How many PCB layers ?

[Andre]

How many layers are normally needed for PCBs using low cost FPGAs ??

I've just been told by a supposed board layout expert that the 256 pin
BGA version of a Cyclone EP1C6 would require an 8 layer board
(apparently having the entire underside of the device covered by balls
with no free space at the centre makes signal routing a big problem).

Is this really true ??

 

[John Adair]

If you were using Xilinx I would point you here
http://support.xilinx.com/bvdocs/appnotes/xapp157.pdf .

John Adair
Enterpoint Ltd.

This message is the personal opinion of the sender and not that necessarily
that of Enterpoint Ltd.. Readers should make their own evaluation of the
facts. No responsibility for error or inaccuracy is accepted.

"Andre" <armcc@lycos.com> wrote in message
news:ae5c06e9.0402120029.4ea5405d@posting.google.com...

How many layers are normally needed for PCBs using low cost FPGAs ??

I've just been told by a supposed board layout expert that the 256 pin
BGA version of a Cyclone EP1C6 would require an 8 layer board
(apparently having the entire underside of the device covered by balls
with no free space at the centre makes signal routing a big problem).

Is this really true ??

 

[Gregory C. Read]

At least 6 layers. And 8 isn't unreasonable.

--
Greg
readgc.invalid@hotmail.com.invalid
(Remove the '.invalid' twice to send Email)


"Andre" <armcc@lycos.com> wrote in message
news:ae5c06e9.0402120029.4ea5405d@posting.google.com...

How many layers are normally needed for PCBs using low cost FPGAs ??

I've just been told by a supposed board layout expert that the 256 pin
BGA version of a Cyclone EP1C6 would require an 8 layer board
(apparently having the entire underside of the device covered by balls
with no free space at the centre makes signal routing a big problem).

Is this really true ??

 

[Uwe Bonnes]

Andre <armcc@lycos.com> wrote:
: How many layers are normally needed for PCBs using low cost FPGAs ??

: I've just been told by a supposed board layout expert that the 256 pin
: BGA version of a Cyclone EP1C6 would require an 8 layer board
: (apparently having the entire underside of the device covered by balls
: with no free space at the centre makes signal routing a big problem).

With 0.3 mm drills, 0.6 mm vias and 0.14 mm lines spaces, the FBGA256 should
be routable with a ground plane on 4 layers...

More layers will give you supply layers while with four layers you have to
route the supplies with what is left after the signal routing.

Bye
--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------

 

[Leon Heller]

Andre wrote:

How many layers are normally needed for PCBs using low cost FPGAs ??

I've just been told by a supposed board layout expert that the 256 pin
BGA version of a Cyclone EP1C6 would require an 8 layer board
(apparently having the entire underside of the device covered by balls
with no free space at the centre makes signal routing a big problem).

Is this really true ??

A layout technique called "channel routing" can reduce the number of
layers significantly.

Leon
--
Leon Heller, G1HSM
Email: aqzf13@dsl.pipex.com
My low-cost Philips LPC210x ARM development system:
http://www.geocities.com/leon_heller/lpc2104.html

 

[Uwe Bonnes]

John Adair <newsreply@loseinspace.co.uk> wrote:
: If you were using Xilinx I would point you here
: http://support.xilinx.com/bvdocs/appnotes/xapp157.pdf .

The examples omit all decoupling capacitors in the layout...


--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------

 

[Kenneth Land]

If you can sqeak by with 240 pins I've got a custom 4 layer board running
Nios with SDRAM and USB 2.0 incredibly stable at 110-120 MHz.

Stack is Signal-GND-PWR-Signal. Made 10 with ground pours, 10 without.
Both sets function the same. I have not compared the external signals on an
analog scope, but the clocks look good, the logic analyzer looks good, and
they all work without glitch.

If you go to CompUSA and buy a few highspeed PCI cards you'll see they're
doing their magic with only 2 layers with ground pours.

I was originally told by the board expert the same 8-layer requirement on
our design. I talked him down to 6 and then went with 4 based on a lot of
reading, looking at examples, and applying common sense.

I say listen to experts, but beware the advice of people who make more money
if your design is more complex.

IMO
Ken

"Andre" <armcc@lycos.com> wrote in message
news:ae5c06e9.0402120029.4ea5405d@posting.google.com...

How many layers are normally needed for PCBs using low cost FPGAs ??

