Help with Minimum Blind Via Ratios

[AdLav]

I'm somewhat confused regarding what the criteria to pass a DRC check is with regards to the Min. Blind Via Ratio. Referencing several of the responses to this post from November, I was under the impression that this flag only appeared if the width of the drilled hole/the depth of the hole was greater than the MBV Ratio. My design has 5.03938mil of dialectric material between layers 1 and 2 and a minimum drill size of 11.81102mil. The ratio between those works out to 0.4267 and the listed ratio is at the default of 0.5 which, given the comment from a developer on that thread, which I will paste below, should be fine.

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1. The Via Depth is calculated by adding up all dielectric thicknesses between the via start and stop layers (the copper thickness is not added);
2. The Blind Via Ratio is calculated as the ratio between the Via Depth calculated above and the current Via Drill size and then is compared against the Min Blind Via Ratio from design rules, like: Via Depth / Via Drill Size > Min Blind Via Ratio if this inequality is true then a DRC Blind Via Ratio Violation is created. The DRC Blind Via Ratio Violation is only reported if there is no micro via drill size violation.

"

I'm still quite new to PCB design and Fusion 360, so I may be misunderstanding something here. I would appreciate any insight that could be offered in this, as I'm quite confused as to what's happening.

[jorge_garcia]

Hi @AdLav,

If you are new to PCB design and having to deal with blind vias then I feel for you. You have the documentation so I'm guessing that the concern is why is this needed?

Blind vias (especially micro vias) require very thin drill bits to make the holes. The deeper that drill has to go into the board the higher the chance of the drill bit breaking do to vibration, misalignment, etc. So manufacturers will specify a preferred ratio of depth to diameter.

If you look at our default setup of 0.5 you could interpret it like this. 0.5 = t/d -> d*0.5 = t

So you could say that for a given drill bit diameter the deepest you can drill is 1/2 (0.5) of that value. In your case you are only drilling to 0.4267 of the drill bit diameter which is less than 0.5 so you are good.

That's how this check works. Let me know if there's anything else I can do for you.

Best Regards, 

[AdLav]

Hi, Jorge. Thanks for the reply. I'm glad to know that I'm doing the calculation right, but that only makes me more confused as to why the DRC is giving me an error. Are there any other factors that could cause this error? I can check my settings again tomorrow to give more details if needed.

[constantin.popescuXD3CL]

Hi w0436473,

I have checked the Micro / Blind via ration violation generation and the calculation is correct. For micro-vias we calculate the via depth using only the dielectric layers between the via start and stop layers. Can you please post a picture showing these errors in the DRC Errors dialog? Also if you could send me your design then I would have as look and see what the issue might be in this case. My email address is: [email protected]

Please let me know if there is anythign else I can help you with.

Kind Regards,

[AdLav]

Thank you very much for your offer, Constantin, but I've discovered that the problem was a misclick on my part followed by insufficient caffeine to notice the issue. I had accidentally set the via for 1-15 instead of 1-2, making the via distance too long for a blind via. The error message I received only gave the issue as being the Blind Via Ratio, so I failed to recognise the issue being that I selected the wrong layers.

Thank you both for your time and assistance.