Back to max thermal and power for XC4VLX200's

On 1 Feb 2006 11:01:23 -0800, fpga_toys@yahoo.com wrote:

>
>Allan Herriman wrote:
>> Is it difficult to measure typical values of the voltage drop on
>> actual hardware though?  Just set some outputs to be CMOS highs and
>> lows and measure their voltage at some convenient spot on the board.
>
>Measuring VccInt from an I/O pad?   ... some trick?

No, but you can measure the GND voltage on the die that way.  That's
half the answer you want.

Allan.

Allan Herriman wrote:
> No, but you can measure the GND voltage on the die that way.  That's
> half the answer you want.

No. But one can measure the carrier ground plane, which is half way to
half of what I want :)

fpga_toys@yahoo.com wrote:
> Allan Herriman wrote:
> 
>>No, but you can measure the GND voltage on the die that way.  That's
>>half the answer you want.
> 
> 
> No. But one can measure the carrier ground plane, which is half way to
> half of what I want :)

  You can measure the GND on an IO ring, and that can still be usefull -
that is the method used to test for crosstalk.

  Measuring via a sense-routed bump, will likely get you much more than
'half way' to the Core Vcc - because there are very many, very short 
bumps from the carrier plane, to the die, and the PCB traces/vias that 
go back to the regulator are included in the other leg.
  The powersupply controller can also do what it likes with the Sense 
Feedback, and Current information, so you can, (if it really matters to
you), regulate a virtual point ahead of the sense point, to a stable
voltage.
  - jg

Austin Lesea wrote:
> All,
>
> More heat is conducted out the bumps, through the substrate, through to
> the pcb than through the backside heat spreader (without a heatsink).

Howdy Austin,

Could you provide a bit more detail on this?    UG112 seems to say that
theta JB varies too much from situation to situation to be worth
publishing.  If it has even more impact on cooling the device than the
theta JC, it seems like more information should be provided.

Furthermore, how much closer to 0 degC/W could the thermal resistance
be, compared to the ~0.6 degC/W of the flip-chip packages?  Or were you
referring to everything except flip-chip?

> Even with a heatsink, as much as half of the power is going through to
> the pcb.
>
> I know that is hard to believe, but the heat is much closer to the
> bumps, the bumps are metal (ultra low alpha lead), and they go directly
> to a copper plane in the substrate (package pcb).  FR4 and epoxies are
> pretty good at conducting heat.
>
> The lead balls to the copper pcb completes the (best) heat conduction path.

Isn't the heat spreader on the flip-chips also copper?  It seems like
going  through one tiny layer of thermal grease and one layer of
heatspreader would have less thermal resistance than  bumps + epoxy +
substrate + ball + pad + via + ground plane.  I don't see mention that
the substrate has a substantal amount of copper in it, but that doesn't
mean it isn't there and just not well documented.

> The backside of the die is almost 1 mm of SiO2 away from the area that
> is hot, and has to then go through a thermal compound to get to the top
> heat spreader, and then has to be mechanically bonded to a heatsink (if
> you really want to get power out of the top of the package).

I think you are referring to non-flip-chip here?

Thank you!

   Marc

Marc,

I was only talking about flip chip.

The die vias, metal, bumps, planes, vias and balls are all metal.  They 
conduct heat very well.  They are very shot lengths.  The epoxy and pcb 
material also is a great conductor of heat.

The grease and copper top is good, but not as good.  Especially after 
800 microns of glass (the backside of the die).  Further to go.  And 
then one more interface to the air (teriible) or to a heatsink (also may 
not be very good).

For all the details, you would have to contact your FAE and have them 
discuss a particular case with our packaging department.

Austin

Marc Randolph wrote:

> Austin Lesea wrote:
> 
>>All,
>>
>>More heat is conducted out the bumps, through the substrate, through to
>>the pcb than through the backside heat spreader (without a heatsink).
> 
> 
> Howdy Austin,
> 
> Could you provide a bit more detail on this?    UG112 seems to say that
> theta JB varies too much from situation to situation to be worth
> publishing.  If it has even more impact on cooling the device than the
> theta JC, it seems like more information should be provided.
> 
> Furthermore, how much closer to 0 degC/W could the thermal resistance
> be, compared to the ~0.6 degC/W of the flip-chip packages?  Or were you
> referring to everything except flip-chip?
> 
> 
>>Even with a heatsink, as much as half of the power is going through to
>>the pcb.
>>
>>I know that is hard to believe, but the heat is much closer to the
>>bumps, the bumps are metal (ultra low alpha lead), and they go directly
>>to a copper plane in the substrate (package pcb).  FR4 and epoxies are
>>pretty good at conducting heat.
>>
>>The lead balls to the copper pcb completes the (best) heat conduction path.
> 
> 
> Isn't the heat spreader on the flip-chips also copper?  It seems like
> going  through one tiny layer of thermal grease and one layer of
> heatspreader would have less thermal resistance than  bumps + epoxy +
> substrate + ball + pad + via + ground plane.  I don't see mention that
> the substrate has a substantal amount of copper in it, but that doesn't
> mean it isn't there and just not well documented.
> 
> 
>>The backside of the die is almost 1 mm of SiO2 away from the area that
>>is hot, and has to then go through a thermal compound to get to the top
>>heat spreader, and then has to be mechanically bonded to a heatsink (if
>>you really want to get power out of the top of the package).
> 
> 
> I think you are referring to non-flip-chip here?
> 
> Thank you!
> 
>    Marc
> 

Austin Lesea wrote:
> Marc,
>
> I was only talking about flip chip.
>
> The die vias, metal, bumps, planes, vias and balls are all metal.  They
> conduct heat very well.  They are very shot lengths.  The epoxy and pcb
> material also is a great conductor of heat.
>
> The grease and copper top is good, but not as good.  Especially after
> 800 microns of glass (the backside of the die).  Further to go.  And
> then one more interface to the air (teriible) or to a heatsink (also may
> not be very good).

Austin,

I understand now.  Thanks for the response - among other things, the
glass on the backside of the die was something I overlooked.

Thank you,

   Marc