RE: [SI-LIST] : micro BGA SI vrs PCB consideration

Dave Hoover (dhoover@mccurdy.com)
Thu, 14 Oct 1999 08:21:32 -0700

You can use that stack-up from a PCB fab standpoint.
The microvias can go to layer 2 (or layer 3 or 4).
The real issue is the following:
1) The microvia needs to be <=.7:1 Aspect Ratio.
This is to guarantee the plated hole quality. (+/- 3 sigma)
2) The depth of the microvia needs to be evaluated from an
assembly approach. For example, for via-in-pad will
the microvia create a huge bubble during reflow? If so
does the solder void violate the 20% max rule?

I agree that for CSP (<.8mm pitch grid array packages) that
microvia is the best approach. It allows more rout channels
for signals.

You can have signals on the outerlayers also. Via-in-pad
provides more room for that. You can even have a plane
on layer 2 with a signal on 3 to have the plane act as an
EMI shield. (Get noisy clocks under a plane) like...

sig (c/s)
pln
sig
...

There are MANY reasons I've seen for MicroVias. Here's just
a few.
1) Fine Pitch BGA. (Like CSP, FPBGA, DSP. Pitch's less that 1.0mm)
2) Via-in-pad. (To free up real estate under the BGA's so termination
resistors and caps can be mounted as close as possible to the device)
3) Dropping a noisy clock/signal below a plane (to lyr 3) for EMI/EMC
reasons.
4) Providing distributed plane capacitance right at the solder ball
(no lead inductance which can degrade electrical performance on
high speed devices)
5) Separating Logic types on the PCB on one side only with something
else on the other. With microvias you could leave the planes intact
with no clearances or "swiss cheese" effect.
(i.e., Analog, Digital, RF, Control, or Microwave)
6) Connecting directly to planes for heat dissipation (or pwr)
without "swiss cheesing" the plane(s).

That's just a few. It looks like when the PCB (or substrate) get's
greater than 130 Holes per square inch, then microvias (or
buried/blind vias) are necessary.

Common PCB types using microvia are:

Portable Consumer Products
like GPS, PDA, camcorders, PCS, and Cellular Phones.

Interposer/Adapter Boards
like BGA to CSP, QFP to BGA, CSP to BGA (The skys the limit here)

Organic Chip Carrier Packages (FlipChip, PBGA, MCM-L)
like CSP or microBGA

Wireless Products
like Wireless Base stations

Memory Modules
like SIMM

Computer Networking Cards
like PCI, Compact PCI, Mother Boards

What have I forgot?

Dave
-----Original Message-----
From: Ilan Adar [mailto:ilan.adar@seabridge.co.il]
Sent: Thursday, October 14, 1999 5:02 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : micro BGA SI vrs PCB consideration

hallo

We run into some problems when using micro BGA.

We used PTH vias between the micro BGA pads and this leads us to
low yield in the manufacturing .

the PCB people tell us that we must use micro via technology, but this
requires us
to change the PCB stack to :
CS
SIG
SIG
GND
..
..
..
..

can I use such a stackup ? or is there a mother solution to micro BGA PCB
layout.

thanks very much

Ilan Adar
Ilan.adar@seabridge.co.il <mailto:Ilan.adar@seabridge.co.il>
tel 972-9-7751239
Fax 972-9-7751212

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