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April 2002

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Subject:
From:
JaMi Smith <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 2 Apr 2002 12:40:39 -0800
Content-Type:
text/plain
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text/plain (106 lines)
Janine,

The first and probably most important question is whether the controlled
impedance is for high speed logic or is it actually an RF design.

Then the next question is whether or not you have 50 ohm traces on both
sides of the board.

Are any if your 50 ohm lines going to be stripline (internal, between
planes), or are they all microstrip (external, above a plane).

Once you know those, and can estimate the remainig needs in terms of
layers, you can probably look at what your controlled impedance stackup
is going to have to be (trace width, dialectric constant, height above
ground plane / distance between internal planes, etc.).

Somewhere in the middle of all that, you have to address the issue of
termination. What kind of termination is required and where are you
going to put it.

There is much much more to it, but this can be a starting point.

All this in a 71 mil board ... have fun.

JaMi Smith

* * *

-----Original Message-----
From: Janine Badcock [mailto:[log in to unmask]] 
Sent: Tuesday, April 02, 2002 4:28 AM
To: [log in to unmask]
Subject: [TN] BGA/Highspeed Design Technologies

We are just about to embark on our first BGA/highspeed design.

Board specification : -

Double Side Surface Mount Board.
With 225 pin 1mm pitch BGA's on each side
Datalines @100Mhz and impedance matched to 50 ohms.
Powerplanes 3V3 and DGND (digital ground)
Military specific applications
Maximum Board Thickness 1.8mm

Could anyone with any experience of the above please comment on the
following : -

1.      Layer stack (blind & buried vias?)
2.      Technology (microvias?)
3.      Via size
4.      Track size
5.      Materials

We have been advised that for high speed digital designs that we should
limit the
number of vias through ground and powerplanes.

If you have any additional information/comments about the design you
feel would be
useful for us, we will be grateful.

In anticipation

Many Thanks



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