Re: ADC125, signal dist module, 2eSST tests

["Fernando J. Barbosa" <barbosa@jlab.org>]

Hi Gerard,

I can model the thermal transfer through vias but that's very tedious 
and does not provide any major benefit. I would leave them out as a 
first check and add them later if we feel the performance is marginal. I 
will need the component heights and these can be added in PCAD as 
attributes. Otherwise, I was planning on getting the standard package 
heights and enter them separately. It would be helpful if you could 
provide a short table with these values.

It is great if the ADCs are in a QFN package. The heat transfer  is 
excellent!

Indeed heat sinks can cause problems if they obstruct the airflow, 
especially for parts placed in rows along the air flow, like the ADC 
chips. I plan to perform simulations for the second versions of the 
fADC250 and the F1TDC, as well, for exactly those same reasons. Just as 
a final check, I will qualify the thermal simulations with multi-point 
thermocouples and flow measurements under realistic conditions. I could 
also perform the same type of measurements on your fADC125 prototype for 
consistency.

The software that I have takes into account the fluid dynamics and I can 
also set the boundary conditions, as for adjacent boards, for example. 
It has been shown to be very accurate in industry tests and I have 
verified that on recent tests with the preamp card against thermo-couple 
measurements. I can say I have a high degree of confidence in this 
package once the details of the model are well understood.

I believe the layer stack info is included. If not, I will request it 
from you later.

Regards,
Fernando



Gerard Visser wrote:
> Hi Fernando,
>     It would be great to get a thermal simulation... I will prepare a 
> P-CAD file with the principal heat-generating components for the main 
> & mezzanine boards. Do you intend to model details such as thermal 
> trasfer through vias, or only the component and board and some 
> idealized connection between them? Do you need component height 
> attributes in the file?
>     The ADC's _are_ indeed in a QFN package.
>     I understand what you're saying about turbulent flow being 
> important, but still I think that if the hot region is obstructed by 
> too much stuff (such as heatsinks or turbolators of some sort) it may 
> just divert the flow and yield worse results than a more "open" 
> design. Does your thermal simulation software solve the fluid dynamics 
> problem, or just the board thermal conduction problem with some ad-hoc 
> assumptions about transfer into the air?
>     Thanks,
>
>     Gerard
>
> p.s. The layer stack info is all in the P-CAD file. Or maybe that 
> doesn't come through in the P-CAD2002 format, I don't remember - I can 
> send it separately then, let me know.
>
>
>
> Fernando J. Barbosa wrote:
>> Hi Gerard,
>>
>> The VME spacing of 8 mm between modules is very wide by today's
>> standards, given the low profiles of the SMT components. Good heat
>> transfer, via forced convection, is accomplished by turbulent flow
>> around the components and so most of the air in the ~8 mm gap doesn't
>> contribute much to the cooling of the components. That is, the cross
>> section of the air flow responsible for cooling is small and a small
>> percentage of the total flow. Therefore, the use of mezzanine boards is
>> a very good approach to increasing  packaging density while maintaining
>> proper cooling of the components. Packages such as QFN provide great
>> heat transfer efficiency to the board but are not available on your ADCs
>> but increasing the mass connected to the chips will also help.
>>
>> In any case, I can perform the thermal simulations for your layout once
>> you have settled on the placement. All I need is the PCAD PCB file,
>> power dissipation per component and type and layer metal content
>> (#layers, percentage of metal, thickness, etc.) Just let me know when
>> you have all these information.
>>
>> Regards,
>> Fernando
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