto, you can see how much current the via can take.. but also put in more than needed if possible..

hope this helps..
robertferanec , 04-18-2023, 07:59 AM
For normal PCBs you may want to use VIAs which are smallest and cheapest your PCB manufacturer can manufacture. Standard smallest VIA is usually 18mil (0.45mm) pad / 8mil (0.2mm) hole, standard smallest via for cheap PCBs is like 24mil (0.6mm) / 12mil (0.3mm). Why? VIAs can take a lot of space, that's why smallest. And you don't want to go into a very small hole as that can become expensive (e.g. 0.15mm drill bit will drill fewer holes than 0.3mm drill and then it has to be replaced) and also via aspect ratio may become a problem.

For powers, you can use any bigger VIAs, often I just use the same VIA as for signals I just place more of them (based on Saturn PCB via current calculation)

Paul van Avesaath , 04-18-2023, 08:08 AM
totally agree.. my default via is 18/8 for everything.
gyuunyuu1989 , 04-27-2023, 06:27 AM
By the way, won't having several smaller decoupling capacitors instead of one large one, also lead to less inductance and thus less impedance in the PDN?
robertferanec , 05-01-2023, 01:59 AM
yes, but just adding more capacitors is not always the best solution. there are many factors to consider e.g. price, space, capacitor characteristics, is it really needed, do we really need lower PDN, ....

PS: Please, when talking about different topic, create a new topic. Thank you.
qdrives , 05-01-2023, 12:45 PM
Placing multiple capacitors also often means longer tracks. That increases the inductance.
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