Optimizing VIA Drill Pairs for Multilayer PCB

Design and Layout Design for Manufacturing
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In the context of a 6-layer PCB with the layer sequence specified as:

1 - Signal

2 - GND Plane

3 - Signal

4 - Signal

5 - VCC Plane

6 - Signal

  • When planning drill pairs for VIAs, what factors should be considered to ensure optimal performance and manufacturability?
  • Is it feasible to establish VIAs that traverse from the 1st layer to the 3rd layer and from the 1st to the 4th layer without encountering any issues? If not, what is the recommended approach for interconnecting these layers effectively?
  • Can anyone offer practical recommendations for drill pairs in multilayer PCBs that strike a balance between manufacturability, minimal layer traversal, and cost-effectiveness?
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A lot of this depends on density, Via diameter and distance between layers but here are some options:

• Single lamination with a controlled depth drill from 1 to 3 and 1 to 4. Easy to do and it’s a blind via so doesn’t affect layers 5&6 and can save space. Limitation is the aspect ratio can only be around 1:1 to ensure good plating.

• Single lamination with plated through holes then back drill to break connection on layers 4,5 & 6 and layers 5 & 6. Easy to do and aspect ratio isn’t an issue but through holes take up more space.

• Next options are Sub-assembly/Sequential lamination which gets into blind and buried vias. Can be complicated and expensive but great if needed for high-density.

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Rachel … I apologize if what I say is stuff you already know, but I think steve.carney might be assuming more background knowledge than I have, and some readers will have even less than I do. Hopefully, he’ll correct anything I get wrong, including on pricing. [Yes I’m being “wrong on the internet” in hopes of drawing out a better answer.]

If cost-effectiveness is important, I think the first choice is just a plated through-hole, with antipads (etched-away copper at least as large as an annular ring would be) on any layers that should not connect.

It does traverse (and use space) on all 6 layers, and there will be some stubs where signals intended for layer 3 can go on to layer 6 and bounce back. But this is the most basic drill hole you can get, and anything else goes up to at least the “let a salesman contact you” level.

If the stubs are a problem, you can backdrill, for extra cost. I think this is reasonably standard, but it does seem to move into “let a salesman call you back” territory, so I’m guessing the job becomes about 20%-30% more expensive. (Anyone willing to correct me, please do.)

It sounds like you might also be able to use controlled depth drilling. This sounds a lot like backdrilling. But for reasons I don’t understand, this is the first I’ve heard of being able to use controlled depth drilling to create a blind via, so there might be additional complications that make it more expensive in practice.

Also note that since you would only be able to plate the hole from one side, you might also need more space on the top level for larger holes to get the right aspect ratio. Freeing up space on layers 4-6 at the cost of using more space on layers 1-3 is likely a bad trade.

If you do need to route over that space on the 6th layer, or even the 4th for a 1-3 drill, then I guess you do need blind vias. Hopefully, you could do that with controlled depth drilling rather than micro-vias or sub-assembly/sequential lamination. And if you need microvias anyhow for other reasons, it could make sense.

But if leaving an intact power plane is the reason you’re going even to laser drilling, let alone to multiple lamination, then I think you’ve discarded the cost factor. This is well into “contact us for a quote” level, but I got the impression it added a digit to the price. It isn’t clear how much of that is for the drilling itself, vs the tighter tolerances and registration that usually come with it.

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Hi Jim, you’re not totally wrong but there’s more to it. We’ve used the methods I described for years to solve the problem outlined in the question. For us they are best practice but there can be other solutions especially if you don’t have our resources. There are even special drills designed for this application.

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To add a bit more to this…

• If a through hole is used it’s easy to get rid of the internal connections but layers 1 & 6 still need to be connected for the hole to plate up so breaking the connection on layer 6 after plating is the issue. The only option besides back drill is to etch a clearance ring around the vias. This is a dry film/imaging function which takes up about 3X more space than back drill. Also takes a CAM operator more time to make sure all the image data is correct but the biggest problem is the rings are small, isolated resist features which have a high risk of falling off and creating shorts. Reworking a panel full of shorts definitely puts a dent in cost effectiveness especially if it fails and the panel is scrapped.

• Mechanical controlled depth drilling to form blind vias is probably the best solution of this type of design if the via diameters and depth meet aspect ratio requirements. This process has been around for years and all the drill bit manufactures make several types of tools specifically designed for this application. They even have conical drills to replicate the shape of a laser via. See a lot of blind via pictures that look like laser but were actually done mechanically. Not a total replacement for laser or a good option for 6mil diameter and below but you can go as big as you want. You can also drill, metalize and plate blind vias along with through holes at the same time to minimize process steps.

