Interesting.

Here is what I am doing. I am working on a design that is very very
high speed. Putting deocupling capacitors 2 inches away from the
BGA pad I might as well throw the caps in the garbage. What we
have done in the past without issue (however have we gotten lucky with
entrapment issues?) In the past we dogbone out of the BGA and put the
caps right on top of the vias on the bottom of the board. So the 
via on top underneath the BGA is tented and the via on the bottom is clear
because it is right in the middle of a 0402 capacitor pad. This
has worked quite well but with a recent design I have been working
with fabricators and some have expressed concern about the issues
that have come up so far. The board will be Enig. I think I am just
going to switch over and do a via plug with plate over process.

1. This has become a very common and robust process yes?
2. I am also thinking about just killing the dogbones to eliminate
the thermal resistance and the inductance. I have spoken with one
vendor about using a 19 mil pad with a 8 mil hole plugged and plated over

But when you have a IC running internal PLL's at 10 Gigahertz and several
hundred IO's switching at 250 Mhz  plus 8 or 16 lanes of serdes lines
switching at 3.125 Gigahertz there is a real need to get a lot 
of capacitance on the power without a lot of inductance. 

I was looking at a print the other day from someone and I saw them call
out the via plug without the plate over.

The Del Mar show is tomorrow in San Diego so I will be able to speak with
a lot of fabricators in one place.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Stadem, Richard D.
Sent: Tuesday, May 01, 2007 10:44 AM
To: [log in to unmask]
Subject: Re: [TN] unsymetrically masked vias


Depends on which side it is. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of [log in to unmask]
Sent: Tuesday, May 01, 2007 10:58 AM
To: [log in to unmask]
Subject: Re: [TN] unsymetrically masked vias

You create a cup for entrappment of chemicals with only one side being
covered.
Kat
Sent via BlackBerry from Cingular Wireless  

-----Original Message-----
From: John Foster <[log in to unmask]>
Date: Tue, 01 May 2007 08:48:28
To:[log in to unmask]
Subject: [TN] unsymetrically masked vias

Is it problematic for a fabricator if a via

.062 board   10 mil finished via


is masked on one side and not the other?

Any input will be appreciated

Thank You
John Foster

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