Re: gEDA-user: Blind and buried vias?

[Bill Gatliff <bgat@xxxxxxxxxxxxxxx>]

Gabriel Paubert wrote:
>> I'm asking out of plain curiosity - I hope that I never have to make a
>> board with such vias as I've heard that they add a bit of sadomasochistic
>> flavor to board bringup/debug efforts - but then I guess some boards are
>> so cramped for space that you can't avoid them...
>>     
>
> Indeed. Although I have never needed them.
>   

It isn't always that the board itself is cramped, though that can 
definitely be a reason.

The latest generation of BGA parts have so many pins on the package, 
packed so tightly together, that it isn't possible to get all the 
signals out of the chip in two layers and still have the traces large 
enough to meet specs.  A sophisticated chip in an 0.5mm-pitch BGA 
package, like the OMAP3430, is an example.  I have never seen the 
underside of one of these guys, but I bet it's pretty scary.

I haven't had to lay one of these boards out myself yet, thankfully!  
:)  I'm told that the OMAP3430's Package-on-Package configuration 
requires at least six layers to get all the signals out.  Ugh.


b.g.

-- 
Bill Gatliff
bgat@xxxxxxxxxxxxxxx



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