CMX Heat-Sinks FPGA and MiniPOD ------------------------------------- Original Rev. 18-Feb-2013 Current Rev. 25-Apr-2014 The CMX card requires heat sinks for two component types: the Virtex FPGAs and the MiniPOD optical components. These two applications will require different types of heat sinks and will be described separately. Virtex FPGA Heat Sinks: ----------------------- The Virtex FPGAs on the CMX card are in the Xilinx FFG1759 package. - This package has a 42.5 mm square metal top thermal contact. - The absolute maximum junction temperature of the Virtex FPGAs used on the CMX card is 85 deg C. - The expected thermal resistance from the junction to the top metal thermal contact lid is about 0.1 Deg C per Watt. There is a parallel junction to board thermal resistance of about 2.0 deg C per Watt. - The heat dissipation in the CMX Virtex FPGAs is estimated to be > 7 Watts < 30 Watts. - The surface of the top metal thermal contact lid is between 2.80 and 3.50 mm above the upper surface of the pcb. I do not know how parallel we can assume that the FFG1759 lid will be to the CMX circuit board. - To be within specifications, the maximum height of any component on a VME card is 13.716mm. If we assume the maximum 3.50mm mounting height for the FFG1759 this implies that the heat sink can be at the most 10.2 mm tall ( 0.402 inches tall ). This is not enough height to use a heat sink with a built in fan - The expected air flow velocity in the VME card crate is at least 200 feet/minute or 1 meter per second. - I will assume that the air inlet temperature is 30 deg C or less. - One must control the force of the heat sink against the top metal thermal contact lid of the FFG1759 package. A rational force is 30 pounds. Numbers range from 5 to 70 pounds. Typically you need 300 kPa (43 lb/sq_in) to get full thermal connection with a thermal grease TIM. The contact lid of the FFG1759 is about 2.8 sq_in which implies 120 lb of clamp force for full thermal contact. - There is not enough space on the back side of the card to include a stiffener aka force spreader under the BGA package. Thus we are limited in how much force we may use to clamp the heat sink to the Virtex FPGA. - Springs are used on the top side of the heat sink to control the force of the heat sink against the lid of FFG1759 package. - Extra attachment bolts are used out, near the perimeter of the heat sink, that do not apply clamping force, but rather just control / limit the movement of the heat sink relative to the card. - The bolts that apply clamping force to the heat sink are located symmetrically around the FPGA package so that the heat sink will remain flat against the FFG1759 lid. - If done right the expected thermal resistance in the joint between the FFG1759 lid and the heat sink is 0.1 deg C per Watt. - The lowest expected mounting height of the FFG1759 ( 2.80mm ) allows enough space between the circuit board and the bottom of the heat sink to mount all of the capacitor types that need to be close to the FPGAs. Specifically the expected capacitor heights are: 0.87mm for the 100 and 220 nFd, 1.40mm for the 4.7 uFd 2.10mm for the 33 uFd tantalum. The bottom of the heat sink needs to be milled to provide clearance for any taller components that it covers. - The heat flow out of the silicon will be on two main paths: junction to top lid through interface to heat sink then from the heat sink to ambient and a parallel path of: junction to board then board to ambient I have manufacturer's numbers for all of these thermal resistances except for the board to ambient. The board to ambient thermal resistance obviously depends on the details of the board design and the air flow around the board. For the CMX, its board to ambient thermal resistance is probably pretty low because of the 10 full coverage Ground layers and because some of these layers are close to the surface of the card. This thermal resistance probably not more than 3 deg C per Watt. The 10 ground planes are 1/2 oz which gives an overall ground thickness of 5 oz or about 7 mils of copper. - The Xilinx white paper 258 indicates that a typical heat sink design will result in *about* 25% of the heat going out through the board and about 75% of the heat going out through the package lid and heat sink. - What is the temperature of the air coming out of the heat sink with this 30 Watt load and with 30 deg C air going into the heat sink ? The heat capacity of air is about 36 Joules per cubic foot deg C i.e. 36 Watts will rise 1 cu_ft 1 deg C in 1 second. The cross section of the air flow through the heat sink is about 0.013 sq ft. With 200 linear ft/min flow through the heat sink this gives about 2.5 cu_ft / min or about 0.04 cu_ft / sec flowing through the heat sink. There is about 75% of 30 Watts or 22.5 Watts going into the heat sink. This is enough heat to rise 1 cu_ft per second of air 0.625 deg C. But we have only 0.04 cu_ft per second flowing through the heat sink which gives an air temperature rise of about 15.6 deg C. - What is the power limit without a heat sink but running vertical in the crate with 250 LFM air flow ? Without a heat sink but with 250 linear feet/min air flow the FFG1759 package has a junction to ambient thermal resistance of 4.7 deg C per Watt. This is in parallel with the 2 + 3 deg C / Watt thermal resistance for heat flowing out through the circuit board. This gives an overall thermal resistance of about 2.42 deg C / Watt junction to ambient. With a 30 Watt load and 30 deg C input air this would give 103 deg C silicon temperature. With a 22 Watt load and 30 deg C input air this would