PCB manufacturing process, how buried and blind vias are made?

Discussion in 'General Electronics Chat' started by Dong-gyu Jang, Jan 10, 2016.

  1. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015

    I've studied PCB manufacturing process in googling and most of the articles represent PCB manufacturing process for 4 layer case.

    The article said some mechanical drill bit is used to drill the hole through the PCB as first step of making Plated Through Hole, corridor to connects all electrical layers in PCB. Making through hole via looks very clear so I have no confusion about this.

    The question arises when I read some article from PCB manufacturer website saying that they don't use depth-controlled drill bit to make via so not all combinations of layer connections with blind and buried via are possible (for ex, connection of blind via from 1st to 3rd layer is allowed while 1st to 2nd is not). The Ultiboard (PCB design software) also allows only some of layer connections with blind via.

    Then...how they can they make blind and buried via? The most of the PCB article shows only how to make through hole via, not other types of via. I need to know this process to trace back why not all layer connections are allowed.

    Thanks to read this post and please give me some comments.
  2. Picbuster

    Well-Known Member

    Dec 2, 2013
    look at www.eurocircuits.com
    they have a document with all the ins and outs.
    Interesting stuff for the pcb designers.
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    First think of a traditional two sided board, and say you need to add more traces. The easiest thing to do is to glue two PCB boards together to get an extra two layers "inside" the structure.

    That is exactly what a multi layer board is. Just a series of double sided boards stacked up and glued together.

    A via may be made on any individual board or when the boards are joined together. Vias made on an individual board will be blind as it only connects some of the layers and does not travel from one side to the other.

    Note the vias are made in an extra step from thru and thru vias, which is why such boards are more expensive.

    Blind vias made this was can only connect thru certain layers: say you have a 6 layer board and have blind vias from layer 1-4. It is now not possible to use this technique to make blind vias from 3 to 6. To do so needs another technique where you make a hole partially the way thru, typically using a laser.

    Google "pcb blind via fabrication process" and you should get some good hits.
    Dong-gyu Jang and Roderick Young like this.
  4. RichardO

    Late Member

    May 4, 2013
    A project I was involved with used a 4-layer PCB. It was not 2 2-layer boards glued together. Instead, the center was a 2-layer board and the outer layers were each single sided boards.

    When you make the PCB this way, the making of buried vias is easy. First make a 2-layer board with the buried vias plated through. Then sandwich it with the 2 outer layers and do the rest of the vias.
    Dong-gyu Jang likes this.
  5. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    Thanks to comment me!

    So, as vias are made basically by drilling through the stacked PCBs in conventional method, for example, in order to have blind via connecting 1st to 4th layer in 6 layer PCB, it can be made in the following; 1. two double-sided PCBs stack up 2.drilling through stacked PCBs and plating (of course, some clearance from this via to traces on other layers must be done before) 3. other double-side PCB is glued so this via becomes blind.

    Is this what you conventional way of making blind PCB you mentioned?
  6. SLK001

    Senior Member

    Nov 29, 2011
    This is the way multilayer PCBs are usually made, except the outer layers do not start as single sided boards. Instead, the stackup is:
    PRE-PREG (consists of thermal setting dielectric material, usually some fiberglass weave - but no copper. It's basically a layer of glue.)

    The core is drilled and etched, and if it is a non-sequenctial board, all blind vias are plated, then the 4 layer is assembled, then the board is pressed under heat to activate/set the pre-preg layers. The completed subassembly is then drilled, plated, then etched.
    Dong-gyu Jang likes this.
  7. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    Thanks to help!

    It looks conventional method has limitation of having combination of layer connections with via. You mention sequential board, could you please give me more info of what it is? Brief search give me some concept but I didn't find detailed article.