I am a new bee to PCB designing. While reading articles I have found people talking about "covering even number of layers with blind via". I have talked to a local PCB manufacturer and they said the same thing and they said they can't do via like 1-2 and 2-3, but they can do 1-3 and 4-6 (Assuming 6 layers board). For it seems that if I connect an internal plane to an external layer (Say 1-2), then I can't connect the internal plane (2) to any other layer. Is it the case? I am really confused with this. Can someone explain in detail with possibly a picture?

  • \$\begingroup\$ I have had many PCBs fabricated with micro-vias from layer 1 to 2 and Layer n to n-1 (where n is the bottom layer). Please reference the articles where you have seen these statements. \$\endgroup\$ – Peter Smith Sep 29 '17 at 6:51
  • \$\begingroup\$ The question you need to ask yourself is why do you need it? How dense is your board going to be? \$\endgroup\$ – Sachin Sep 29 '17 at 6:59
  • \$\begingroup\$ @PeterSmith, I don't have the links right away with me, but here is what the PCB manufacturer said. Dear Sir, We are using normal drilling method to get a blind drills. So, always we can do drilling in even no of pairs like 1-2, 3-4, or 1-3, 4-6. We cannot provide blind drill in layers like 1-2 and 2-3 or 1-3 and 3-4. So, please provide us modified data accordingly. Thank you for your kind support. \$\endgroup\$ – Rakesh Mehta Sep 29 '17 at 6:59
  • \$\begingroup\$ @Sachin, this is not a question of "If I need it or not", its about clearing my confusion (for future may be?). I want to clear my confusion about limitation of doing blind vias, so that I can keep it in my mind while doing complex PCBs (In future). \$\endgroup\$ – Rakesh Mehta Sep 29 '17 at 7:00
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    \$\begingroup\$ @RakeshMehta Ask youself how the board is built. E.g. for a 6-layer board, you can either start of with one core in the center and then do a build-up with prepreg and foil, you can laminate together 3 cores or you use two cores and and do the outer layers in prepreg. If the use conventional drilling for the blind vias, they can only drill through cores or or through combinations of them. When you use laser drilling for blind vias, you can drill real blind holes and then fill them with copper. Familiarize yourself with the production process involved, so you see what's possible. \$\endgroup\$ – Manu3l0us Sep 29 '17 at 7:05

Normal vias go through the whole stack. Once the board stack has been fabricated, a hole is drilled through all layers, and plated up. This is the usual and cheapest option. Making buried vias increases the number of manufacturing stages, so increases the cost and manufacturing time.

Which combination of layers can be served by buried or blind vias depends on exactly how the board is built.

In a 6-layer board made from 3 cores, any core can be drilled and plated after etching and before assembly. This means you can have 1-2, 3-4 and 5-6 vias. You can also drill after partial assembly, so a 1-4 buried via is possible. However, if you have one of those, it's not possible to have a 3-6, as that would result in an impossible build sequence.

If your 6 layer board is built from 2 cores, with 2 outer foils on pre-preg, then you can have 2-3 and 4-5 buried vias.

With the latter construction, you can have micro-vias, which are laser drilled 1-2 or 5-6 through the pre-preg. These, as the name suggests, are much smaller than conventionally drilled vias, and are often used for routing away from dense BGA footprints.

Buried vias would only tend to be used on advanced boards, which are pushed for space or isolation. If you're new to PCB design, then it would be good to start on a few less demanding boards first.


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