I've just been told by a supposed board layout expert that the 256 pin
BGA version of a Cyclone EP1C6 would require an 8 layer board
(apparently having the entire underside of the device covered by balls
with no free space at the centre makes signal routing a big problem).

Is this really true ??

 

[Greg Steinke]

"Kenneth Land" <kland1@neuralog1.com1> wrote in message news:<102nc03snlh3i67@news.supernews.com>...

If you can sqeak by with 240 pins I've got a custom 4 layer board running
Nios with SDRAM and USB 2.0 incredibly stable at 110-120 MHz.

Stack is Signal-GND-PWR-Signal. Made 10 with ground pours, 10 without.
Both sets function the same. I have not compared the external signals on an
analog scope, but the clocks look good, the logic analyzer looks good, and
they all work without glitch.

If you go to CompUSA and buy a few highspeed PCI cards you'll see they're
doing their magic with only 2 layers with ground pours.

I was originally told by the board expert the same 8-layer requirement on
our design. I talked him down to 6 and then went with 4 based on a lot of
reading, looking at examples, and applying common sense.

I say listen to experts, but beware the advice of people who make more money
if your design is more complex.

IMO
Ken

"Andre" <armcc@lycos.com> wrote in message
news:ae5c06e9.0402120029.4ea5405d@posting.google.com...
How many layers are normally needed for PCBs using low cost FPGAs ??

I've just been told by a supposed board layout expert that the 256 pin
BGA version of a Cyclone EP1C6 would require an 8 layer board
(apparently having the entire underside of the device covered by balls
with no free space at the centre makes signal routing a big problem).

Is this really true ??

Hi Andre,
You should be able to break a 256-ball 1mm BGA in 4 routing layers and
perhaps even 2 or 3 routing layers depending on the pin usage and FPGA
design pinout choice. The details depend on your exact board
technology, such as trace width. These are the signal layers, not
including VCC and Ground planes. You may have VCCIO be different from
VCCINT so you would have to take that into account. Potentially you
could have a split VCC plane to cover VCCIO and VCCINT to save a
layer.

The center pins are VCCint power and ground so they should be
connected directly to planes. Via breakouts should go out in four
different directions opening a space shaped like a cross to place
VCCint decoupling caps under the BGA pattern.

It's hard to give a fixed answer for all circumstances as it's very
dependent on both the FPGA design and the board technology that is
available.

Ken (above) has a good suggestion about the Q240 - actually the EP1C6
Q240 and the EP1C6 F256 both have 185 IOs, so you would not lose any
IOs by going to the Q240. However the F256 does have the advantage of
being much smaller, so you can choose between board cost and size.
Another option to consider.

Sincerely,
Greg Steinke
Altera Corporation
gregs@altera.com

 

[Kenneth Land]

Hi Greg,

I was thinking more in terms of SI rather than routing difficulty.

In that case let me share how we handled that on only two signal layers. (We
did to a split PWR plane, BTW)

It was a challenge with all the pins and signals and we wound up using a
routing service. The company does nothing but apply a full up SPECTRA
auto-routing system as a service to others. They were so knowledgeable and
helpful and the whole transaction was done by email and phone over a few
days. The price was incredible and now we'll save much more than their fee
every batch of boards due to the fewer number of layers.

I suspect their routing experience has a bit to do with the stability of our
boards as well.

Email me privately if you'd like the contact info. (remove the 1's)

Ken

"Greg Steinke" <gregs@altera.com> wrote in message
news:5c1de958.0402121904.62f8897@posting.google.com...

"Kenneth Land" <kland1@neuralog1.com1> wrote in message
news:<102nc03snlh3i67@news.supernews.com>...
If you can sqeak by with 240 pins I've got a custom 4 layer board
running
Nios with SDRAM and USB 2.0 incredibly stable at 110-120 MHz.

Stack is Signal-GND-PWR-Signal. Made 10 with ground pours, 10 without.
Both sets function the same. I have not compared the external signals on
an
analog scope, but the clocks look good, the logic analyzer looks good,
and
they all work without glitch.

If you go to CompUSA and buy a few highspeed PCI cards you'll see
they're
doing their magic with only 2 layers with ground pours.

I was originally told by the board expert the same 8-layer requirement
on
our design. I talked him down to 6 and then went with 4 based on a lot
of
reading, looking at examples, and applying common sense.

