Buried vias and their effect on PCB cost

[Simon]

I'm trying to get a quote from PCB cart but their form is a bit confusing to me. There is a box asking me if I have lined or buried vias and then there is a selection box asking for "times of buried via" I'm not sure what this means I can choose between one and 5 and the price increases steeply each time I add one. Are they asking how many types of buried via I have? In this case it would seem that have completely through vias is much much cheaper?

[free_electron]

if you make a 4 layer board you can maximum have 1 buried vie : layer 2 to 3
IF you make a 6 layer board you can have max 3 types : layer 2 to 3 , layer 4 to 5 and layer 2 to 5.  OR , if the stackup is different : 3 to 4 and  2 to 5. this depends ont he stacking order. first case makes two double side pcb's. one holding layer 2 and 3 , one layer 4 and 5. these get glued together , then redrilled to make 2 to 5 connection.  second case starts with a doublesided board holding layr 3 and 4 , then two single sideds are added forming 2 and 5.

Make an 8 layer board ... 3 cores and a stackd rill : 4 buried types.   use sequential : 3 types
and so on.

it depends on the layer stack

[jmole]

They may also be counting blind vias as buried for costing purposes.



on a 4 layer board, i could see blind vias as being a much more useful interconnect. not sure on the actual cost difference between the two in terms of maufacturing steps.

[Simon]

Yes they are putting burried and blind together for costing. So presumably by "times" they mean types and anything that is not a through via adds to cost. I think I'll convert to through only as my board is not that compact that I to put traces over buried vias. I thought involving less layers may have been cheaper.

[helius]

I am not an expert in this area, but I think that blind/buried vias are more expensive because the board needs to go through drilling and plating multiple times. When you use only through vias, there is only a single drill step and a single plating step, which is after the stackup is glued.

For completeness' sake, I'll add that tented vias are orthogonal to the above types. Tenting/plugging vias is a soldermask step and happens after the stackup is glued, whether the via underneath is blind or not.

Tenting types: http://www.pcbcic.com/docs/Via_and_SMask_design_guide4.pdf

[kizzap]

In case it isn't obvious: back in primary school we learnt all our times-tables. 

[IconicPCB]

Some manufacturers offer a special deal on 100% buried via only designs.

[Psi]

PCB Cart changed their website recently so it wouldn't surprise me if they are still tweaking the English so it makes sense.

[Simon]

PCB Cart changed their website recently so it wouldn't surprise me if they are still tweaking the English so it makes sense.

Yes the english does need tweaking, i noticed it's different from last time i used them, my worry is that doing 2oz boards i want to make sure I and they understand what it all means so that there are no mistakes.

[free_electron]

I am not an expert in this area, but I think that blind/buried vias are more expensive because the board needs to go through drilling and plating multiple times.

it depends. The buried via's are typically done in 1 shot. so it's not like on a 6 layer they will bury 3 to 4 and 2 to 5. That would be very expensive.
The burieds run through the entire board and go from the first inner layer to the last inner layer. or the second inner layer to the last-2 inner layer.

if you then can get away with only having laser via's you are good. PCB manufacturers like laser via's. Why you ask ? because the laser can shoot up to a 2000 holes a minute with a much larger registration accuracy than a mechanical drill can. Drilling 3000 holes mechanically takes an hour ... and if the bit breaks and tears up the hole the board is scrap ...

To give you an idea : you need a 10 mil drill in a 22 mil pad just to allow for the mechanical drill to 'wander'
Laser via ? 5 mil hole in a 6.5 mil pad ... not a problem.

So in that respect : blind and buried is not really a cost adder , in volume it is actually the opposite. This is driven by the smartphone/tablet market. those are all blind and buried boards. they laser the outer layers. you can even stack via's on top of other via's using laser technology.

another nicety is that laser via's can be grown shut so you can put them directly in component pads.