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Registered: ‎01-17-2020

Kintex 7 - Full Dimensions for Heat Sink Design

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Hello!

I'm working with the Kintex-7 FFG676 package and am looking for a specific physical dimension. Please see attached diagram

I can find the major dimensions in the "Xilinx UG475 7 Series FPGAs Packaging and Pinout Specification, User Guide," but the distance from the underside of the lid to the bottom of the BGA is not listed. 

I also see that you do not provide 3D models to customers, so I am unable to find an answer to this questions myself.

Can you help me find this demension (with tolerances)? Please let me know if you have any questions on my request, and when I should expect to hear back from you.

Thank you for the help!

Best,

-Will

 

Kintex-7, Needed Dimension.png
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Teacher
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Registered: ‎07-09-2009

Re: Kintex 7 - Full Dimensions for Heat Sink Design

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Id forgotten about these small heat sink clips,
but they still clip to the bottom of the package, which is a specified dimension.
This user wants to clip onto the gap between the heat spreader and the package, which is not specified, I guess mainly as the TIM under the heat spreader is a variable, or because no one else wants that dimension.

I'd not be happy mounting a big heat sink to just the heat spreader, I'd imagine one bump during transit the heat spreader would pop off, There is certainly no mechanical spec I know of as to how much force the heat spreader can take.


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Teacher
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Registered: ‎07-09-2009

Re: Kintex 7 - Full Dimensions for Heat Sink Design

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Are you aiming to take the lid off the FFG676 package ?
If not , Im intrigued as to what your going to do with the number
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Registered: ‎01-17-2020

Re: Kintex 7 - Full Dimensions for Heat Sink Design

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Hello, drjohnsmith.

Thank you for the clarifying question. I am looking to mount a heat sink to the top of the FPGA, hooking into this portion of the lid.

  • I need to determine this dimension to know how much clearance I will have between the PCB and the "hook" portion of my heat sink.

 

My goal is to get official dimensions from Xilinx, and these forums are the support path they have directed me to.

  • In addition, if Xilinx makes surface-mount heat sinks for this series of FPGA, I would be interested to learn more.
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Registered: ‎07-09-2009

Re: Kintex 7 - Full Dimensions for Heat Sink Design

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I don't think the dimension you are after then is a fixed, but a variable.

A heats sink is either stuck to the top, or mounted to the board.
A large heat sink needs to be board mounted, as the weight shock / vibration could rip off the lid. Not a good solution.

A heatsink mounted to the board, is normal for this reason, and you have the dimensions for that.

heat sinks if they have a thermal material under them ( TIM ) , need to be fixed with a given pressure , to little and the TIM will not flow, to much and you risk bowing the board and de padding the FPGA . This is one reason why you often see springs under the mounting screws on heat sinks,

have you seen chapter 3 here
https://www.xilinx.com/support/documentation/user_guides/ug112.pdf

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Registered: ‎01-17-2020

Re: Kintex 7 - Full Dimensions for Heat Sink Design

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Thank you for the reply, Dr. John.

I understand that heat sinks are often mounted to the PCB or adhered to the top. These are both non-ideal options for my application, and I would thus like to explore others - including the "clip-on" option I mentioned. One example of such a solution is in figure 3-12 of the document you linked (which was useful, thank you). 

With this type of retention, the heat sink mouting bracket clips to FPGA itself. I was assuming this clips to the lid of the package, but it could be clipping to the substrate.

My question is: what is the thickness of the portion of the lid this would clip to? (illustration in my initial post) Is information provided officially somewhere?

 

 

Clip-on Heat Sink (Black outer clip secures to FPGA, retains black Al heat sink).png
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Registered: ‎07-09-2009

Re: Kintex 7 - Full Dimensions for Heat Sink Design

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You will see that photo you shared is of a 4K series, in a PGA package,
Those , the heat sink was designed to clip to the bottom of the package, as there was a gap between the package and the PCB due to the pins. The package depth was defined in the specs of the data sheet.

Surface mount chips dont have that "advantage" ,

I think we have been around this loop , unless some one else knows other wise,

to recap:

a 3D model is a random sample of one, not a specification that the manufacturer and has no tolerances. Its no better than you getting a chip yourself and measuring a random sample of one.

Heatsinks depending upon the packages and use, either stick to the heat spreader, or affix to the board using screws and springs to ensure the correct pressure on the TIM.