I say listen to experts, but beware the advice of people who make more
money
if your design is more complex.

IMO
Ken

"Andre" <armcc@lycos.com> wrote in message
news:ae5c06e9.0402120029.4ea5405d@posting.google.com...
How many layers are normally needed for PCBs using low cost FPGAs ??

I've just been told by a supposed board layout expert that the 256 pin
BGA version of a Cyclone EP1C6 would require an 8 layer board
(apparently having the entire underside of the device covered by balls
with no free space at the centre makes signal routing a big problem).

Is this really true ??

Hi Andre,
You should be able to break a 256-ball 1mm BGA in 4 routing layers and
perhaps even 2 or 3 routing layers depending on the pin usage and FPGA
design pinout choice. The details depend on your exact board
technology, such as trace width. These are the signal layers, not
including VCC and Ground planes. You may have VCCIO be different from
VCCINT so you would have to take that into account. Potentially you
could have a split VCC plane to cover VCCIO and VCCINT to save a
layer.

The center pins are VCCint power and ground so they should be
connected directly to planes. Via breakouts should go out in four
different directions opening a space shaped like a cross to place
VCCint decoupling caps under the BGA pattern.

It's hard to give a fixed answer for all circumstances as it's very
dependent on both the FPGA design and the board technology that is
available.

Ken (above) has a good suggestion about the Q240 - actually the EP1C6
Q240 and the EP1C6 F256 both have 185 IOs, so you would not lose any
IOs by going to the Q240. However the F256 does have the advantage of
being much smaller, so you can choose between board cost and size.
Another option to consider.

Sincerely,
Greg Steinke
Altera Corporation
gregs@altera.com

 

[Andrew Greensted]

I've got 30 spartan2s sat next to me now. I went for the TQ144 package.
We soldered them on ourselves here, onto two layer boards.

BGA is of course a completely different issue, but if you're using 'low
cost' FPGA's you'll probably find one with a more 'sensible' package
(such as TQ144)!!

Andy

Andre wrote:

How many layers are normally needed for PCBs using low cost FPGAs ??

I've just been told by a supposed board layout expert that the 256 pin
BGA version of a Cyclone EP1C6 would require an 8 layer board
(apparently having the entire underside of the device covered by balls
with no free space at the centre makes signal routing a big problem).

Is this really true ??

--
Andrew Greensted Department of Electronics
Bio-Inspired Engineering University of York, UK

Tel: +44(0)1904 432379 Mailto: ajg112@ohm.york.ac.uk
Fax: +44(0)1904 433224 Web: www.bioinspired.com

 

[William Wallace]

"Kenneth Land" <kland1@neuralog1.com1> wrote in message news:<102ok9hpug47378@news.supernews.com>...

Hi Greg,

I was thinking more in terms of SI rather than routing difficulty.

In that case let me share how we handled that on only two signal layers. (We
did to a split PWR plane, BTW)

It was a challenge with all the pins and signals and we wound up using a
routing service. The company does nothing but apply a full up SPECTRA
auto-routing system as a service to others. They were so knowledgeable and
helpful and the whole transaction was done by email and phone over a few
days. The price was incredible and now we'll save much more than their fee
every batch of boards due to the fewer number of layers.

Did you make it past emissions testing? At what rate do your signals
change? How is signal integrety? Getting a few boards to work in the
lab is not as tough as getting a design to work in production in
different envirinments while meeting goverment regulations.

 

[Kenneth Land]

Hi William,

Our fastest signals on the board will probably be the 32 bit SDRAM at
100-120 MHz. Our ADC's are operating at 6 MSPS (10 bit samples). USB 2.0 is
operating at 30 MHz. Nios will match SDRAM.

I need to look at the signals on an analog scope, but the logic analyzer is
showing very low jitter operation and the clocks (both input and PLL
generated output) look very clean and stable on my digital scope. The
boards are operating at 110 MHz for as many days as I care to let them, so
I'm fairly confident.

If you stay tuned, I'm sure I'll have a fun emissions testing story in a
month or two. NTI was great help with our first board 3 years ago. I'm
sure they'll be a big help again. We've shipped the products to 40+
countries since then with the biggest problem being that we once had to fax
the emissions report to a diligent customs official.

Believe you me, my intimate intimate life-force-draining familiarity with
production realities and EMC/Safety regulations is why I'm drooling over
this new board. Lower parts count and layer count is key. Anyone who
understands joint probability can see why.

