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buried bias and power planes v/s old fashion IC power

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I just send a budget for a PCB that have tons of buried bias (never done it before) and are WAY more expensive than a regular PCB, and with regular I mean draw tracks to power chips and stuffs, and not getting the power from power planes.

I did it because the specifications for RF chip recommend some 4 layer PCB as best, and impedance matching for RF tracks for certification later (CC3200MOD).

There is a way to manage the buried bias and not get plucked on the process ?, o a better alternative not so expensive ?

I have no experience on this, so any tip is welcome.


Can't you just make the vias go all the way through the PCB, instead of burying them? Or am I misunderstanding the question?

BR Jonas

Edge rates these days mean that my default starting position is 4 layers (Two layers is only for almost literally DC power and smallish audio things and even then....).

Buried via is insanely expensive, sometimes you have to, but on anything you can reasonably route in 4 layers you should be able to avoid it.

4 Layer is pretty much the minimum for acceptable performance if the board is even slightly non trivial, and should not add that much to the cost if you are doing a reasonable sized run (There is rather more setup involved).

If you want sane prices at small volumes then you must make sure the options you select match the board house defaults for a 4 layer, but for RF you will want to use someone who specifies the layer stackup so that you can calculate the track widths for the appropriate impedances.

I can and have done two layer RF boards (One routing layer plus ground plane on the back, the trouble is if you do this with a 1.6mm FR4 board, a 50 ohm trace becomes insanely huge, and even a 1mm or 0.8mm thick board does not really solve this, a buried ground plane say 0.2mm under the outside routing layer however, and all of a sudden your RF tracking is a sane size.

The ground plane is kind of non optional IMHO so a two layer stackup only has a single routing layer, where a 4 layer has two (And you can sometimes sneak some routing onto the power plane remembering that signals on the back of the board will be using the power plane as a reference so chopping it up is not a great plan, so effectively two and a bit).

Regards, Dan.


--- Quote from: JJalling on March 15, 2016, 10:38:24 am ---Can't you just make the vias go all the way through the PCB, instead of burying them? Or am I misunderstanding the question?

BR Jonas

--- End quote ---

Yes I can, but the real problem is my projects become more and more complex and I have to deal with this problem, plus I expend too much time arranging tracks. And as far I know, vias (TH) are not the best solution to get contact to mid planes.

You should first think about going 6-8 layers or HDI before buried and blind vias. If you have to use a 4 layer stackup for some magic RF reason, than I would rather use a 8 layer board and skip layer 2, making it 7 layer board, with similar stackup:
4-5-6-7 is up to you.  Maybe even sacrifice the third layer. Buried and blind via only when PCB size is defined by the marketing department (giant phones, anyone?). This will probably change in a decade or so.


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