Any other special information you are after, you are going to have to set up a NDA with Xilinx, probably signed by the company director. Your first contact for an NDA is your FAE.

https://forums.xilinx.com/t5/General-Technical-Discussion/How-do-I-contact-a-FAE/td-p/746194

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Scholar
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Registered: ‎04-26-2015

Re: Kintex 7 - Full Dimensions for Heat Sink Design

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Some BGA heatsinks do actually clip on between the PCB and the FPGA body - example. There's not much gap there, but most FPGAs don't require a very big heatsink and so thin plastic wedges are sufficient. Obviously if you've got a big chip that needs to dissipate 20W+ heat (or withstand substantial vibration) you'll need to provide appropriate mounting holes on the PCB, but for lab use on small chips these heatsinks do a nice job.

 

 

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Registered: ‎07-09-2009

Re: Kintex 7 - Full Dimensions for Heat Sink Design

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Id forgotten about these small heat sink clips,
but they still clip to the bottom of the package, which is a specified dimension.
This user wants to clip onto the gap between the heat spreader and the package, which is not specified, I guess mainly as the TIM under the heat spreader is a variable, or because no one else wants that dimension.

I'd not be happy mounting a big heat sink to just the heat spreader, I'd imagine one bump during transit the heat spreader would pop off, There is certainly no mechanical spec I know of as to how much force the heat spreader can take.


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Registered: ‎07-23-2019

Re: Kintex 7 - Full Dimensions for Heat Sink Design

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will.hilzinger@ni.com 

The distance you mention depends on how the solder balls melt down and reshape, that depends on many factors and I believe it's "highly" variable (maybe plus/minus 0.1 mm).

my advice: you don't need that distance. At least a precise figure. Merge with mechanical engineers. They talk of "nominal" measures and tolerances, so everything has a max and a min. Your heatsink will attach to the FPGA lid and will be at some (min, max) distance to the PCB. Use the min to check is not touching any component. The heatsink will probably be held by screws tightened a bit more or less depending on the distance. You may want some o-rings with the spacer to accomodate for the tolerance. I think that's it.

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Re: Kintex 7 - Full Dimensions for Heat Sink Design

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I understand from the diagram at the start, the user wants to clip onto the heat spreader / lid , not the package, and certainly not the board.
.
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Registered: ‎07-23-2019

Re: Kintex 7 - Full Dimensions for Heat Sink Design

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will.hilzinger@ni.com 

So is your plan to decapsulate packages for alternative heatsink? Good luck. why don't you use lidless packages? The dimension you want may be in lidless packages documentation. Have you stumbled upon UG475 Appendix B? I hope you are aware of the implications of attaching heatsinks to the bare silicon. Lidded packages exist for a reason. Do you think a fraction of a mm of metal will add too much thermal resistance, keeping in mind "thermal adhesive" is not that much conductive, is just thin? Have you pondered the risks and associated cost of heatsinking the silicon? Have you done a cost-benefit analysis? My take on that is you should have serious reasons for doing that.

I was in a company doing some tests on chip cards. One of the operations was decapsulation: tear the contacts away (that was easy), then swell the card with acetone and scratch *carefully* with an xacto knife till there was a thin layer above the chip, then rub *very gently* with a cotton bud. The silicon side exposed is actually the back, where there are no transistors, just bulk silicon, nevertheless it's amazing how easily the chip is damaged. And I guess the bigger the more fragile. a card chip is some 3 mm, an FPGA die can be 10, 20, 30 mm (?) with similar thickness I guess. Again: good luck. 

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Registered: ‎04-26-2015

Re: Kintex 7 - Full Dimensions for Heat Sink Design

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@drjohnsmith I suspect that he's just trying to clip the heatsink on anywhere it'll fit, and is under the (mistaken) understanding that it should be clipped onto the heatspreader.

 

As above, the standard way to attach a heatsink (when no PCB mounting points are available and adhesive is not acceptable) is to clip onto the FPGA package directly.

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Registered: ‎01-17-2020

Re: Kintex 7 - Full Dimensions for Heat Sink Design

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Hello all,

Thank you for the input. One heat sink manufacturer had specified that the "clips" should be mounted to the heat spreader, hence my original line of questioning.

From this discussion, it sounds like this is not recommended (due to shock/vibration concerns), and the preferred approach it to mount to the substrate. As @drjohnsmith  mentioned, I do have those dimensions so should be able to get what I need.

I appreciate the discussion and the help you've provided.