Creating a large cavity using blind via

I would like to place a large (20 mm) round pad on an inner layer of a 4-layer PCB, and then open that pad to the outside by removing layer 1 over the pad. I don’t want to plate the sides of the hole. There doesn’t seem to be way to restrict edge cute to a single layer, so I am trying to do this using a blind via. I think I see how that works, asking fo0r a blind via between layers 1 and 2. Then I just make the via big. I expect a disk-shaped depression 1 layer think with the copper pad from layer 2 showing. (I have made each layer 1 mm thick to work this out). The preview shows a circle of no copper, and without the top layer of FR4 removed. Has anyone attempted this? Maybe its just that I don’t really understand how to drive the blind via process.

It does not work that way. Things like blind via’s have very specific production processes, and those processes are highly standardized (although different PCB manufacturers may have different standards).

I don’t know the exact technical term, I tried to search a bit with “PCB pocket milling” or “PCB depth milling”, but I got a lot of false positives for hobby level CNC machines.

But the general procedure is you first contact your PCB manufacturer of how they want this non standard information defined. They will likely tell you to make a drawing on one of the spare (In KiCad “User”) layers, and then add a note for the depth this layer has to be milled.

But most often, it’s better to search for some other solution. It is for example often possible to make the whole PCB thinner. Maybe you can make a sandwich of two thin PCB’s and use THT parts to connect them together. Also keep an eye of the costs when you want something non-standard. Whenever your PCB deviates from something that can be easily manufactured with standard methods, prices rise rapidly.

That’s not surprising, cuts are made using a milling tool so have to be through the entire thickness.

OP: You might want to watch a PCB manufacturing workflow to see how a board is formed. Then you’ll have a better idea of what to ask the manufacturer for provided it’s feasible and you accept the substantial extra cost. I’d post a link to a video but all of them seem to be made by fabs and I don’t want to favour any particular one.

Blind vias are sometimes made by cutting through the outer copper and one dielectric layer with a laser. This doesn’t scale to big apertures like the OP wants.

@pboisseau

And IF the manufacturer is willing/able to make this.
Generally, the thickest layer is between 2 & 3. They are manufactured (etched) first. 1& 4 (usually much thinner layers) are later glued to 2 & 3 before etching.
You may have difficulty finding a manufacturer.

Is the depression really needed?
What is its purpose?

From what I have read in the literature as well as manufacturers websites, what you describe (cutting with laser thorugh one dielectric layer) is a microvia.
A blind via is a conventional via (drilled) that does not go all the way through the board.

A buried via eg 2-3 on a 4 layer board is drilled before the outer prepreg and copper is added. A via 1-2 or 3-4 on a 4 layer is also known as a microvia and is laser cut. Then it is a matter of what do you mean by a blind via.

Here is one explanation of the terminology, with pictures:

Don’t forget the OPs request folks:

A 20mm Dia. non-plated blind via 1mm deep.

Hmm, 2016 lithium cell cavity? :thinking:

No idea.
I asked 13 hrs. ago. No answer.
Another half story.

That might be a difficult one.
Usually the outer layers on a 4-layer board are separated by a very thin prepreg dielectric layer, around 0,1 - 0,2mm or so.
If he needs 1mm between the outer layers, it will be a non-standard process. Probably possible in volume from some manufacturer if really necessary, but might be at a premium.

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Yes, I could read people were getting off track, that’s why I repeated the OPs request.

I’d change your comment to:

:laughing: :laughing: :laughing:

Amusing myself…

Unlikely to find someone to make it but, if really wanting a Pocket buried in a PCB;
General Steps…
Load the Kicad PCB into StepUp
Create a Solid from it
Compound it
Draw the Pocket shape
Create a Pocket
Set Transparency for better visualization

Last screenshot basically shows step sequence…

We have done something like this in one of our semi-professional (Formula Student) motorsports projects some years ago together with with a local pcb manufacturerer here in Austria. The idea was to weld the tabs of lithium polymer pouch cells directly to massive copper inlays embedded inside high current PCBs. Therefore we had multilayer PCBs for the BMS / intercell connections where the the high current bus bars (for >200A) were embedded inside. To gain access to the high current busbars, all low current layers were milled down in some specific areas.

What i can tell you from my experiance: this is not for cheap hobbyists projects and typical manufacturers like JLCPCB or PCBWay won’t do this. There is a lot of communication between you and your manufacturer required and also this is not directly related to a “how to do in Software XY” question. Instead you need direct contact to your manufacturer’s engineers and they will either tell you how to engineer your projects according their demands, or instead, they most likely will do it for you.

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