Ken




"William Wallace" <msm30@yahoo.com> wrote in message
news:7e4865b7.0402130820.482bb3a6@posting.google.com...

"Kenneth Land" <kland1@neuralog1.com1> wrote in message
news:<102ok9hpug47378@news.supernews.com>...
Hi Greg,

I was thinking more in terms of SI rather than routing difficulty.

In that case let me share how we handled that on only two signal layers.
(We
did to a split PWR plane, BTW)

It was a challenge with all the pins and signals and we wound up using a
routing service. The company does nothing but apply a full up SPECTRA
auto-routing system as a service to others. They were so knowledgeable
and
helpful and the whole transaction was done by email and phone over a few
days. The price was incredible and now we'll save much more than their
fee
every batch of boards due to the fewer number of layers.


Did you make it past emissions testing? At what rate do your signals
change? How is signal integrety? Getting a few boards to work in the
lab is not as tough as getting a design to work in production in
different envirinments while meeting goverment regulations.

 

[William Wallace]

Sounds like you already selected the layer count.

I'd recommend the following two books if you haven't already looked at
them. They are both right on the money.

1. High-Speed Digital Design: A Handbook of Black Magic by Howard
Johnson
(Title is a misnomer, it is not black magic, and it is not presented
as black magic.)

2. High-Speed Digital System Design: A Handbook of Interconnect
Theory and Design Practices by Stephen H. Hall (Author), Garrett W.
Hall (Author), James A. McCall

Reducing layers often results in voids on the plane layers which leads
to impedance mismatches and cross talk. Using ADCs, as well as
digital ICs that have VCCOs different than internal VCCs can lead to
less than ideal use of plane islands. Add to this some application
notes put out by vendors that give bad advice (when compared to the
tried, true, and tested advice given in the above two books), and you
can run into trouble. I.e., you should always follow application
notes regarding layout recommendations. Some are on the money, others
mis-apply rules of thumb.

Also watch for board designers that use via anti-pads that are so
large on the plane layers that they result in copper voids (see
above).

Finally, if you can, get a digital scope with a FET probe and use very
low impedance grounds for the probes for lab validation of your
simulations and calculations (final sanity check).



"Kenneth Land" <kland1@neuralog1.com1> wrote in message news:<102q798km86rn0d@news.supernews.com>...

Hi William,

Our fastest signals on the board will probably be the 32 bit SDRAM at
100-120 MHz. Our ADC's are operating at 6 MSPS (10 bit samples). USB 2.0 is
operating at 30 MHz. Nios will match SDRAM.

I need to look at the signals on an analog scope, but the logic analyzer is
showing very low jitter operation and the clocks (both input and PLL
generated output) look very clean and stable on my digital scope. The
boards are operating at 110 MHz for as many days as I care to let them, so
I'm fairly confident.

If you stay tuned, I'm sure I'll have a fun emissions testing story in a
month or two. NTI was great help with our first board 3 years ago. I'm
sure they'll be a big help again. We've shipped the products to 40+
countries since then with the biggest problem being that we once had to fax
the emissions report to a diligent customs official.

Believe you me, my intimate intimate life-force-draining familiarity with
production realities and EMC/Safety regulations is why I'm drooling over
this new board. Lower parts count and layer count is key. Anyone who
understands joint probability can see why.

Ken




"William Wallace" <msm30@yahoo.com> wrote in message
news:7e4865b7.0402130820.482bb3a6@posting.google.com...
"Kenneth Land" <kland1@neuralog1.com1> wrote in message
news:<102ok9hpug47378@news.supernews.com>...
Hi Greg,

I was thinking more in terms of SI rather than routing difficulty.

In that case let me share how we handled that on only two signal layers.
(We
did to a split PWR plane, BTW)

It was a challenge with all the pins and signals and we wound up using a
routing service. The company does nothing but apply a full up SPECTRA
auto-routing system as a service to others. They were so knowledgeable
and
helpful and the whole transaction was done by email and phone over a few
days. The price was incredible and now we'll save much more than their
fee
every batch of boards due to the fewer number of layers.


Did you make it past emissions testing? At what rate do your signals
change? How is signal integrety? Getting a few boards to work in the
lab is not as tough as getting a design to work in production in
different envirinments while meeting goverment regulations.