• There seems to be some misconceptions regarding controlled depth/back-drill so here is a basic overview. They are essentially the same thing requiring control over the X,Y and Z axis on the drill machine.

• On older machines, X&Y position starts at a Table Home/Zero point so registration is dependent on how the panel is pinned to the table. The machine knows where the table is but could care less about the panel location. This makes registration difficult especially in the case of back-drill where you have to hit a previously drilled hole. Z axis travel is just a set number and drill depth is established through trial and error. There is no compensation for drill length or thickness variation in the panel, back-up or entry materials so depth control usually exceeds +/-.002”. Setup is all manual and can easily tie up a machine for an hour. All things considered a nasty and expensive process.

• Our new machines control X & Y using a camera/vision system that targets off of four holes drilled with an X-ray drill. The X-ray locates and drills to internal fiducials which follow material movement. The Drill machine now uses the camera system to automatically align to the internal registration and could care less where the table is. A back-drill program can align to the same targets the first drill program used so registration is quite good. Every panel is aligned and scaled individually. The Z axis is controlled by an electronic feedback system that reads conductivity between the top of the panel and the tip of the drill bit. The machine knows the exact contact point for each hole and controlling depth is just a matter of telling the machine which drill you want to use and how deep you want it to go. Depth control is excellent and is usually less than +/-.0005”.The machine can also run an entire program regardless of how many tools are in it and just use the ones selected for controlled depth. We still verify the depth with a cross section but the process is now automatic, simple and cost effective.

• We typically run 2 to 3 and as many as 5 controlled depth jobs a day and have the front end/engineering process automated. Used to take hours to get one of these jobs to the floor but now it’s only a matter of selecting the desired options and all the programs including the depth verification coupons are automatically generated. Running a job right now with 32 controlled depth programs so this is a pretty big deal.

• Not all shops have these resources but for us this has become a pretty simple and cost effective process. I don’t get into pricing but we always try to keep cost down so suggest people send us their stack-ups and see what we come back with.

Hope that helps.

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That’s quite an answer! Thanks, Steve!

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A Drill Pair definition in Altium Designer describes a drilled hole’s span between board layers, defined by a Start Layer and a Stop layer. They are used to define the drill hole layer span for Vias such as the default type (top to bottom layer), Blind Vias (a surface layer to an internal copper layer) and Buried Vias (one internal layer to another internal layer).

The application and management of Drill Pairs has been enhanced with the implementation of defined layer combinations based on the board’s layer stack, replacing the approach of nominating individual drill start and end layers when configuring Vias. Access has also been improved to the Drill-Pair Manager, which is the central location for defining Drill Pairs for the current design.

I had “Drill Pairs” as an option for an output on my PCB software. I have never used it, nor has anyone I know ever even mentioned it. Sine the holes are all controlled by drills and drill fie callouts never understood the need or reason. So above is Altium comment.

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@allank What do you mean about the holes being controlled by drill file callouts? Just that you create a separate drill file for each layer pair, and this doesn’t seem like something you need to track either with individual layers or with via types? Or something more that I haven’t thought of…

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Thank you @steve.carney – that was exactly the sort of information I was looking for. I hadn’t thought about the annular rings breaking off, or the drill bit length/height being an unknown that you could now account for.

Are the X-ray internal fiducials something the designer can/should supply, or do you just figure you can always add a fiducial in the tooling strip?

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All the drill diameters in a program are assigned a number. When setting up for controlled depth it’s just a matter of going into the program, selecting the drill you want to control, adding a command so the machine knows it’s going to control the depth and defining how deep you want to go. Looks something like this: T6 .020 G92 .015. The machine will now use tool #6 to drill .015” deep.

In regards to the internal target fiducials, our scripting automatically generates all the targets that are needed for each layer so not something a designer needs to be concerned about.

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I was referring to the designers options. In Altium, you click “fabrication outputs”, then “create drill drawing” No selection or discussion or definition of drill pairs. Not mentioned, but when the drill drawing is created, on the table the last column happens to read “layer drill pairs” Never paid any attention to it, never needed to. I assumed it was just something some fab houses might use because I’d never been asked about it either. So my answer was in the designer lane but not in the fabricator lane.
After all, there’s many things the fabricator does the designer may be unaware; nor even care about if it produces a good board. Best example is calling out a ten mil hole for some location. Well the uninitiated would logically assume they drill a ten mil hole. Never. They drill maybe a twelve mil hole, the plate it to achieve ten mils “finished hole size”. Hope this helps explain it a little.

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