Blind and Buried Vias
Blind and buried vias are used to connect between layers of a PCB where space is at a premium. A Blind Via connects an outer layer to one or more inner layers but does not go through the entire board. A Buried Via connects two or more inner layers but does not go through to an outer layer.
- Not all combinations are possible. This is explained in this blog.
- Blind and Buried Vias add considerably to the cost of a PCB. They should only be used when absolutely necessary. To help designers of tight boards, we offer via holes down to 0.15 mm in our pooling services and down to 0.10 mm as a non-poolable option. These need minimum outer layer pad sizes of 0.45 mm and 0.40 mm respectively.
Use the “smart menu” Buildup Wizard to check what Blind and Buried Via options can be produced for your design and to work out their cost. Choose from over 700 pre-set multilayer builds, accessed by number of layers, board thickness, build and copper weight. Then add your blind and/or buried via requirements. If you can use a pre-set build you get your offer faster, the boards are easier to build and so the price is lower.
How a Multilayer PCB is Built.
Every multilayer build-up is constructed out of Cores, Pre-pregs and Copper Foils.
You can think of a core as a double-sided PCB. It is a rigid piece of base laminate with copper pre-bonded onto each side. If you go to the pre-defined build-ups for a 4 layer, you will see that the standard build has a core in the centre containing the copper tracking/planes for layers 2 (= inner layer 1) & 3 (= inner layer 2).
On each side there are one or more sheets of pre-preg and then a sheet of copper foil. Pre-preg is glass-fibre cloth pre-(im)preg(nated) with uncured resin. The sheets of pre-preg and foil are bonded onto the core using heat and pressure. This cures the resin in the pre-preg as well as bonding the whole build. The resulting “sandwich” is then drilled and plated like a 2-sided board. For more information on the process see our Movies on PCB Production.
How Blind and Buried Vias are Made.
We do not use depth-controlled laser drilling to manufacture blind and buried vias. We first drill one or more cores and plate through the holes. Then we build and press the stack. This process can be repeated several times.
1. A Via always has to cut through an even number of copper layers.
2. A Via cannot end at the top side of a core
3. A Via cannot start at the bottom side of a core
4. Blind or Buried Vias cannot start or end inside or at the end of another Blind/Buried via unless the one is completely enclosed within the other (this will add extra cost as an extra press cycle is required).
These rules are incorporated into the Buildup Wizard.
So on a standard 4-layer build, we can only drill buried vias between layers 2 & 3. If we do this, blind vias become impossible.
For Blind Vias you need to select a reversed build. Here instead of a single core at the centre between layers 2 & 3, there are two cores on the outside of the board between layers 1 & 2 and 3 & 4. Now we can drill Blind Vias between 1 & 2 and 3 & 4. Buried Vias are no longer possible as the dielectric between 2 & 3 is pre-preg which cannot be separately drilled.
Higher layer counts work in the same way, but now it is possible to combine Blind and Buried Vias.
Blind and Buried Vias can be overlapped provided that one is completely enclosed within the other (but note the extra press cycle).
A special build follows the same procedures but with different thicknesses at higher prices.
To avoid issues with Bow and Twist after the pressing/lamination process the build should be symmetrical,.
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