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Blind Vias, Buried Vias, and Micro-Vias
There are four types of vias:
Through-hole via: These go completely through the board. They are your standard vias.
Blind via: One side of the via is exposed on the top or bottom side of the PCB but doesn’t go all the way through.
Buried via: Makes connection between inner layers and is not accessible on either side of the board.
Micro-via: A micro-via is a really small via that is cut with a laser. They have special requirements known as aspect ratio that limits them from cutting through thick material such as PCB core material. Typically microvias go through prepreg material only and then the prepreg gets laminated to a core material.
| Via Type | Via Diameter (max) | Via Diameter (min) | Via Pad | Annual Rings | Aspect Ratio |
|---|---|---|---|---|---|
| Blind via (mechanical) | 0.4mm | 150μm | 450μm | 127μm | 1:1 |
| Blind via (laser) | 0.1mm | 100μm | 254μm | 150μm | 1:1 |
| Buried via (mechanical) | 0.4mm | 100μm | 300μm | 150μm | 1:10 |
| Buried via (laser) | 0.4mm | 100μm | 225μm | 150μm | 1:12 |
As a PCB manufacturer, designers with blind and buried designs will send us several NC drill files, one for each layer pair.
Some things to keep in mind:
Stacked micro-vias is when you place two micro-vias directly on top of each other. This is a common thing to want to do when you have to get through several layers in one shot but this isn’t something PCB manufacturers are able to do reliably. The reason has to do with the structural integrity of the joints. If you must have stacked micro-vias then you may have to pay for conductive via fill called copper-filled microvia or find a PCB fabrication company that can do ‘any-layer’ PCB fabrication.
Staggered micro-via layout is the most economical option for high-density PCB layouts. The difference between stacked micro-via and staggered is simply that no two adjacent layers have vias on top of each other. Keep this in mind when doing a BGA or high density PCB layout.
Blind and buried vias design limitations
This rule is a concern for PCBs with 6 layers or more. UL specifies that three thermal press cycles is the maximum for PCB fabrication. This means you may not design a PCB such that it would take more than 3 lamination steps to assemble them. For example, the 10-layer board requires 2 lamination steps.
The first step is to laminate layers 3-8 and then drill and plate the holes.
The second step is to laser cut the blind holes through the prepreg material and laminate all the layers together
Drilling and plating the through hole via isn’t a lamination step.