Example Heatsink Design

Designing Heatsinks and Thermal Solutions for Xilinx Devices (XAPP1377)

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Using the information discussed up to this point, this section briefly describes an example heatsink design for the VC1902-VSVA2197 device fitting the needs of the application in terms of device resources, I/O, GT, and package size.

  1. Perform an initial thermal evaluation of the package.

    Get an initial power estimate from XPE for the desired application. Obtain the thermal simulation model from the Power Efficiency page of the Xilinx website. Perform a quick simulation using a simple representative boundary condition and ensure there is some margin in the operating junction temperature. If adequate margin is not found, iterate with different acceptable device, package, power and environmental conditions until a workable solution is found. For the purposes of this example, it is found that there is adequate thermal margin.

  2. Identify the associated documentation and package lid type, contact area, and height requirements.

    Because this is a Versal device, we used the Versal ACAP Packaging and Pinouts Architecture Manual (AM013) to determine the target package is a 45 x 45 mm lidless package.

    Table 1. Example Table from the Packaging and Pinouts Guide Specifying High Level Information of a Target Package
    Packages Description Package Specifications
    Package Type Pitch (mm) Size (mm) LSC Ball Grid Size (balls)
    SFVB625 Super-fine pitch with forged lid BGA 0.8 21 x 21
    VBVA1024 Fine pitch bare-die 0.92 31 x 31 4 x 4
    VFVB1024 Fine pitch with forged lid 31 x 31 4 x 4
    VFVB1369 Fine pitch with forged lid 35 x 35 5 x 5
    VSVE1369 Fine pitch lidless with stiffener ring 35 x 35 5 x 5
    VSVA1596 Fine pitch lidless with stiffener ring 37.5 x 37.5 5 x 5
    VIVA1596 Overhang fine pitch lidless with stiffener ring 40 x 40 6 x 6
    VFVA1760 Fine pitch with forged lid 40 x 40 6 x 6
    VFVC1760 Fine pitch with forged lid 40 x 40 6 x 6
    VSVD1760 Fine pitch lidless with stiffener ring 40 x 40 6 x 6
    VSVA2197 Fine pitch lidless with stiffener ring 45 x 45 7 x 7
    VSVA2785 Fine pitch lidless with stiffener ring 50 x 50 9 x 9
    The package physical dimensions are located within the mechanical drawings section of the document.
    Figure 1. Package Physical Dimensions for Example Target Package from the Packaging and Pinouts Guide

    VC1902-VSVA2197 is a lidless design. As previously defined, the contact area is defined by the area of the die surface. This is represented in the drawing with the dimensions of 25.75 × 17.78 mm, and is the minimum dimension of the island on the heatsink base. The maximum dimension in the X direction is 45 – (2 × 4.00) - 0.20 = 36.8 mm and in the Y direction 45 – (2 × 6) - 0.20 = 32.8 mm. However, you cannot have it overhang this much in both directions due to the protrusions in the inner corners of the stiffener ring. Thus, it is suggested to slightly oversize it, but not to the point that it touches the corners of the stiffener ring. Anticipating the uncertainties in the manufacturing of the island as well as the placement of the heatsink, we allocate a little more than 2 mm to each dimension resulting in a 30 × 22 mm contact surface for the island. This should ensure that the entire die surface area is covered under all circumstances without interference from the stiffener ring.

    Next, the height of the island is determined by the offset of the die plane to that of the stiffener ring. This would be the MAX of the A3 dimension of 1.15 mm. The height of the island must be sized so that when assembled, the heatsink base does not touch the stiffener ring so some additional margin can be added to ensure this. Finally, the A dimensions is used for the minimum height of the heatsink base from the PCB.

    A 3D STP file is then requested from Xilinx so that these sizing assumptions can be tested with the model to ensure good fit without interference in your CAD program. Further fine tuning could persist. It is also a good time to order a mechanical sample of this package so that it is available in time for receiving the first heatsink prototype for proper fitting analysis.

  3. Design heatsink base, fins, airflow, and attachment.

    The heatsink base for this example design is simple aluminum because adequate airflow and not excessively high ambient conditions are expected. If the expectation is to operate in a more challenging environment, other materials and/or 2-phase cooling should be evaluated. We plan to allocate 100 × 70 mm to the heatsink base area which should allow enough area to a fansink, and fins to allow for sufficient cooling. These parameters can be refined after thermal simulation. Using the parameters for the island gathered previously, the bottom center of the heatsink base is designed. An etched surface is specified into the island surface area in contact to the die to further improve effectiveness.

    Several fin designs were discussed and evaluated and eventually modeled to determine an optimal arrangement. A 2 1/5 inch diameter fan is recessed into the fins to accommodate height requirements for the overall design.

    To allow for even and proper pressure compensation for the life of the product, four equally spaced spring screws are designed into the corners of the heatsink as well as the proper keepouts specified for the PCB design.

    The spring constants for the springs are calculated. Initial assumptions are target pressure of 30 PSI for this device/package, area is the die area of 25.75 × 17.78 mm = 457.835 mm2 and the amount of spring compression is 0.5 inches. The first step is to convert the die area to inches: 457.835 / 645.16 = 0.7096 in2 .

  4. Choose TIM and determine application.

    VC1902-VSVA2197 is a lidless design. A TIM 1.5 is suitable for this application. In order to get the best contact resistance we will select a phase change material (PCM) composite. Taking into consideration availability, contract manufacturer capabilities, and prior experience, we have selected the Laird 780SP material. The maximum possible coplanarity deviation for the die is 0.1 mm and thus enough material should be dispensed so that it covers the volume of the maximum deviation of this mating surface or 25.75 × 17.78 × 0.1 mm = 45.8 mm3 . Additional material is specified to ensure proper coverage and margin. Details of the dispensing of material is worked out later with the contact manufacturer.

  5. Determine thermal parameters for simulation and validate thermal solution.

    Using the thermal model obtained in the initial evaluation, now elaborate the thermal model to more accurately represent the thermal system. Perform, evaluate, and iterate as necessary to gain as much thermal margin as possible. If adequate thermal margin is not obtained, re-evaluate prior decisions to obtain adequate thermal margin before proceeding.

  6. Finalize the design.

    After a final design is achieved, the heatsink design is then sent to manufacturing. More details need to be worked out with the contact manufacturer for assembly and test. The details of this is dependent on the manufacturer. After the heatsink is received, it should be tested for mechanical fit as well to ensure that pressure and board strain meet the design goals. Then thermal characterization should proceed to validate that the thermal simulation and design are still within design margins.