package choice, temperature and obsolesence issues with a xi

[jean-francois hasson]

Hi,

I am involved in a project dealing with fpgas and the pcb may only be used
on one side for component placement (actives an passives). Therefore I am
considering using a QFP package instead of a BGA package for decoupling
purposes mainly. My application runs at 40 MHz and the IO need is small
(about 100 IOs). I am considering either a 208 QFP or a 256 FBGA for a
Xilinx part like spartan IIE. The other issue is about obsolescence. I
noticed the QFP package is still available for Spartan III. Is it a good
indicator for the life of this package for Spartan IIE or is this idea
irrelevant ? Finally, the temperature of the environment will border 85°C so
I believe the regular industrial grade parts we use may be out of our specs.
What is the rule for Xilinx parts like spartan IIE or else ? Since the
junction temperature is 100°C what happens if the part is in an ambient
temperature above 85°C ? Will it keep working or will performance be
degraded ? Does it depend on the process of the FPGA for a same grade ? Is
there a derating one can apply if the FPGA is out of specs but not about to
be destoyed because junction temperature was not reached ?
Any idea is welcome !!!

JF

 

[Austin Lesea]

JF,

Wow. Not enough questions?

See below, I will try to do my best.

Austin

jean-francois hasson wrote:

Hi,

I am involved in a project dealing with fpgas and the pcb may only be used
on one side for component placement (actives an passives). Therefore I am
considering using a QFP package instead of a BGA package for decoupling
purposes mainly.

A QFP package has significantly more inductance, and is harder to get a
good decoupling design done right. A fgXXX package (fg256) is a much
lower inductance value package, and with all components on top, is not
impossible at all to do a power distribution decoupling scheme well.
You may need two more layers in the pcb stackup to do it right (in
either case). If you are having problems with your layout, you may send
it into the Hotline, and someone will look at it for you, and advise
you. My Lab gets invovled in many pcb cases. I would prefer you learn
how to do it right and succeed the first time!

My application runs at 40 MHz and the IO need is small

(about 100 IOs). I am considering either a 208 QFP or a 256 FBGA for a
Xilinx part like spartan IIE. The other issue is about obsolescence. I
noticed the QFP package is still available for Spartan III. Is it a good
indicator for the life of this package for Spartan IIE or is this idea
irrelevant ?

The QFP will be around for quite awhile. It is too popular, and too
high a seller. For extreme SI applications, it is not used, however.

Finally, the temperature of the environment will border 85°C so

I believe the regular industrial grade parts we use may be out of our specs.

85C to 100C is 15C junction to ambient. In a QFP package, that might
limit you to about 1 watt of power dissipation in the part! This will
be tough.

What is the rule for Xilinx parts like spartan IIE or else ?

The 'rule' is that above 85C for commercial, and above 100C for
Industrial, the device is not expected to meet specifications (but it
will still function up to 125C).

Since the

junction temperature is 100°C what happens if the part is in an ambient
temperature above 85°C ?

Specifications may not be met.

Will it keep working or will performance be

degraded ?

Functionality will continue to ~125C junction, and beyond that we make
no statements or assurances.

Does it depend on the process of the FPGA for a same grade ?

No.

Is

there a derating one can apply if the FPGA is out of specs but not about to
be destoyed because junction temperature was not reached ?

In the old days, when everything was simple, simple derating formulas
could be applied to the CLB, routing, etc. With DCMs, BRAMs, etc. this
no longer works at all. Perhaps you need the automotive version of the
parts which is characterized (has a speeds file) for 125C abs max?
Contact your disti or Xilinx FAE.

Any idea is welcome !!!

Contact your disti or Xilinx FAE. They can be more helpful than I.

JF

 

[Eric Smith]

Austin Lesea <austin@xilinx.com> writes:

A fgXXX package (fg256) is a
much lower inductance value package, and with all components on top,
is not impossible at all to do a power distribution decoupling scheme
well. You may need two more layers in the pcb stackup to do it right
(in either case). If you are having problems with your layout, you
may send it into the Hotline, and someone will look at it for you, and
advise you. My Lab gets invovled in many pcb cases. I would prefer
you learn how to do it right and succeed the first time!

Is there an appnote with recommendations and/or examples? That would
be really useful!

 

[Symon]

Eric, try XAPP623. Cheers, Syms.

Is there an appnote with recommendations and/or examples? That would
be really useful!

 

[Austin Lesea]

Eric,

The SI webpage on support.xilinx.com lists all of the collateral
material that we can provide (and there is a lot). There are a couple
of app notes that cover this topic.

Austin

Eric Smith wrote:

Austin Lesea <austin@xilinx.com> writes:

A fgXXX package (fg256) is a
much lower inductance value package, and with all components on top,
is not impossible at all to do a power distribution decoupling scheme
well. You may need two more layers in the pcb stackup to do it right
(in either case). If you are having problems with your layout, you
may send it into the Hotline, and someone will look at it for you, and
advise you. My Lab gets invovled in many pcb cases. I would prefer
you learn how to do it right and succeed the first time!


Is there an appnote with recommendations and/or examples? That would
be really useful!