give 83 deg C silicon temperature. With a 16 Watt load and 30 deg C input air this would give 69 deg C silicon temperature. - What is the power limit without a heat sink and running flat on the bench without forced air flow ? Without a heat sink and without forced air flow the FFG1759 package has a junction to ambient thermal resistance of 7.8 deg C per Watt. This is in parallel with the 2 + 7 deg C / Watt thermal resistance for heat flowing out through the circuit board. The 7 deg C / Watt board to ambient comes from the board being flat on the bench. This gives an overall thermal resistance of about 4.2 deg C / Watt junction to ambient. With the 7 Watt quiescent heat load and 30 deg C ambient air this gives a silicon temperature of 59 deg C With a 9.5 Watt heat load and 30 deg C ambient air this gives a silicon temperature of 70 deg C. - Some of the possible errors in the above design estimates: The BF heat sink is down stream from the hot air that could come out of the TP heat sink and thus BF FPGA silicon temperature may be hotter than indicated above. The heat sink is close to the pcb and thus has limited air flow under it. The published thermal resistance data for the heat sink assumed air flow all around it. About 18% of the heat sink's surface area is the surface on its bottom side next to the pcb which may have limited air flow. The board to ambient thermal resistance is only an estimate and needs to be studied or measured. The heat sink may work slightly better than the manufacturer's data because the heat is passed to the sink over 42.5mm x 42.5mm rather than over 1 sq inch. There are many more sources of error in these estimates. - A stock heat sink extrusion with good fin design for the CMX Virtex application is QATS types: ATS-EXL1-254-R0. The ATS-EXL1-254-R0 is $51.71 ea in small quantity. - Specifications of the ATS-EXL1-254-R0 material: 100.0mm wide 10.0mm height overall 2.0mm base thickness --> 8.0mm tall fins 2.5mm fin center to center aprox. ?mm width of a fin ?mm width of the gap between fins 40 fins in the 100mm width, edge fins are full width 5 deg C per Watt thermal resistance for a 76mm long section and 200 linear feet/min air flow. 31.0 sq inch per inch of length total surface area calculated from perimeter 33.2 sq_in / inch - The final decision is to use ATS Part No: ATS-EXL1-254-R0 Heat Sink Extrusion for the BF and TP Virtex FPGAs. This is a 100mm x 254mm x 10mm extrusion. - The final drawings of the BF and TP Virtex FPGA Heat Sinks and the details of mounting these heat sinks are in: http://www.pa.msu.edu/hep/atlas/l1calo/cmx/hardware/drawings/heat_sinks/ m1_base_function_fpga_heat_sink.pdf m2_topological_fpga_heat_sink.pdf m20_fpga_heat_sink_mounting.pdf - After manufacture in the MSU Physics Machine Shop the CMX Virtex heat sinks were black anodized. - The details of mounting these Virtex heat sinks onto the CMX card and the hardware, springs, and thermal compound used to mount them are given in the Final Assembly document. MiniPOD Heat Sinks: ------------------- - The MiniPOD heat sink from Avago is 17mm x 20mm mounts on top of the MiniPOD and has 20 prongs that stick up into the air flow. - Each of these 20 prongs has a 2mm x 2mm cross section and is 10 mm long. 80 sq_mm per prong. (20 x 80 sq_mm) + (17mm x 20mm) = 1940 sq_mm total exposed surface on the Avago MiniPOD heat sink. - I have not identified exactly what manufacturer and part number the Avago MiniPOD heat sink is but from looking at similar parts it probably has a thermal resistance in the range of 20 to 25 deg C / Watt at 200 LFM air flow. - The Avago heat sink uses a spring clip mount and there are no specific mounting instructions that I know of. - Avago data sheet specification: Maximum Operating Case Temperature: 70 deg C Module Top Surface Load Limit: 3 Kg Transmitter 2.5 Volt Current Max: 400 mA Transmitter 3.3 Volt Current Max: 160 mA --> Transmitter Heat Max: 1.6 Watts Receiver 2.5 Volt Current Max: 525 mA Receiver 3.3 Volt Current Max: 90 mA --> Receiver Heat Max: 1.6 Watts - So with 1.6 Watts of heat and 30 deg C cooling air and a maximum case temperature of 70 deg C we need a case to ambient thermal resistance of 25 deg C / Watt or less. - We need to use some kind of heat sink that is in the range of 20 to 25 deg C / Watt thermal resistance. Obviously because of its height, the Avago supplied heat sink can not be used on the CMX card. - Our MiniPOD heat sink design fits down over the sides of the MiniPODs and attaches to the MiniPOD with thermal epoxy. - As placed on the CMX there is enough space for an 18mm wing to the East and enough space for a 4mm bar on the North and South side. This the heat sink stock material needs to be at least 18 + 18 + 22 = 58mm East-West and 4 + 4 + 18 = 26mm North-South. - The heat sink sits on the bottom edge of the side skirt and thus can be at the most 8mm high. - The aluminum extrusion bar stock that was used to make the CMX MiniPOD heat sinks is: Alexandria Industries Extrusion Part Number: MM12854 This material is 38mm wide and comes in 6 ft bars. - To make the heat sinks the bar stock is cut into 28mm lengths and then wire EDM is used to cut the fancy shaped hole in the center that fits down over the MiniPOD. This is a nice snug fit on the shoulders of the MiniPOD - The final drawings of the MiniPOD Heat Sink is in: http://www.pa.msu.edu/hep/atlas/l1calo/cmx/hardware/drawings/heat_sinks/ m21_minipod_heat_sink_blank.pdf m22_minipod_heat_sink_hole.pdf - The finished heat sink is anodized black and then attached to the MiniPOD using Wakefield Part Number: DeltaBond 155 thermal epoxy.